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Poor Performance Troubleshooting

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I have a fairly odd problem. Started after replacing the head gasket. First, the car runs fine. But - when putting the car in 3rd or 4th gear below 2,000 rpm's (with the clutch in), it stalls. That's the simple explanation. Example: Sitting still (not moving), pushing in the clutch, put the car in 3rd gear. stall. when I step on the gas so it runs at anywhere below 2,000 rpm, the same thing happens. If I go above 2,000 rpm, nothing happens--the rpm's don't drop, or anything else. I'm thinking it may be an oil pressure issue - but why would it happen only on 3rd and 4th gears? and why below 2,000 rpms? I'm at a loss.

Frank: this problem is characteristic of USA cars of MY 68 or later. One of the smog controls implemented on these cars shuts off the injection pump under certain conditions, most notably coasting in 3rd or 4th gear at engine speeds above 1,250 rpm. Apparently, your pump is being cut off even though you aren't coasting. As a quick check for this, simply disconnect the lead to the lower pump solenoid. Your throttle housing switch is probably out of adjustment, defective, or its leads are shorted.

I still can't get my engine running right on my US 1970 280 SL although it starts up easily. It idles much too low (350 rpm in drive), misfires upon deceleration and eventhough it revs up fairly well, it seems down on power and feels like it's sputtering too much. The spark plugs are of the correct heat range and quickly develop a layer of soft sooty carbon deposits even after freeway-only runs, leading me to believe that it is running too rich. Valve and injection pump timing are correct. I adjusted all the throttle linkages. The Venturi throttle valve and fuel injection pump are in sync and the fuel injection pump rack has not been changed since the car ran well. Oh, compression is good and uniform as well. I have a strong spark and I set the ignition timing to 10 BTDC without and 8 ATDC with the vacuum connected. Even though my distributor says 0 231 116 062 (US emission version), it only has the single vacuum advance diaphragm. I thought that this unit is supposed to have a two compartment vacuum diaphragm? Is it posible that the prior owner replaced just the diaphragm to a single unit? In that case should I use the timing numbers for the standard engines? (I have tried using those numbers as well and the engine still doesn't run any better).

I would also check that you don't have a cold start valve leak by. That will give you rich run no matter what.

Do you know an easy way to check for cold start leak by. The terminal has been disconnected all this time, but obviously the fuel pipe has still been connected to it. I guess I have to disconnect it since I can't just pinch off the line?

Take the valve off the intake, leave the fuel line connected, no electric power (which you already have unhooked) and turn the engine over with the starter. If it leaks by, you should see it even at cranking speed. They can be re-stoned and seated.

Since your car is a '70 you might want to check the ignition amplifier (this is the 'black box' that sits underneath the battery tray or thereabouts). Mine grew old and caused the problems of missing on deceleration, poor power and missing after it was driven for a while. This is a best a guess.

Frank: deceleration backfires in 3rd or 4th can be eliminated by reconnecting the fuel shutoff system, which has probably been disabled. However, in my experience this is frequently a sign of bad points. The idle speed can be increased by backing out the idle air screw, and turning in the idle mixture screw a few clicks. If you have an auto trans, it might require some additional adjustments. If it's running too rich over the full range, and your cold start system is all working OK, my solution is to remove shims from under the barometric compensator.

Joe Alexander: make sure the engine is in good tune. Good idea to check cold start valve for leaks as posted previously. I saw an old German mechanic blocking off fuel lines on a 300-SL once by inserting a small steel ball in the fitting then snugging up the line. The ball becomes a leakproof barrier. The ball must be the right size, not too big, and do not over tighten! It works good I have a nice assortment of steel balls in my tool box for this purpose. Check the fuel pressure! Make sure the fuel filter is clean. I unhook the cold start line and clamp a pressure gauge on the end. Make sure the warm-up valve on the injection pump is not stuck. If you need the procedure on this let me know. Check to see if the rack moves freely on the injection pump. I am also a little suspecious about the injectors if you had them cadmium plated. This may have caused some problem. If you need a cheap injector tester; find a used steel injection line, hook it up to your injection pump. Route this line to a convenient position so you can hook up each of your injectors one at a time to observe the spray pattern while someone cranks the car. Caution!! eliminate all ignition sources first! So you will not ignite the fuel! Keep an extinguisher handy and do it outdoors if youíre working at home. Unplug ignition. I usually remove all spark plugs to make cranking easier and eliminate fuel in other cylinders. Catch the fuel from your injector in a container while cranking. It will take a few revolutions to purge the air. A good injector will give a nice fine vaporized cone-shaped pattern of fuel and a distinctive churp noise.

Will: a bad coil will give similar problems to the ones you describe. And: I thought there was constant fuel pressure to the cold start valve as long as the electric fuel pump was running? So, should not need to crank the engine.

Some ideas on rough running diagnosis. This is for a car that starts and runs but is rough running:

  • fouled or old spark plugs
  • defective spark plug wire(s) or plug terminal(s)
  • burnt points
  • point gap wrong
  • clogged fuel filter
  • stuck injection plunger
  • bad fuel
  • bad fuel pump

A quick way to isolate some of the above is while the car is running pull plug wires off one at a time. The idle should fall off. When it does not, that is the bad cylinder. Then check the ignition system from cap to end of plug wire or pull the plug and install new one. If this does no good, while the car is running unscrew the injection line going to the injector for that dead cylinder. Fuel should come out in little spurts. If it does not, look for clogged line or stuck injector. Fuel pump should put out 10-15 psi and one gallon a minute flow. I do not believe it remains pressurized after you turn it off. I have never heard a release of pressure when changing a fuel filter or undoing an injection line.

Let me give a little more info on my problem. The car runs well for about ten miles and then it starts running a little rough. After resting it runs well again for a little while and same thing. I replaced the fuel filter about 1000 miles ago. I just put new plugs in prior to the last drive. The points have less than 200 miles on them. Gap is good. I wish I had a ignition oscilloscope. I am pretty sure the fuel pressure in the system should stay there for a while after shut down. It prevents vapor lock when trying to restart while warm. I did check the flow rate of the pump at the cold start valve connection, it seemed to put out enough. I was mainly interested to see if there were any air bubbles getting into the system. When the engine ran rough it would idle fine and then just drop off almost dying and then pick up again.

My 230 SL normally performs flawlessly in all weather with immediate starting and good performance. But there is one frustrating characteristic that my experienced mechanic here in Germany tells me is normal. When driving at part throttle and slow speeds (around 20-30 mph), lifting off the throttle and then getting back on it (tip in / tip out in auto speak) can cause the car to 'buck.' I have noticed this behavior from time to time in more modern cars, but it seems to be somewhat stronger in the SL's case. It's not really troublesome, just annoying. Any suggestions?!?

Will: you did not say, but I bet you have a manual transmission? I say that because my 280 SL with manual bucked severely. The 230 SL I have currently is not very noticeable, but still there. My girlfriend has a 230 SL with automatic and it does not buck. So, to answer your question, I have no idea how to fix it or why it does it. If I had to guess, I would say the partial throttle has the injection pump at the exact spot where the plungers are not sure if they are at idle or above idle. The engine is at a point where it wants more fuel, so the engine slows to a point where the fuel flow is correct, engine picks up, then starves, and falls off RPM. This cycles off/on and causes the bucking. This is just a guess and I welcome other ideas.

Took my Ď67 250 SL for a spin to Whistler (curvy mountain highway). Drove fine for about an hour, then started to sputter, but only at speed on hills. When the road would flatten out, it would run fine, but when accelerating up hill, it would sputter, and I couldn't maintain speed over 50 mph. On the return trip, the same thing happened; about an hour after departing, the engine sputtered on acceleration and up hills. Looking for a little feedback on possible problems... ambient air temp was 21 C (70 F) and altitude would be 0 - 2000 ft.

Sounds like fuel starvation, not enough getting through under load. If you haven't changed the fuel filter in a while, that'd be the first thing, cheap and neccesary. It could be other fuel related stuff: tired or plugged fuel pump, plugged or leaky fuel lines (sometimes only leaking under load) gas tank vent or possibly injector pump problems, but look for the easy things first.

Any idea what might cause a slight but rapid bucking or lumpiness when driving slowly in first or second gear? The problem is most noticeable when attempting to maintain a low speed in traffic or on side streets at about 2000 RPM (or less) in second gear. It feels like the car wants to pull forward several times a second. The lumpiness deteriorates into bucking as the RPMs drop from 2000 and approach say 1200. Idle is a bit lumpy as well. It runs fine at higher RPMs and when accelerating. This condition existed prior to, and after installation of a rebuilt engine.

Will says: I had the same problem with my 280 SL with ZF 5-speed transmission. The car was perfect in everyway, but would buck as you described. I have not seen this with automatic transmission 113's. I think it is a function of the injection pump rack transitioning from idle position to partial running position. Automatic transmission cars have the fluid coupling or torque converter to absorb or hold energy during this transition stage. Manual transmission cars do not have this. At least that is my best guess of the problem. I do not know of a solution.

I also had this condition on my 250 and got rid of some of it with mid range FI adjustments. However, it was completely cleaned up when I replaced the linkage cross arm bushings and all ball joint connectors. This was also done at the same time I converted to Crane point conversion. So I am not sure which one or combo rectified it, but it did.

My guess is the linkage adjustment is a little loose when moving from the idle position to the first movement of the gas pedal.

Rough idle - this took me some time to fully resolve but I settled on NGK BP6ES plugs, new leads and I actually just put in a transistor ignition - a maplin kit for about £13 here and have found that excellent and very reliable. Also I do change the fuel filter regularly- the first time I did it there was a noticeable difference. Here in the UK I use an octane booster/ lead substitute (castrol) as these cars just love octane. BTW the BP6ES do foul a little but burn off quick and as I tend to cruise at around 80mph they are very clean after a long trip. I also spent a lot of time cleaning and then adjusting the warm-up device on the injection pump and had to lap the starting valve to stop it leaking and then actually enrich the mixture at the pump as it was too lean. Plus adjust the anti-stall device - which is better when both the pressure switches on the auto are working. I guess the lesson here is that you have to take the time to figure out all the details, these cars are so robust they will keep going even when lots of little things are wrong.

If you know that a particular cylinder is missing it is almost always electrical. I would run the engine and then pull the spark plug and see if the plug is wet with gas. You could also hook up a timing light to see if you have spark or not. Fouled plugs, spark plug wires, cap & rotor would be the usual suspects, unless, of course you have a serious combustion problem due to a bad head gasket, valves, etc.

I have a Ď68 280 SL with a unique problem. I drive in the city and it performs fine. Take it out on the highway and after 30 to 45 minutes the car appears to run out of gas but the tank has plenty of fuel. The fuel pump is 1.5 years old. I have talked to Hans at H&K and he stated that the fuel tank maybe dirty. I just got back from the mechanic and the tank is clean and the vent system is also operating correctly. Has anyone dealt with this problem before? What other things should I look at.

I had the same problem and it was the gas cap. The next time it happens, loosen or remove the gas cap and see if the problem goes away. I got a new gas cap and never had the problem again.

Dan Caron says: I see this from time to time. The fuel pump gets hot and quits. After you let it cool off it works again, but this is more common on old pumps. Just looking in the sender hole isn't enough to tell if the tank is clean. You need to take the supply hose to the tank off and see how much fuel is coming out. How much fuel is in the tank has a lot to do with this problem. Tank full it's OK. Tank 1/4 Full and it's trouble. There's a reason. The fastest test is to disconnect the return fuel line where it goes back into the tank. You may need to plug the line coming out of the tank so the fuel won't drain out. Have someone turn on the ignition key but don't start the engine. Place a can at the end of the hose and catch all the fuel coming out for 15 seconds. It should have about 1 liter in the can. The fuel should come out in a nice strong flow and not have any slowing down or air gaps. Normally I make this test up in the engine bay but this tests all the lines and everything. No or little fuel means plugged lines, plugged filter, dead pump or the screen in the bottom of the tank is clogged. There is one more deal: the flower pot inside of the fuel tank that sits right over top of the screen. It keeps fuel in it when the tank runs low and the fuel is sloshing around. The pump moves so much fuel through the system that it would suck in air so those clever Germans came up with a really neat idea. In the side of the flower pot is a small hole and the return fuel line shoots it's fuel jet right into this hole. This causes a vortex that actually pulls fuel in from outside of the pot. All of the return fuel just keeps going around the system and any extra gets pulled in as itís needed. If this small hole gets plugged then you will have running problems but only when the tank is low. Now that's clever!

While you're doing all the checks, empty the tank and check the gauze filter that is part of the drain plug on the tank. Also blow through the return line from the outside of the tank to the collection pot. Mine was completely clogged and caused a lot of stalling problems.

I have a '71 280 SL which lately has been stalling sometimes at a very low speed (fwd & reverse) or sitting at a light--but which has fresh plugs and otherwise runs well. Any suggestions please? increase idle? use lead substitue in gas? help!

I have a Ď65 230 SL and it has an idle increase solenoid, which raises the idle when you put the car in gear or turn on the A/C to prevent stalling. These should keep the idle within 50 rpm of the park idle. You should investigate your particular model before just increasing idle because that will introduce all kinds of other problems.

Interesting problem! I just took my Ď67 230 SL to my mechanic for stalling problem at low RPM. I had the engine quit on me about three times in about 3 weeks, always after I had run it for a while ~10 minutes, and after a stop (on my way home from work). It would start up again. What did the mechanic do?

  1. Check my points and found them burnt. In checking further he found the point capacitor was defective (low resistance) and he believed that contributed to the burning of the points. R&Rd them (points and capacitor).
  2. Added a fuel additive to ensure that I didn't have water in the fuel (it is cold here and water can cause fuel line freeze)
  3. Removed the plugs, cleaned them, and replaced them. I had just had new ones put in a month ago (~100 miles)
  4. Cleaned out the top end of the engine with a "3-part induction system cleaner kit" (I might add that I had quite a bit of maintenance on the car the month before and the fuel system had all new filters replaced)
  5. Charge me $280 for parts and labor

The result: it runs great!! better than ever before. I don't put a lot of miles on it - about 40 miles a week - but it starts and runs great. No stalling in the last 2 weeks.

Thanks one and all for the tips, which I used until the problem was resolved. I tried fuel additive for four tanks and got slightly better results. I switched out the plugs and that helped but the car continues to die occasionally (after it had always run like a top). When I brought it to the mechanic to examine the idle, electronics and to perform a diagnostic, it ended up being the points as the culprit.

If it was your points, consider replacing the condensor as well (which could be contributing). It will cost you a couple of bucks and will avoid future burning out. I have also recently replaced the coil, points, plugs, condensor and ballast resistor. When the car is at this age it is worth doing. Be sure to use Bosch parts, they tend to be better and would you believe often cheaper than aftermarket parts.

My 250 SL was having the same problems of stalling at idle in gear; at last tune-up in October my mechanic increased the idle slightly, and I havenít had any stuttering/stalling since then, nor any performance problems.

I took my Ď68 280 SL out last Saturday on the highway for about 20 minutes. Upon exiting the highway the engine started to sputter. I pulled to the side of the road where it stalled. It started up but I could only push the gas pedal a short distance until the engine would cut out and almost die. I went through this routine a few times and turned the engine off. I restarted the engine a minute later and it operated fine.

I had a somewhat similar experience not long ago. Driving at highway speed on a warm day, slowed for a right turn, and the engine sputtered a bit coming out of the turn, then cleared up and ran fine. I figured it was vapor lock, but was puzzled by it happening after slowing and turning. It sounds like maybe that was what happened to you, too. This is complete conjecture, but I wonder if vapors somehow accumulate in the top of the fuel filter and slowing/turning allows them to escape, causing a brief vapor lock. The later cars have the vapor relief back at the fuel pump instead of in the injection pump. I've wondered why that change was made, maybe it was done to eliminate this problem?

I have also experienced a similar occurrence. Prior to having the car repainted it would sometimes sputter, spit, and generally disagree with the thought of rapid acceleration. This would usually occur above 4500 rpm, no amount of gas pedal pumping would get the engine running. Went through the expense of a tune-up, still had the issue, of course the mechanics could not recreate it. Let me add while the car was sputtering, I could be at 60 mph I could turn the key off and back on, the engine would fire immediately and off I would drive. I replaced the fuel pump thinking it might be a fuel delivery issue. Well car went in for paint and stuff ... two years later I've driven the car about 200 miles without an issues. About a week ago I started on a nice run up to about 90 mph where she started sputtering and kicking, same thing turn key off and on and she runs. I have now had it occur a couple of times at lower RPM. My issue sounds electrical to me although I guess it could be fuel oriented as well??

I had a similar problem with my Ď68 280 SL. My problem turned out to be the fuel return line plugged in the gas tank. Pulled the tank and cleaned it out. Runs better no more stalling or running poorly. If you decide to pull the tank make sure to remove shroud that covers the gas tank filler tube located in the trunk. You will find small lines located under it that must be disconected. If the tank has not been out in a long time it might be a good thing to try.

Engine backfired and died, now won't start, Help! My 230SL has been down hard for about the last three weeks. It died while out on a drive (two saturdays in a row). I had lots of rust and gunk in the fuel tank, and I have since repaired that and flushed the system. I also had two in-line fuel filters, an aftermarket one before the fuel pump, and the one on the engine. But itís just not starting. Fuel Pump is pumping nicely (fuel is coming back down the return line nice and strong), the timing and points were set correctly before it broke down. New plugs and wires. But it won't turn over (not even a sputter). Any suggestions? Before it broke down it would sputter, miss really badly and then lose powerand eventually die. I thought it was just really badly contaminated fuel, hence my work on the fuel tank, ect. What should I try next? Maybe really badly carbonized valves? Or could my coil or condenser have failed? The vacuum advance is working fine (checked it just before it broke down). Also the air filter on the injection pump was fine (no blockage), linkage seems to be adjusted correctly... Any help would be greatly appreciated.

Sounds like an ignition problem.

First you need to confirm if it the problem is gas or electric. Pull a plug and see if it has gas on it. If it has gas on it you need to look at the electric system. Pull a plug wire and hold it a few inches from the valve cover. Crank the engine and look for spark. The spark should be visble and audible. I had a similar problem recently where I had burnt out the points becuause I set the gap too wide. If you are not getting a spark, I would start with the points, rotor, condenser and distributor cap. They are all realitively cheap and if they are not part of the problem, the parts will be used in the future. The next step after this is the coil and ballast.

I checked the #2 and #6, cranked the engine and both had a decent spark. They were also both wet with gas. Is there some other part of the ignition system that might be faulty that would prevent the car from starting? The points, rotor and cap all look good, points are set correctly. The rotor snaps back into place when you deflect it side to side. Anyone else out there have a suggestion?

Sounds like your timing is off for the distributor, your spark plug wires are mixed up or your sparks are weak and not strong enough to overcome the wet plugs. I would go over the entire ignition system again with a fine toothed comb. Clean the wet plugs with brake cleaner and then light them with a match or lighter to burn the gas and then use a bit of fine sandpaper to lightly sand the spark surfaces and crank the engine a few times to blow out the fuel(please remove the high tension coil from the coil itself before cranking). And redo everything you before when it ran fine.

A backfire would indicate unburned fuel in your exhaust. You have spark at the plug and since your car was running the wires are almost certainly correct. I wonder if you are getting too much fuel? Injector or fuel pump problems perhaps?

Almost sounds like the distributor hold down loosened up now it is out of time. See if you can rotate the distributor by hand. If so, then it must be retimed. You can do this by sound and feel, but first must move it to a postion where the car will start. The hold down uses a 5mm allen head screw. Let us know what you discover.

Joe Alexander: check the porceline ballast resistors on the ignition. The later 113's like all Mercedes of the era, had a starting circuit for the ignition which by-passed the ballast resistors for starting purposes only. The running circuit would be knocked out and would produce a no run situation. The ignition would test OK during starting but would not run after the ignition key is released. The easiest way is to check them is with an ohm meter. If bad one will be completely open. This ignition arrangement was added around 1968-69. If these resistors are ok, try squirting a little starter fluid in the intake to see if the engine tries to start. If it does it may be a fuel delivery problem. Just remember the basics; to run you need, fuel, compression and spark on time. Check the timing to make sure something in the distributor or gearing did not jump. If you have a mechanical problem like a jumped timing chain, check the compression. Good luck.

I discovered that there is a wire from the key switch "crank" position directly to the coil. When the key is in the "on / run" position, the coil gets its primary voltage through the ballast resistor. I assume that this arrangement is meant to compensate for the drop in voltage that occurs when the starter is cranking. In any case, if the ballast resistor is open as mine was, then you will get a good spark as long as your cranking the engine over but as soon as you let go of the starter switch you get nothing. However, if this is your problem, I would expect that you would hear the engine catch as your cranking but just in case it's a simple thing to check out the resistor. B.T.W. as a reference to my technical credentials, my car is presently in the shop being attended to by someone who hopefully truly does know what he's doing.

My '67 250 is in great shape and runs like a dream. However, whenever I coast downhill, the exhaust sputters intermittently which sounds like mini-backfires. There is no loss of power or any other noticable effects. The sputtering ceases once I accelerate. Is this due to a hole somewhere in the exhaust path or something else?

My 280SL (1969) does the same thing - and has for the past 15-20 years or so. This "burbling" or grumbling and backfiring happens on deceleration and there is no other noticeable performance problem. Actually, I think that it gives the W113 something that it does not have naturally: a macho sound. So, no change, unless one of our tech advisors, more skilled than me, suggests otherwise. I have always assumed that it was something worn in the fuel injection pump control system, allowing fuel to slip through when it should cut off. Tom (et al), have you a diagnosis for us?

Tom Sargeant: I think that the technical experts (I am not including myself in that group) would suggest that several things could cause this backfiring: Tuning (ignition timing, followed by points and air/fuel mix) Small hole in exhaust. My car has done this in the past but now that it is properly tuned with a new exhaust-it does not have the backfiring.

Pete Lesler: this burbling back through the exhaust is usually caused by a slightly overrich idling condition. You will only notice it on coast down conditions. I am surprised to hear any 280SL owners have it since their US pumps have a fuel shut off solenoid that is activated on coast down conditions. ( mandated by the EPA). However, many 280 owners have disconnected this solenoid as the car runs better with it disconnected. Both my early 250's do this as they do not have the fuel shut down solenoid. I have somewhat eliminated the problem by doing some idle richness adjustments. The later (after VIN 2980) 250's will have the second solenoid on the fuel injection pump for the fuel shutdown. If this really bothers you. I suggest you first check your ignition timing, then if the situation continues, you can check your fule injection linkages, adjust to spec, then lastly eitherr adjust idle screw at back of pump, (please read manual so you do not adjust while the engine is running) or go to someone who has a CO meter. The pump should be adjusted to 3.5 to 5.5 PPM at idle.

Up until a few weeks ago my 230 has been running well. About that time I began to notice a slight miss at idle, particularly before the car has warmed up. Then last week the miss became more severe, noticable at higher RPM's and with a noticeable loss of power. The problem seemed to lessen after 10 or 15 minutes of driving. Since the car runs rich, I thought I would check the plugs and then maybe the points/timing. But being blessed with 10 thumbs, I would really appreciate any tips and advice from all the experts out there. A basic primer on setting the timing would be great.

With the help of an experienced mechanic, I have done a good bit of work to the engine. It's mostly good, but there are lessons learned I want to share. When I had the car restored, I did all of the interior, body and all of the non-engine mechanical. Non engine mechanical includes all new breaks, fuel pump, fuel lines, gas tank boiled, radiator re-cored and a bunch more. I did not have any engine work done, as the mechanic tested compression, drove the car and said it was all in good shape. In trying to learn about the car, I hired a mechanic to come to my garage and teach me about the car. We pulled the valve cover to adjust valve clearances and noticed that the cam was not centered within the cam brackets (also called cam bearings or cam towers). So the cam was not wearing evenly. Couple of weeks later we pulled the cover again to reset the valves (started clicking louder) and noticed the cam had become scored and had gouged the rocker arms.

Lesson learned#1--I suspect that I caused the advanced wear to the cam by cleaning the pipe that lubricates the cam. I suspect that I simply dislodged sludge from the interior of the oil feeder pipe, which clogged the orifices that feed oil to the cam.

Lesson learned #2--I suspect (but can't be sure) that a switch to Mobil 1 probably helped dislodge the junk. While I think Mobil 1 is the right oil, I should probably have changed the oil excessively following the first move over to synthetic. So spent Saturday replacing the cam, the cam towers and the timing chain (for insurance). Incidentally, the timing chain was stretched by about 4 degrees prior to replacement. Last weekend I replaced the FI Pump. So in the last week, lots of good stuff done to the car. Results: Car runs great. When driving at 60 MPH, I cannot hear the engine (no more valve clatter)-just that dreaded Time Valve exhaust! At idle, I can barely hear the engine-mainly the vacuum from the engine. I was previously contemplating a hood pad (even though it is not original). Not needed now.

Some of you may have read my previous posts about the erratic behavior exhibited by my 65 230SL. Here is a description of the problem. The engine starts well and runs perfectly when cold. There is plenty of power and not a hint of any hesitation or "flatness" when accelerating. The engine continues this way until the temperature needle hit running temperature (about 170). As soon as that temperature is reached, the performance diminishes substantially. At worst,the car hesitates, accelerates poorly, and sometimes even backfires slightly. But...and this is a big but....about half the time the drop in performance is not as severe. It's still there (compared to the cold performance) but more tolerable. The guys at Southern Star here in Greenville have spent a fair amount of time screwing with the injectors. This resulted in some improvement, but not a real cure. They have checked the compression (both cold and hot) and found low compression in the range of 75lbs to 90lbs, which they claim (and I agree) could cause the poor performance. However, if the low compression was the culprit how can the car run so strongly when it is cold? And why is the poor performance not so bad some times and very bad other times? The entire engine was rebuilt about 11,000 miles ago by a competent shop. We checked the cam lobes and they seem OK (though we didn't actually mike them). The mechanic is leaning towards bad rings, but I am not so sure. For one thing the car doesn't use any oil. It smokes a bit upon initial start, but I've always figured that was leakage of the valve seals. My guess is that the engine has some kind of timing flaw, created when the engine was rebuilt. I am speculating that the previous owner never uncovered the problem because he seldom drove the car. The engine was rebuilt 11,000 miles ago but twelve years ago! I talked to the guy I bought it from and more or less confirmed this. I would welcome any comments or suggestions....

Replace the coil and see if that helps I have had several that when hot perform erratic.

I have always found Joe Alexander's technical posts very helpful so hopefully he, Arthur Dalton or one of the other technical experts will read the post and offer some thoughts. I will offer my non-technical expertise with a couple of thoughts...take them for what they are worth. It sounds like the cold start system is working well and delivering extra fuel during warm up. After warm-up, the extra fuel (and air from the air filter on the FI pump) goes away. Seems like this could mask a timing problem when cold. Might need the FI pump serviced or rebuilt. Just guessing on this. Timing could be ignition, the engine/valve timing, or the injection pump timing. You can test the advance with a timing light to see if ignition timing across all RPM's is normal. Engine timing-with valve cover off, the slot in the cam flange should line up with the slot in the cam at or near TDC on the crankshaft damper (within 3 or 4 degrees). Messing with the FI pump would be the last resort. For the FI pump, I don't know how to check this timing easily. If you go through the trouble of pulling the FI pump, the procedure calls for *first* setting the engine timing at 20 degrees *after* TDC -- i.e. the pointer on the crankshaft should point to 20 degrees ATDC. When the pump is removed, there is a notch on its flange and a notch on the gear-these should line up. If they don't, either they got mis-aligned upon removal or your timing on the FI pump was off. While you have the thing out, might want to send it off for a rebuild-H&R Fuel Injection in Bohemia, NY can do it in a week. While I have never done this, I understand that you can losen the FI pump and turn it while still installed to adjust timing, similarly to how the distributor works to adjust timing. Spark plugs can offer some clues on engine condition. If it were the spark plug wires, you would not have the good cold start performance. That's why timing or fuel seem to be candidates for further review. Hope this rambling sparks some ideas from others...

You mention a compression of 75lbs to 90lbs. I assume that this is actually PSI which would give a 517 - 620 kPa compression in metric terms. I think the compression on these motors should be about 1000 kPa = 145PSI. I also seem to remember that the absolute minimum is around 700kPa = 101PSI. If yours is really that bad, the thing is not ever going to run properly. 11,000 miles is not much but it is a hell of a lot if the car was started from cold every 2 miles or so. That will take this motor out because of the fuel washing away the oil on the cylinder walls. What also happens often if these cars are not driven for long distances and get stored a long time, is that the carbon seizes the rings in the piston and they no longer push outwards. that will also result in low compression. Ok, so now what? Well, see if it is the rings. Drive the car to temperature. Take the breather pipe off the top of the motor and check for substantial smoke and pressure coming out there. Stick your finger in the pipe and see if there is oil wetness in there. If yes, it is the rings and cylinders. If this pressure becomes enough, the car will not run properly because enough emissions are fed into the intake manifold to prevent the motor from getting enough air. You will also see smoke coming out of the dipstick and there will be oil wetness there too. Also, when warm, check the compression again and then again after squirting about a teaspoon of oil into the cylinder. If the compression improves quite a bit then, it is also the rings or cylinders. If this is the case, you have found a problem. In this case I would suggest changing the oil and filter and then going for 600-700 mile non stop drive. That will allow the motor to actually come to the correct oil temperature. The last 150 miles, you must push it to about 85 mph. The engine should run at about 70 - 75% of its maximum revs for those last miles. I have cured a few motors of similar symptoms with this treatment. The theory comes from my dad. Basically if the rings have seized because of carbon and old oil due to long standing, the proper operating temperature together with working stress may loosen them again. So, do this and then re-check the compression. If it is better, good, if not, your motor is still shot. If there is a lot of pressure on the breather pipe, keep it disconnected for this drive and put a pipe on that can vent into a bottle to catch any oil. Ok, if you do not have any pressure on the breather pipe and if you also do not get an increase in compression if you put oil in the cylinders: then there is a problem with the valves. Before doing the suggested drive or anything, check the valve clearance. You can also throw money at it and get a cylinder leakage test done. It will come to a similar conclusion as the above. Bottom line: No good debugging the fuel system if the motor is not in order to begin with.

Another thought on sitting motors for long periods when compression is low you can run a break in procedure on the engine you can find the specifics in almost any shop manual but the procedure that i use is to start the engine, wait for oil pressure to come up and then raise the rpms up to above 3000 for at least 20 min. After this drive the car above 50mph veering speed and load for the next 50 miles or so come home and run the compression test again. I had the same problem with compression on my 66 230sl it had been sitting for 11 years and when i picked it up the compression was at 95psi. Now after performing this it is at an acceptable 135psi.

Although I have heard of these tests before, I don't understand them. Please enlighten me! Maybe I don't understand something basic about internal combustion engines.

  1. If you have pressure at the opening of the valve cover breather tube, how does that relate to a piston/ring problem? I would think that pressure there would result from cylinder compression pressure leaking UP through the valve system into the valve cover. If the rings were leaky, I would think the compresion pressure would leak DOWN past the rings and come out the dipstick hole or someplace else.
  2. If you have oil wetness at the opening of the valve cover breather tube, wouldn't that be normal? After all, isn't there oil flying around under that valve cover? Some of this should get sucked up into the breather tube, I would think.
  3. I have heard that if you plug the opening of the valve cover breather tube, you can learn something by checking the engine performance with this blocked. The engine may even die. What is this telling us?
  4. to make it simple the oil that you put into the motor goes in through the valve cover is all one big open box excluding the cylinders. If your rings are liking compression then is entering into the crank case which is open to the valve cover and is the highest point. This is why you can look at the vent tube to see if you have smoke coming from inside the crank case. A small amount is normal but allot with oil mixed in is not it.
  5. On the inside of the valve cover a baffle is mounted to the inside just for that reason. For oil to come out it must be under pressure not just flung around by the moving parts.
  6. I don't know maybe someone else can help.

I think I get it, partially. When I put oil in through the valve cover, it must get down through the head to the crank case some how. This means there are some open passages that oil (and air pressure) can pass through. So, if the rings are not tight to the cylinder walls during compression, the air passes down into the block and then back up around the head to the valve cover area. BUT, would the same not be true if valves were not seated well? Can't compression be poor due to bad valve components? I still think that would lead to pressure in the valve cover as well. Finally, I don't know the details of how, but the valve train is lubricated by the oil pump sending the oil to this area and it gets applied somehow. With all the furious movement in there, I would assume that it does get flung all about. Would you run your engine without the valve cover? Wouldn't it make a big mess? Therefore, I would think that some of the oil gets into the breather tube.

You got the part about the oil!! But the valves open into the intake manifold or the exhaust manifold not into the valve cover. Well that is true if your valve guides are not worn but that is another story. The tops of the valves go through the head into the area covered by the valve cover but this is sealed of from compression gasses. Yes you are correct about the flinging oil but that is what the baffles are for the oil can not get around the baffle unless it is under pressure.

If the rings leak, it is like pumping the engine up with gas and it is then under pressure. This means that these gasses have to get pushed out somewhere ot else something might burst or as you mentioned if they canít escape, they could leak back up past the of the cylinders that are sucking air in. The there would be gas there instead of air. The result is that the cylinder would not fire and the engine will stall. That answers your poinnt3. However, if the engine is under too much pressure because of blowby, you will find that oil start being pushed out in all kinds of places like seals, gasket, dip stick, valve cover breather etc. etc. Not to mention this gas coming out the breather goes back into the engine's intake again, further displacing valuable air intake.

First I want to thank everyone who responded with excellent help on my problem; which was that the car would idle at about 1500 when it was cold but sometimes would die when it got warm. The idle air adjustment on the manifold was all the way in (i.e. no air). This was apparently a case where the symptoms appeared not because something broke, but because it started working. The Cold Start Thermostat was apparently stuck in the rich/cold position when the previous owner had the car tuned up. So the idle was turned all the way down in a failed attempt to get to idle. As I started to drive the car the termostat started to work, although not reliably or completely. When it did close off the air, naturally the car died. Opening up the idle adjustment on the manifold was all that it needed, and a new thermostat.

I found a hissing noise when moving the small air hose going from the air cleaner top to the idle mix screw on the engine. I had replaced this (and forgot about it!) because the old one was very old. The hose I replaced it with was too big of a diameter, so the clamps still left a small gap. Replaced this with a smaller diameter hose, actually too small, but I worked it on there for a temporary fix. I started the car, which had been running very rough, and it ran 99% like normal. I took it for a little drive and blew a lot of smoke out of the exhaust. This seemed to help, and now it runs just as before.

Normally the engine in my 280SL runs very well, but intermittantly it will begin to sputter and run rough, eventually stalling. This happens sometimes during moderate to hard acceleration, particularly when the engine RPM's are low, but has also occurred while running at a constant speed and while idling. It seems to happen only after the engine is well up to full operating temperature (180 or above). When this happens the car looses power, sometimes continuing to run, sometime eventually stalling. When the problem occcurs, I turn off the key, then it restarts after cranking for a couple of seconds and it will run fine until the problem reoccurs after several miles (5 to 10 minutes or longer). The coil, condensor, point, rotor, cap, plugs and wires are new or in good condition. I have rebuilt the fuel pump and the fuel injection pump is adjusted and working well. I plan to remove the fuel tank plug to check the screen as the fuel pump inlet screen had some crud on it when I removed the pump. Has anyone had such a problem, or have some advise on what could fix it?

I am having a very similar problem except my engine seems to clear up when I accelerate hard. Also, I have experienced the problem at any temperature. One thing, if you remove the fuel sensor which is accessible from inside the trunk you can see the fuel screen in the bottom of the tank. This may give you an indication of the condition without having to drain the tank. Let us know how it turns out and if I ever figure out what's happening with mine I'll do likewise.

It sure sounds like you may have some rust flaking off the inside of your fuel tank and restricting the fuel flow at the bottom screen. If you do find rust flakes, you'll need to remove the tank and recoat the inside. Others have written about this. Draining the tank and cleaning the bottom screen should give you a temporary fix.

A dirty tank could be the cause. But I had a similar problem, and it was a clogged fuel return line. You can check how many liters are returning to the tank in 15 seconds, it should be around 750cc or 1 liter. If you have a poor flow returning, then you should check the complete fuel lines and circuit until you find the source of the problem. On previous messages about similar symptoms, you can read: "I too remember reading about this problem. It turned out to be a blocked return fuel line. The fuel could not return to the tank so it just sat in the pump and lines and boiled into a vapour, instead of flowing back and cooling the pump and lines. Someone also suggested changing the small hoses between the main tank and the vapour tank.

Ran into my first problem after driving around all day in stop and start traffic. Noticed puffs of smoke at take offs lasting only a few seconds and a strong gas odour from the exhast. Also a lot of black soot mixed with water condensation from the exhast on the garage floor upon cold start the following morning. Pulled the plugs to find 5 the color of toasted coconut and one black with soot and wet with gas. Cleaned the bad plug and the problem seems to be solved. Anyone have any idea about what the hell just happened? Is it common to foul plugs in stop/go driving or did I break something on my wild ride home?

A common reason for this: rich fuel mixture caused by bad engine coolant thermostat, or more likely: bad injection pump thermostat. To chec: remove the pump thermostat air filter at the back of the injection pump. With the engine cold and idling, blocking the intake where you unscrewed the filter should cause the engine to slow down (strong suction). With the engine warm, doing the same thing should result in little or no suction and no change in idle speed or wuality. If when warm there is strong suction and engine slows down with t'stat air intake blocked, the pump thermostat needs to be replaced.

Pete Lesler: I had a friend experience something very similar after many hours of hard driving. What happend is one of his valve adjusters had loosened and his the valve was not opening, resulting in a five cylinder car, at least until we re-adusted the vale clearance. Check to see if you are running on all six after you replace the plug.

It might be leaking intake valve seals. If you use engine braking while slowing down you suck in oil which burns when you step on the gas again. Hence a puff of smoke until it burns off.

My 250sl (4 speed) has a fast idle problem every time I come to a stop. As I slow down to stop and push the clutch in the motor races to over 2000 RPM. I have to kick the accelerator to get it to drop back to under 1000. I have stopped the car and checked the linkages several times with nothing looking out of place. When I push on the linkages by hand to race the motor everything drops back in the same place but the motor rpm's drop. What am I missing? I have owned and driven this car for over twenty years so I am not new to it and how they work, but this has got me at a loss. Any ideas are most welcome.

Extra air entered into the system would raise the idle. For example, pressing on the brake pedal when the vaccum brake booster has a leak will raise the idle. Could you have a cracked air hose or vaccum hose? Try coming to a stop with engine braking (down shifting) and then in 1st gear, use the hand brake. This eliminates the brake booster from the equation, see if it still has a fast idle. I'm sure there are lots of possibilities the list can think of, this is just the first I thought of because you mentioned coming to a stop.

I tried your suggestion. I drove the car till it warmed up then did the brake test. Still has the fast idle. It is at about 1800. When I kick the accelerator it drops to about 900. Checked all the linkages once more, everything checks out. There is a crack in the rubber of the boot on top of the valve cover, but I don't think this could be related to this problem.

Dan Caron: look at the distributor too. Sometimes they hang up and won't return to the idle position due to weak or worn return springs.

Take a look to see that the linkage lever is on the stop screw at the venturi when the car is at that fast idle. It should be hard against the stop or else something is holding the linkage slightly open, (loose part, vacuum dashpot, something binding). Also during that fast idle try poping the linkage apart on that cross-over-rod. This will isolate the linkage going to the venturi from that going to the injection pump and this may tell you something. Try disconnecting the linkage going downt to the accelerator from the cross-over-rod to see if the fast idle goes away. If you do not have a linkage problem or a vacuum leak, that distributor could be the problem as Dan has mentioned.

I've been following this thread curious to find out what your problem was. I think the last thing was that you were going to check the distributor to see if the advance was hanging up. Assuming that that is not the problem, could it be that the linkage inside the fuel injector pump itself is hanging up? As I understand their operation, there are springs and weights and all kinds of complicated mechanical things controlling the "rack" which is what meters the fuel. Is your pump one of those that has its own oil dip stick? Have you check it? I believe that it is possible to check this theory as follows: there is a rubber cap on the front end of the fuel injector pump (closest to the radiator). If you remove this cap you will have access to the end of the rack. You can use a long (I think) 5mm screw which will screw into the end of the rack. This will allow you to manually move the rack by pulling and pushing on the end of the screw. Now when you have the fast idle, instead of kicking the throttle just push in on the end of the screw. If its the pump itself that is hanging up, the idle should return to normal and then stay there after you let go of the screw.

I would disconnect all the linkages and start over with the adjustment of each one. Start the engine and see if it will idle properly. Then connect one by one till you have the fast idle situation. Most likely will be some binding inside the injection pump, like Chuck says. Do you engage the kick down solenoid when you drive? I.E. press the pedal to the floor? Could be an electrical fault. Also the racing idle could be the brake booster leaking. Plug up the vacuum line leading to the booster and see what happens(careful the brakes will be very hard here).

I have had my 1970 280 SL for about 3 months now(it has been virtually fully restored) and no problems. However on the weekend drive it ran rough with a very low idle speed,particularly when warm.It has stalled at traffic on a few occasions(very embarrassing since it takes some time to position the key and restart). The worst time was on a hairpin bend and going downhill and it suddenly stopped on the apex of the corner.

Cold and warm starting

I have a Ď71 280 SL and it has to crank for awhile to start when it's cold. Any suggestions on what I should check.

Dan Caron says: cold start valve. Run a wire from the battery over to the terminal on the CSV and see if it clicks. Crank the engine and have someone do the wire to the CSV thing. If it fires right up you have a relay or thermo time switch problem. That system has a fuse on it so check that too. No. 6 I think. I may be other things too but start there.

The long and short comes down to that once the car warms up, it runs very poorly, as if it is out of gas. No power, not just running on 5 cyl, but no power at all. Tonight I will pull the plugs, check the cap and wires. The car has a Pertronix electronic ignition system, so I'm sure the stuff inside the distributor is good.

Check the head gasket. A small leak would generate exactly those symptoms -- that happened to mine a few months ago. Mechanic told me it was fine, hand me the keys, then I'd be sitting at a gas station calling him to come get it again!

Here's the problem. The car starts to lose engine power after about 15 miles of driving. It will then die and won't start. If I let it sit for about 10 minutes then it will start up again and run another 10-15 miles. (This isn't really the way I planned on driving the car.) Before I bought the car the gas was drained and new gas put in. The points are new as are the oil and fuel filters. The battery was nine years old so I replaced that as well. I would welcome any suggestions before I head to the mechanic.

The first thing to determine would be if you have a spark and gas supply going to the engine when it won't start. First pull off a plug wire and check for a spark when it is in the non-starting mode, if you have a spark then check the fuel supply going to the engine. I would suspect that the fuel pump is stopping for some reason, you can put a test light on the pump terminals (re-build kits available). Your fuel filter canister could also be airbound but that has never happened to me when I changed the fuel filter, there is a bleeder bolt on the top of the canister if that is the problem.

If the car re-starts after a short rest and then dies again after a short wile, I don't think it's an electrical fault. This kind of problem usually occurs because of heat build-up: the cold car starts, then it stops when it's hot, then it starts again when it's cooled down, and so on. Most typically this is due to fuel pump overheat. Anyway, it should be identifiable by the kind of stopping. I mean: if the car splutters before dying, it's fuel, if it dies all at once, it could even be an electrical bad-earth problem.

My suggestion on the topic involves rust in the gas tank. There is a screen over the fuel pickup on the tank, and if this gets clogged up with rust the fuel will not make it to the pump to get to the engine. This has occurred on our new SL, and we have to get the tank cleaned and coated to alleviate the problem. Have you tried restarting the engine immediately afterwards? This could all be due to the gas having water in it and rusting the tank for the years that it was not driven. The car should restart after turning the pump off though because it causes negative pressure which pushes some of the rust off the screen.

My guess is that it's the fuel pump.

I rescued a 230 SL that was stored for 12 years and had the same stalling problem. When the car stalls, open the fuel feed line at the filter head to make sure fuel is coming through (with ignition on). If so, let the fuel run for a few seconds. I found that the fuel got warmer and then I got bubbles. The electric pump pumps about 4 litres a minute and the injection pump uses what it needs and sends the rest back down the return line to the collection well in the tank. The extra fuel also cools the injection pump, I'm told. On my car the fuel lines may have corroded from the inside and all the rust particles entered the tank and clogged up the inlet to the well. Rust from the feed line on the other hand landed in the fuel filter and was filtered out. So you should check if you can blow into the tank thru the return line. In my case even a 100 psi air line could not clear the blockage. Eventually, I took the tank out and a radiator repairer cut a hole in the tank and cleaned out the obstruction. Runs perfect now. I was lucky in that the previous owner had repaired the main injection pump, replaced the electric pump and changed the fuel lines in an effort to overcome the stalling but didn't succeed. The car stalls because the fuel sitting in the electric pump chamber gets hot and evoparates as it has nowhere to go. I would think the electric pump should be okay as it builds enough pressure to start from cold (fuel pressure with the ignition 'on' and engine not running should be between 0.8 and 1.4 bar).

I had a problem with my 250 SL. The car was great at making one way trips. Once it warmed up and was shut off it would not start again. What had been done to the car prior to that was the replacement of the fuel injection pump (the result of the shaft freezing up on the pump). It seems that the new (rebuilt) pump was not calibrated on all six and as a result was running lean on three. After a lot of money spent to remove the pump, send it back to CA to be recalibrated and then reinstalling, the car starts hot or cold and it does round trip trips.

I think I finally got the relays and cold start valves working properly, however it seems to be a never ending quest. Now when the temperature gets to normal the engine just stops and will not restart. When trying to restart, it will only catch and run a few revs and then won't do anything except turn over. Up to the time it stops, it runs fine and accelerates normal with no load (I have the car on a lift). Once it has cooled off it will start again and run until it warms up, then stops.

Check that fuel return line. It may be clogged up making the fuel in the electric pump boil and vaporize. If the fuel return is ok, try adjusting the throttle linkage per the book, and then adjusting the idle fuel/air mixture and speed. I'm guessing idle air/fuel adjustment is off, and it's running only on the thermostatic warmup system until that system shuts off. After adjusting the linkage, adjust the idle fuel a couple of notches richer if possible, and then open the idle air valve a couple of turns to try to keep it running, and then go through the adjustment procedure.

I took all the fuel lines lose and the return line was stopped up. I took the return line lose from the tank and tried to blow through it with compressed air with no luck. I replaced the line and reconnected everything except the fuel injection pump. I tried again to blow through the line before connecting it and it was still stopped up. I went back to the fuel tank and disconnected the line where it went into the tank and tried to blow into the tank only to find that was stopped up. I now have remove the tank and can not see where I can get to the supply and return lines inside the tank to unstop them (I know the supply is also somewhat stopped as the fuel would not run out while I was taking the tank out). I've pulled the sending unit out, however I'm hoping this is not the way to do it. Any help will be appreciated.

First, remove the brass drain plug in the bottom of the tank (it takes a 22mm hex wrench - you can make one from a large bolt). The drain plug will have filter screen attached to it, which may be partially blocked. With this plug out, you can also see the inner opening of the "supply" tube, and cleaning it is easy. The inside opening of the "return" tube is hard to see - I would use a semi-rigid wire to carefully roto-rooter it out. Use something like a strand of solid copper electrical wire. Look down through the fuel level sensor opening while you are doing this, and try not to ruin the plastic "flower pot" assembly inside the tank.

I tried to unclog the return line with an old inner and outer heater control cable that I had just replaced. I had a vice grip on the cable and a hammer on to the vice grip and pipe full of WD40 and that didn't clear it. In the end, a radiator repairer cut a hole on the underside of the tank (near the drain plug/filter), desoldered the pipe at the end where it enteres the tank and removed the pipe to clean it with a drill at the tank end. The tank was in quite good condition otherwise and didn't warrant replacing.

This probably an easy one for the experts, but I'm at a loss. My 1970 280 SL starts right up in the morning when cold. It runs great during the 30 min. commute to work. But 8 hours later when I try to start it for the drive home, I need to crank it for at least 20 seconds before the engine turns over. I just had the fuel injector pump rebuilt (it was threatening to stall when stopped) and no one seems to know the solution to this problem.

The cold start valve is a good suspect. The manual describes a leak test. Even though my valve passed the leak test I found it to leak intermittently. The valve can be rebuilt. On my car I have a warm start solenoid mounted on the fuel injection pump. Mine was not wired up. I do not believe that the later models had this solenoid. It is mounted on the rear of the fuel injector. Some models have two solenoids. The higher one is to reduce emissions during deceleration. The lower one is for warm starts. I hope this information is helpful.

Someone else had a similar starting problem and it was the warm running solenoid (the solenoid on the fuel injection pump). A list of items to check is: cold start solenoid operation; cold start valve operation (it can stick even if the solenoid is operating, can also leak); thermostatic time switch mounted on head behind fuel injection pump; warm running solenoid (your car might not have one); and operation of the relays that control the solenoids. Lastly, the manual describes a modification for later cars that involved adding a relay that activates the cold start solenoid for 1 second even if the engine is warm. A good shop manual (or good shop) may help you determine if this modification was done to your car. If everything else is working right and the problem persists, this might be a solution.

Yesterday was a glorious day in the New York area. I was out running some errands in my '69 280 SL standard transmission. I was in a store for 20 minutes and when I returned the car would not start. The engine was turning over but, it would not catch. I thought I would be going home on the back of a flatbed truck. I let the car sit for an additional 30 minutes, then it started right up. I believe my problem has to do with the fuel injector system. Additional background information for you ace diagnosticians include: I recently replaced the injector thermostat. I did the leak check by the venturi. No leak. I cannot get the car to idle in the 800 rpm range. It idles at 1200. It starts up immediately when cold or hot. I only have a problem when it is warm. The fuel pump is humming. The gas tank vapor lines are clear. I changed the fuel injector bladder several years ago. I would appreciate any suggestions or else I will have to go into my local mechanic who will solve my problem at the expense of several body limbs.

I'm no pro, but here are a couple thoughts. Note that I have a Euro 280, so yours may have US emissions controls that I know nothing about that might be involved. First, intermittent ignition coil failure isn't entirely out of the question, make sure you are getting a spark. Second, from what I've read, the thermostatic system on the injector pump is adjustable by means of some internal stacked washers. It should operate at low coolant temps to give a richer mixture and higher idle speed. If it's staying open too long, i.e., when the engine is warm, it may cause an over-rich mixture and a no-start, and may also be the cause of your high idle speed. I don't know the official way to diagnose this, but with engine cold, I'd try removing the small round air filter on the injector pump, then starting the engine. You should hear air being sucked into the pump where the filter goes. As it warms up, the amount of air being sucked in should decrease and finally stop altogether. If you have a good shop manual, it may give the coolant temp at which the thermo system should shut off. The air filter requires a thin 22 mm wrench (7/8 inch fits, too). Last, the throttle plate should be completely closed at idle, with idle speed controlled by the screw at the front of the manifold. I don't know what you mean by "leak check at the venturi", perhaps you have already checked this.

I do not know if anyone really cares but, I am very excited that I think I solved my problem. I disconnected the coil wire and checked for voltage to the starter solenoid on the fuel injector. There was no voltage so the starter solenoid was never activated. This solenoid is the one on the fuel injector and is not the one associated with the starter motor. I checked the wiring diagram and found the lead on the relay switch that the solenoid should get current from. I ran a line from the relay to the solenoid. I only tried it once and it worked fine when warm. I am hoping that my problem has been solved. I want to thank the group for your ideas and support on this. Perhaps someone else has the same problem and does not realize it yet.

Although your problem is solved, hard starting when warm is not an uncommon problem for these cars. Vapour locks in the fuel line are common in really warm weather. If this occurs, members should simply do what you did. Go away and have a beer or a coffee, and come back later, they usually start first go after a bit more rest. Vapour locks should not be a daily event. If you're having other problems (hesitating, flooding etc) think about the following. Something you may want to consider is if you think at any time in your car's history someone has played with the fuel injection (which can cause problems such as this) then one of the best investments you can make is to get the system recalibrated. Each lever on the system has to be at the perfect position (length), or you risk getting the wrong mixture in at different points of acceleration (e.g. your tune up may have got you idling okay, but running way too rich at 3,000 rpm - which incidentally causes massive engine wear if you don't service at correct intervals). NEVER adjust these levers, as they do not move on a 1:1 ratio. Basically, the repairers will recalibrate the whole system by running the car on a dyno and setting the mixtures at each rpm range. It will cost you a couple of hundred, but the results can be astounding. I did this on a 220SE coupť I owned, and the results were amazing. The biggest problem will be finding a specialist to do this job properly - for example I believe there is nobody in Vancouver who can do this, we have to take our cars over the border. You may want to ask around a Benz club or Porsche club (similar fuel injection system) for advice.

My 1965 230 SL starts well and runs great for two minutes and then wants to die out for three minutes until it completely warms up. I have to keep the revs up to keep it running for that three minutes. Does anyone have any thoughts on the probable cause?

Tom Hanson: the cold start thermo time switch might be going out.

Albert de la Torre Chavalera: It's probably the "Cooling water thermostat" on the fuel injection pump (it's inside the "little tower" connected to two water hoses). This thermostat controls the quantity of fuel and air supplied to compensate for the higher friction in a cold engine and the poorer mixture. As the engine warms up it gradually decreases the richness of the mixture, and finally cuts it off as soon as the temperature of the cooling water has reached 158ļ F (70ļ C). Your probably cuts off at 50 or 60ļ C. It's necessary to adjust this thermostat to the correct temperature.

Rodd Masteller: to check if this is actually closing and at what temperature, Joe Alexander showed me a neat trick. Take the little air filter off the back of the device. This requires a very thin wrench. Start the car. Get out and hold a length of tube or hose near that air intake and listen through it. You should be able to hear the air being sucked into that air intake. As the car warms up and the thermostat closes, you should hear that sound go away. Lean back into the car and see where the engine temp. guage is at that point. Also, examine filter and replace if necessary.

I'm seeking some advice from folks in warm climates re hot starts. We have been hotter than usual in the Ottawa valley with temps of 34C (92F) and I have been experiencing problems restarting after brief stops. It seems to be flooding or vapor locking, contradictions I guess, but the fix is to hold the pedal to the floor and crank for 15-20 seconds, where upon it sputters then roars. I have a new fuel filter and recently determined I was getting adequate fuel delivery (liter in 15 sec.) so that might take care of 2 possible causes as listed in the Haynes manual. The other listed causes are: Injector pump by-pass leaking (can we determine this or is this a shop job?). Fuel pump feed pressure too low (is this different from fuel volume and is the test done at the Cold Start Valve?). Injector pump ball pressure valves leaking (would more than 1 fail at once?). Control rack rod leaking (is this something that could be checked or does this need to be done by a pump shop?). Fuel evaporating from engine heat (fix by modifying start valve with MB 1 second time switch). I'm thinking I'll check the tank pick-up screen and the fuel pump filter for contamination, but these Injector pump possibilities don't sound as simple. Other than the hard start it runs fine and has had recent valve clearance adjustment.

Bernt Damm: hard starting when hot: correct procedure is that you must always give full pedal to the metal when starting the hot motor. Other cause is a leaking cold start valve. That makes for very hard starting sometimes. You need to check that. Also, the thermo-time-switch could be the cause or the time-delay-switch. Put a test light onto the cold start valve and observe when starting the cold and then the hot motor. When cold, the test light must come on for a couple of seconds while cranking. Time depends on the actual temperature and is governed by the thermo-time-switch. When hot or above 35 degrees celsius, the lamp must come on for 1 second only while cranking. This time is controlled by the time-delay-switch. Repeat this test with the solenoid on the rear of the injector pump. It should give exactly the same readings if your wiring is correct since the solenoid and cold start valve are in parallel on your vehicle.

Naj: fuel feed pump pressure is measured at cold start valve because its easy to get to, I think. Its static pressure you want to measure so hook up a pressure gage to the fuel line there and switch on ignition. Pressure should be between 11 and 14 psi. I had borrowed a fancy Snap-on kit to do this test and the gage was calibrated 0 to 100 psi. I got a reading of 9 psi. I thought I needed a gage with smaller scale for more accrucacy (say upto 20 psi) So I got one of them and the pressure was 8 psi. Car runs okay so I haven't looked for a cure! Mine is a 230 with the pressure reg valve on the feeder to the inj pump. With return line blocked pressure rose only marginally and the pump is the later smaller type and looks new.

Dan Caron says, however: That's not right. Measure fuel pressure and volume where the return line goes back to the tank. If you really want to do the whole system you could pinch off the return hose at the tank and then take it off of the return line. Put a short piece of hose on the line and then do the test. You may find that you only have a small amount coming pot at this point. This would indicate a restriction in the return line or some other problem. The cold start valve test is usually inconclusive.

Thanks to those who offered advice on my hot start problem. Here's an update....I checked and found that the inside of my gas tank and the pickup screen are clean and apparently rust free...hooray I did manage to clean out the insides of the tank sending unit which was gummed up with some black goo and my gauge is now working perfectly, no more bouncing needle when it gets below half full. I found that I have a new small size fuel pump, but have the early style setup, without a connected bypass valve and T in the return line. According to the BBB this should be used with a hole in the IP overflow valve. I have to check and see that this is what I have or else connect the bypass system on the fuel pump itself. This may be the root of my problem, but of course the car is behaving now that the temps aren't so high. Bernt mentioned that the normal hot engine start procedure is to hold the pedal to the floor, but I don't normally do this and the car starts fine. When it is difficult to start I do hold the pedal down and the subsequent start seems harsh as the engine races off to 7 or 8,000 rpm. Is it really a good idea to hold the pedal down for each hot start? Bernt also referred to a one second delay from the time start switch to the cold start valve on a hot engine but I believe the book says that an additional part was added on some cars after the problem developed and my car at least does not have this extra component. My thermo time switch is probably incorrect for this climate/car as it is a 5* C switch (from a 300 6.3 ?) instead of the 32*C switch that would be normal, this leads me to wonder if I jumpered the thermo time switch to ground if it would help start the hot engine problem. I'll try this next time it won't start. Naj replied that the fuel pressure should be checked at the cold start valve and Dan corrected to say that a better place to check was at the tank return line. How would this be done if you have a new style pump with a connected bypass T in the return line ?(which I don't have ,but may end up doing if I discover that the overflow valve in the IP does not have a hole) On a different topic: I have done the valve clearance adj. three times in less than 1500 miles since having the head overhauled. Each time there was some slack to remove. Is this normal ? The valves seem quiet after these adj. until I do some sustained highway driving and then it seems like the valves are noisy again and I end up adjusting again. The guides were replaced and the seats ground but I'm still surprised that they could move like this and the valve clatter seems to get worse. Any thoughts about this?

No, it is not normal for the valves to keep needing adjustment. Firstly, did you re-torque the head after around 350 miles or so? If this problem is what you say, I think that something is wearing at a rapid rate and a good possibilty is tht soething is not getting oil. Then again, how much are we talking about? If it actually becomes noisy, then I would have it checked out immediately!

Dan Caron: you're supposed to let go of the pedal as soon as the engine starts to fire so as not to like over rev it I think I said that the normal place to check fuel pressure and volume is after the fuel damper before it goes back to the tank. ( if I didnít then I'm saying it now) you can also check it at the tank. I'd check the return line anyway. You don't need any extra fuel going into the engine on hot start. That's why you have to open the throttle - to give it some extra air. It's not the valves that are moving or seating - it's the rockers . Just keep checking them until everything settles into place. Usually one re-set will do it.

Bernt Damm: the hot start procedure I described is correct and you should adhere to it. You must of course let go the pedal as soon as the motor starts. You should always give it full throttle when hot starting to allow more air into the motor to compensate for a rich mixture which is caused when the motor is turned off. As the motor spins to a halt, the injector pump still injects fuel into the manifold which then evaporates and sits there, waiting to be sucked in when you start. The thermo-time switch you mention is wrong but if it still works, it should be ok because it influence cold start and not hot start. Jumping it to ground will do exactly the wrong thing and seems to be not a good scientific choice. Your car will have the time-switch as well. They all have it since before 1964. It sits next to the relay for the wiper motor, close to the washer bottle. It is normally round. there will be 2 relays and one time switch there. Their order from the front is: Starting circuit relay, wiper relay, time switch. Did you verify the measurements I mentioned with a test lamp?

Cees: when my car is cold, I turn on the ignition and leave the pump running for about 8-10 seconds before I start it. It will start promptly. If I would try to start the car straight after turning on the ignition, it cranks and sputters and finally starts - after 8-10 seconds! The time I figure it takes the fuel pump to get "the system pressurized". Or whatever, at least it always works fine this way. Then, when I turn off the engine when hot and turn it back on within, say, five minutes, it starts right up without pedal to the metal, waiting any seconds or anything. So the system, I figure again, is still "pressurized". When I wait more than five minutes to start the hot engine back up, I have to wait the 8-10 seconds again before it will start right back up. I never floor the pedal (although it for sure tells me to do so in the owner's manual) to start the hot engine, this simply does not seem to add to the success of the above-described procedure.

Background: 1969 280 SL has been running perfectly. Just returned from 250 mile road trip consisting of high speed interstate, 75 to 95 mph, without performance issues. Situation: started car without any abnormalities, and drove approximately two miles. The car stopped as thought there was no fuel. When attempting to re-start the vehicle, the engine would start-up from the fuel injected from the Cold Start Valve, but would then die. Resolution; anyone have a theory, or better yet, the solution?

Joe Alexander: some Mercedes of the era, had an ignition ballast resistor by-pass to aid starting. This circuit by-passes the ballast resistor on the ignition when the starter is engaged. So if your ignition ballast resistor is bad, your car may start only when the starter is engaged. As soon as you release the key the car will die. To test jump a wire across both ballast resistors. If this is the problem the car will start up and run fine. Replace the bad ballast resistor as soon as possible since running with the resistor by-passed will burn the ignition points.

When cranking the car over, the ballast resister is by passed to compensate for the drop in battery voltage. As soon as you let go of the key the 12 volts supplying the coil goes through the ballast resister/s (mine has two). If you car runs at all after you let go of the key then it is probably not the ballast. It is easy to check, with the key on but not cranking you should have some voltage to the plus side of the coil (it may not be 12v if the points happen to be closed. I think?). There have been lots of posts on the subject in the archive. Many of them put their when I was trying to figure out my ballast resister problem. However, one would hope that any professional mechanic, even one who is not familiar with W113s would have found a bad ballast resister in short order. So if this was your problem you probably already know it. Joe: when I jumpered out the ballast resisters my car would run but not very well at all. Lots of back firering and no power. Even if jumper across just the .4 ohm resister leaving the .6 ohm in line it doesn't run well. Any ideas? Normally it runs fine except for an intermittent miss which I believe is a fuel problem.

Joe Alexander: Sounds like your problem may be somewhere else. The ballast resistor by-pass is merely slightly compensating for another problem. Ignitition and fuel system checks should be performed. Start with the inexpensive items first. Ignition points, also look at the spark plugs to see if they give some clues. Change the spark plugs if they look wet down or fouled. If you recently refueled, you may have even gotten a little water! A fuel pressure and volume check may be in order. This should give the status of your fuel supply. Items like coil and condensor could cause problems also. The injection pump will not usually fail all at once, but usually developes problems over a long period of time or after long periods of inactivity.

Thanks for responding. Although I'm not sure I made myself clear. I jumpered across the ballast resistors because the car had died on me with the symptom that it would run as long as I held the key in the start position. I was surprised when the car wouldn't run well at all with the resisters jumpered. Testing with an ohm meter when I got the car home (towing) revealed that the .4 ohm was indeed open (it was not a stock part and just broke. Replacing the resister restored the car to running the way it did before it died. I just assumed that I was wrong in thinking it should be able to run with the resisters jumpered. However, it does have an intermittent miss which existed before the ballast problem. I was just hoping that the additional information of the symptom that it doesn't run well with the resistors jumpered would help identify the problem. So far I have replaced the plugs coil condenser and both resisters. I have tested the fuel pump volume output but not at the tank only at the output of the injector pump after the damper. I also inspected the inside of the tank through the level sensor. Tank looks great but the tank filter appears black (earlier post). My next step is to drain the tank and check out both the tank and pump filters. However, I'm having a new garage built (home for the SL) and until that's done it's not likely I'll get much tinkering in. Thanks again for your suggestions. I never would have gotten this far without this group.

No power and lots of backfiring and dies. Put on a new condenser. About $6 bucks. Mine did the same thing, kind of sounded like the car was choking, big backfires. When I replaced my condenser, the wire fell off of the top of it. Ran fine after that.

I'm experiencing poor performance prior to reaching operating temperature. Car starts fine but will stall at idle for the first 2 minutes or so until the engine reaches operating temp. Also demonstrates sluggish performance and slight "backfiring" during this same 2 minute warm up period. Once operating temperature is reached, it idles perfectly and drives like a dream. I recall someone writing about a temp sensor near the fuel injection. I'm not sure what it looks like or if this is my problem. Any other suggestions about operating changes that occur during and after warm up?

Joe Alexander: the first few things I would check would be; injection pump air filter, engine fuel filter and injection pump warm-up regulator. Coughing or backfiring through the intake is a sure sign of a lean or not enough fuel situation. I am assuming that the car is in basically good tune otherwise. If your car runs ok when warm then the removal of shims or adjustment of the barometric regulator will probably not cure your problem. All tune-up items should first be checked along with the fuel system and the engine linkages. The warm-up regulator on the injection pump should be checked to make sure it is not stuck and the air filter on the pump should be checked. Only after getting everthing else in order, as last resort turn your attention to changing settings on the injection pump. It would be a shame to scramble all the factory settings on the injection pump then find that your warm-up device was stuck, or your engine linkages were set correctly!

It is always best to start with the easy stuff. I would suggest taking a look at the cold start valve. There is a bolt on the side of the valve. Remove the bolt and turn the key to the on position. If gas leaks from the bolt hole your cold start valve needs to be rebuilt.

Bernt Damm: test the cold start valve or fuel injector pump start solenoid with a test lamp. If there is constant power there, suspect the thermo time-switch. Test by disconnecting it. I have opened about a dozen of these switches here and they all seem to blow in the same way which would cause that problem you mention.

Here's my guess: the thermostatic warmup system on the top of the injection pump. When cold, a valve is open inside this thing that supplies more air to the intake manifold to give a fast idle. At the same time, it moves a linkage inside the injection pump to inject more fuel. As the engine warms up, the valve gradually closes and less additional fuel is injected until it's completely shut off. The valve could be stuck open, or the thermostat element inside could have failed. To test, remove the small round air filter on the back of the inj pump (thin 22 mm wrench), start the engine from cold and let it warm up. You should hear air being sucked in where the filter mounts. It should decrease and stop altogether when the engine is fully warm. If it doesn't, it's bad.

Dan Caron: Nope, that really wont do it in most cases. They usually stick in the hot position and the engine is too lean while starting. This causes it to back fire and pop until it gets warmed, at which point it will run normally. The aneroid compersator will make it rich across the whole range but usually it will still be drivable. Most common problem is the IP or the cold start valve. I some cases you can have a bad juel injector but only that cylinder would be rich. This is assuming that the ignition and any other smaller componets are in working order. When the pump goes bad it really rich. Burning eyes kind of rich that doen't go away no matter how hot the engine gets. You can see the cloud of unburnt fuel coming out the exhaust. If no amount of tunning or new parts makes any difference, you probably have a bad pump. In otherv words, If changing the plugs, wires, fuel filters, CSV, injectors.

I think my problem may be from the switch on the intake manifold it appears that the valve is not seating fully and gas may be passing by this will this cause these problems . My car seem to be flooded even when it is cold and I crank it and it becomes undrivable due to soo much gas. I have already checked the cold start valve on the injection pump the one with ball inside and it works fine.

Intermittent Miss

I just purchased a 1966 230 SL and after about 2 weeks of driving it the car began to have an uneven idle. It drops to about 500 from seven & sometimes dies. I checked the thermostat and it seems to be good. I also plugged off the vacuum hose to the brake hose with no change.There is too much oil in the reservoir but I can't see a way to drain some out. The car has since developed a slight miss. I also changed out the fuel filter. What should I do next, check the tappet clearance or timing or what?

I am assuming the reservoir you are talking about is for the fuel injection pump. I had a similar problem and removed the oil by inserting a 1/4" plastic tubing into the fill hole and sipping it out until I filled a length of tube about 2 feet. Kept doing this until oil level was right on the dip stick. Maybe a turkey baster would work if you don't like the taste of oil.

I'm trying to track down the cause of a very slight stumble at idle. The problem has persisted, despite the following restoration work during the least year: new distributor; Crane ignition and coil; new plugs and wires (1K ends); engine rebuild (Metric Motors); adjusted timing and valves; checked cold start valve on intake manifold; checked idle mixture; checked and adjusted throttle linkage. The problem seems slightly worse when the engine is hot, for example in stop and go traffic. What am I missing? Could this be a fuel injection or injector nozzle problem? Ideas?

How much running in has the engine had? Sometimes recently rebuilt motors will get better as they run in - depends on how bad this slight miss is. If it hasn't done much running in, take it on a good run to seat the valves in nicely etc. Personally, I wouldn't have confidence with anything but original Bosch ignition parts, but that doesn't mean these are the cause - I'm just a fussy git. I notice not everything has been replaced - I'd do the lot to be sure (e.g. ballast resistor and all).

Also, one thing that concerns me is that there has been an adjustment made to linkages on the fuel injection system. This is a BIG mistake on these cars unless it is being done by someone who knows how to recalibrate Benz & Porsche mechanical systems (and believe me, such people are hard to find). Fiddling with these can put the calibration of your system way out of whack (the air:fuel relationship is not linear 1:1 - as the foot goes down, too little or too much air/fuel can be sent in even if it's adjusted correctly at idle).

The rebuild has about 2K on it so it should be run in nicely by now. No internal fuel injection mixture settings were touched; the linkage adjustment I referred to is the adjustment of the linkage rods between the FI and the venturi unit. Setting it to factory spec is a fairly simple procedure.

Dan Caron says: you may want to check your return fuel line and see if it's clear. It could be a bit of vapour lock from the fuel getting hot and starting to boil if there's a restriction at the return line. Try adjusting your injection pump out a click or two to see if that helps. I always clean the cold start valve and have injectors tested and cleaned on all engines I work on. This will save a lot of time and grief later on while tuning. I set the plug gap to the standard setting which is about .032 or so. The coil may not be all that great if it's old. Are the wires real wire and not some carbon junk. I've seen new sets with carbon coil wires and the rest of the set OK. You can also look for any vacuum leaks by pouring water over the places air could leak in.

I'd hoped to have solved this by now, since it is prime roadster weather now. However, I continue to bang my head against the wall trying to find the root of my sputtering problem. My problem is driving me crazy. I can drive fine for most of the day, when all of a sudden (usually at speed in 2nd or 3rd gear), the engine starts to sputter and lose power. As soon as you put the clutch in, the revs drop and it stalls. It starts up again immediately however. This may happen 3 to 5 times in succession, or only once in a fifteen minute period. Here are the things that have been done:

  • New fuel pump
  • Cleaned fuel filter (downstream from pump)
  • New distributor cap, condenser, coil, high tension wire
  • New ignition leads and plugs
  • New thermo time switch

A phone call to my mechanic during an occurrence suggested filling the tank up completely; I was at 1/4 to 1/3 of a tank. He thought fuel flow through the filter in the bottom of the tank might be an issue, and by topping up the tank, I might get enough pressure to relieve the situation and diagnose the problem. It did not have the desired effect, and I sputtered all the way home. On latest trip to the shop, the mechanic checked the fuel flow and said that that was good. I am looking for any ideas or suggestions that any forum members have found to correct this kind of problem. I've heard of things like the plugs running too hot, or similar electical issues creating this kind of behaviour.

I had this problem and it turned out to be the cylinder head (gasket). When the engine got to operating temperature, it would start to sputter at higher revs, to the point where the car would not drive faster than 40-50 mph. The first time this happened to me, I took the car to get checked right away. It took the experts (Van Dijk) several tries in all kinds of directions (including replacing the injectors, and the various assorted ignition components that you also list) before they guessed that the cylinder head might have a hairline crack in it. So they lifted it, had it tested but that was not the problem. When they refitted the head with new gasket, correct torque settings etc, the engine was fine and it has been ever since (have driven it some 7000 miles since, at speeds of up to 120 mph) and the problem is gone. This may of course not be your solution, but the problem does sound similar enough to warrant some investigation. Remember Van Dijk's original guess was a hairline crack in the head, this could then also be wortwhile to look into, if all else fails. I hear that these cracks can only be detected by egine rebuild specialists and that they can also be welded.

I would visually check the filter in the bottom of the fuel tank for contaminants. Also, an incorrect gas cap / blocked fume ventilation tube can create a vacuum in the tank as the fuel is pumped out.

I'm trying to track down the cause of a very slight stumble at idle. The problem has persisted, despite the following restoration work over the least year:

  • new distributor
  • Crane ignition and coil
  • new plugs and copper core wires (1K ends)
  • engine rebuild (Metric Motors)
  • adjusted timing and valves
  • checked cold start valve on intake manifold
  • checked idle mixture
  • checked and adjusted throttle linkage

The problem seems slightly worse when the engine is hot, for example in stop and go traffic. What am I missing? Could this be a fuel injection or injector nozzle problem? Ideas?

My car suffers an intermittent miss. Although the symptoms are a little different, the miss occurs at any speed and any temperature (although it seems to occur more frequently the longer I drive it and it "seems" to clear up if I tromp on the gas) I was never the less encouraged by your experience to check out my own tank. Alas, when I pulled the sensor the inside of the tank looked brand new. A testament to the care and expense the previous owners put into the restoration. But probably not helpful in solving this problem. Question: when I check out the tank it was almost 3/4 full. So even though it is clean, I was looking through a lot of gas. I could see the strainer at the bottom of the tank. The screen appeared black (I could not see it in enough detail to see a screen pattern.) I was thinking that it would be like a silver wire mesh. Is it perhaps gummed up or are they normally black? Does anybody have any other ideas? So far I have -

  1. Replaced the spark plugs, Coil and condenser.
  2. Cleaned and set the points (actually done by a mechanic who said they looked fine. If it had been me I would have replace them anyway)
  3. Replaced the main fuel filter. I intend to clean and or replace the remaining two filters, tank and fuel pump, as soon as I use up some of the gas now in the tank.
  4. I "borrowed" some spark plug wires from my boat and was able to temporarily replace 4 of the six wires which appear brand new. No difference. I figure this is 60% test coverage but the miss "seems" to be a complete cut out and not just one cylinder.
  5. Drove around with the gas cap off just to make sure it wasn't a venting problem.

Cees: Considering my recent experience with Miss Intermittent (turned out to be one of the spark plug wires, which were only 2 years old), I would check the resistance on all spark plug wires and caps. Joe Alexander did a post on this some time ago. Maybe it's worthwile to replace the distributor cap and ballast resistor as well. You don't mention whether the ignition timing has been checked, so that would be a possibility, (both at idle and at higher RPM). Finally, perhaps checking the operation of the (vacuum) advance/retard mechanisms would be worthwile. In other words, complete the ignition system check that you have started. On other than the ignition - did you check the fuel flow the pump is producing? And, finally, two years ago I had a very bad Miss I problem that got worse very quickly and it turned out to be that the cylinder head was leaking ...but check the other stuff first.

I would also check the valve clearances. And that tank filter would be brass if the drain plug is brass and if a newer one has been fitted, the plug will be aluminum and the screen white plastic. Black sounds mucky.

Doug: when you hit the gas, does the car immediately respond, or does it stumble or hesitate? An immediate response would suggest itís NOT ignition-related & possibly fuel-related. I'm going through the same thing & this is what I'm told about my miss..... at this point, I think mine is from something screwy with either the fuel pump or the injection pump.

Richard Madison: May not be helpful to your situation but when I first got the Brown One (71 280SL), the engine was missing on one or maybe two cylinders...when I put my hand at the rear exhaust pipes, I could feel an irregular puffing rather than a smooth stream of air. The car was a little sluggish and I was only getting 9 mpg. When the fuel injector pump was examined, it seems that the thermostat (?) that controls the lean/rich mix as the car warms up was not was stuck in the rich position. This caused low mileage...poor performance...and kept a couple of the pistons so wet with gas that they could not fire. After the part was replaced, all the symptoms disappeared and I now get at least 15 mpg in the city.

Thanks for all of the suggestions concerning the intermittent miss. It's always hard to discribe these things but probably miss is the wrong word. A missing problem does suggest one or maybe two cylinders. This problem "seems" to be more like a complete failure of the ignition for a brief period of time. When I was (much) younger, and had a motorcycle we used to refer to this symptom as "breaking up". It was usually a function of Rpm in that it would start breaking up as you approched red line. That's why I began by looking at the coil, points condensor etc. (I once had a condensor that would intermittently short out. Gave very similar symptoms). But in my experience these types of electrical problems usually get worse with Rpms. I'm sure when I find it it's going to be something really simple. In the mean time thanks for the great suggestions; and if you have any more please lets hear them. Hopefully this discription will bring something to mind. I'll keep you informed on my progress. (Unless I give up in which case I'll let you know who fixed it).

Naj: check that the fuel return line is clear. Disconnect return line at damper and blow into it. Should be clear. Sometimes the corrosion from the line ends up in the bit of pipe INSIDE the tank and clogs it up.

I have one cylinder that burns oil. I replaced the oil seals twice and no improvement. Every so often I have to pull that plug (#4) and clean all of the sludge of it. I am thinking of putting in a hotter plug in that cylinder. Does anyone know which one I should put in? Should I replace just that one or all of them?

Dan Caron: standard plug is a Bosch W7DC. I use W9DC most of the time as these engines tend to run rich anyways. You could use a platinum W8DP which will not foul as easily. Have you done a compression and leak down test on that cylinder to see if there's a different problem? Check that plug wire too, it could have too much resistance. 1 to 2 K ohms is more than enough.
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