Main.TrailIndexPage | Engine | Cold Start Valve

Cold Start Valve

This component is part of the Engine. Also see the related topic Fuel Injection. For a comprehensive look at how this all works together, see Engine Starting Aid Tour.

Some of the content on this page graciously provided by the (now defunct) website and George Davis.


Earlier, larger CSV

The Cold Start Valve (CSV) also referred to as the Intake Starting Valve (and most often called the "starting valve" in the BBB) is mounted on the intake manifold near right fender. This device consists of a electrical solenoid and a valve which allows extra fuel to spray into the intake manifold at specific temperatures for starting. The fuel is supplied to this valve by the low pressure electric fuel pump.

The W113 cars had two different size Intake Starting Valves: one was physically smaller.

The later, smaller CSV


Two jets on the early type CSV.

Under cold starting conditions (see: Engine Starting Aid Tour),the solenoid on top of the valve is energised opening the fuel valve allowing two jets to spray fuel into the intake manifold and mix with the incoming air, thus richening the air/fuel mixture to ease cold starting.

The Cold Start Valve (CSV) can therefore be compared to a “choke” for a fuel injected engine. When the starter motor is engaged, the cold start solenoid is activated for a couple of seconds. The CSV is fed by a fuel line from the high pressure fuel pump. It also has an air flow pipe attached, presumably to help blow the fuel spray deeper into the manifold. The solenoid is activated by a +12V line, and draws about 200mA.

Spray pattern

A jet showing the spray pattern. Note the fuel is in droplets, not atomised due to the low pressure from the electric pump.

Needle valve

The fuel flow in the valve is controlled by a spring loaded needle valve and spring.

The circuit to energize the cold start valve is something like this: a relay is energized when the key is in the crank position, but the relay is grounded through the thermo-time switch which is mounted on the head or block, depending on the year of your car. If the thermo-time switch is closed (as it should be when the coolant is cold), the relay is grounded and the cold start valve is energized. If either the relay or thermo-time switch are bad, the cold start valve will not be energized.


The complete CSV Assembly

The CSV Solenoid

The Cold Start Solenoid (P/N: 0-330-106-001) is available as a separate part.

Cold Start Valve Seat

Cold Start Valve and spring

Internal Jets

Solenoid Port

Fuel Line Filter Screen

Solenoid O-ring in place

Solenoid O-ring dimensions

The O-ring(P/N:001 997 75 40) is available from Mercedes. The outer diameter is 11.75mm, the inner diameter is 7.75mm and the ring diameter is 1.98mm. Information and photographs provided by Paul de Geus.


Common Cold Start Valve Problems and how to diagnose them are described below.

Leaking Valve (Internal)

The worst condition is when the cold start valve leaks fuel. In this case, the engine will run rich, but most importantly raw fuel can run into the head, causing severe wear in the valve guides and bore, not to mention what this would do to oil quality.


Remove the small bolt (marked by the arrow in the component view photo) and turn on the ignition. There should be NO fuel leaking out with the fuel pump activated.


  1. The brass valve or the seat is damaged or dirty and needs to be re-seated.
  2. The valve is too tight and has stuck open.
  3. The solenoid is stuck “out” opening the valve.

The leaking valve can be caused by it sticking open, or it may be in need of lapping to restore the seat. Disassembly is by undoing the fuel line (Lower right in the first picture), and then unscrewing the fitting that that the fuel line attaches to. Before trying that, though, get a rod about the same size as the solenoid rod and push on the valve gently. If it doesn't move easily, it's probably just stuck and a good cleaning is all it needs. Remove, disassemble, and roll up a strip of fine sandpaper or emery cloth. Soak well with WD-40, insert into the valve and twist and move in and out gently. Test the valve for fit, it should not wiggle around but should move freely. Repeat as necessary until it's right. You're not trying to remove metal, just the crud that has built up over the years. Clean out well and reassemble. Easiest to do if you remove the CSV, and you'll need a new o-ring to reseal the CSV where it joins the manifold. The valve/seat sealing can be refreshed by lapping the valve. A mild abrasive paste (such as chrome posish etc.) is strong enough to create a good seal.

Once reassembled and reinstalled, test it by removing the 7mm hex screw in the side, and then turn on the ignition but don't start the engine. There should be no fuel coming out of the hole. If there is (more than 3 drops per minutes), it's still leaking.

Leaking Valve (External)

In this case the seal on the solenoid shaft fails, and fuel can pass trough the body of the solenoid and out the top.

You will have to replace the seal on the shaft of the solenoid.

You might try the usual suppliers or an MB dealer for the o-ring; it might be fairly expensive and it is an odd size not commonly available. I had no luck getting the part, so used a small “o” ring sourced from a bearing supply company, and modified a small brass washer to hold it in place. This process is described in Repair of the CSV and appears to work fine.

Hard to Start

This may be caused by a CSV malfunction! I would suspect there are several things that could cause a malfunction of the Cold Start function:

  1. Blocked fuel line. There is a mesh debris collector in-line between the fuel feeder line and the actual valve. Remove and clean the fuel line and the valve holder (shown on the right of the components photo above).
  2. Valve frozen closed. Remove the whole assembly and carefully tap out the brass valve – insert a 2mm diameter rod into the solenoid port and tap the valve out. Mechanically clean the valve guide with “roll of emery cloth soaked with WD-40.
  3. Blocked CSV nozzle – remove and test for clear flow. I used a time of (pressurised) 5 Star carbie cleaner, removed the plastic nozzle and pressed the nozzle tube into the small bleed port of the CSV. One jet was completely blocked and had to be cleaned with acid.
  4. Solenoid frozen open – “before you replace the solenoid, try squirting a little WD-40 on the shaft and turning it back and forth. Mine was simply stuck, and this got it working again” (by George Davis). Activate a few times (with a 12V feed) until it works OK. You can replace just the Solenoid (Australian ~$300) as the part is listed for another (non W113) engine (P/N 0-330-106-001).
  5. No power to the solenoid - The relays are in the engine compartment, driver's side, mounted on a bracket on the inside fender. There are two, they look the same. The forward-most relay is for the cold start valve, the rear-most for the solenoid on the injection pump.
  6. Thermal sensor failure - The thermo-time switch is screwed into a casting that is bolted to the side of the head, above the injection pump, has two wires running to it. You will need to monitor the wire on the CSV for 12V for a short period of time (1-2 secs) during starting.

Repair of the CSV

After removing and disassembling the whole unit as shown above, I found there was no seal on the shaft of the solenoid, and the jets were blocked (probably by the broken down, missing seal).

Replacement o-ring

I couldn’t source a replacement seal for the solenoid, so I used a small rubber o-ring, held in place by a carefully made brass washer. I used a small brass washer which easily fitted over the solenoid shaft, and was just smaller that the outer o-ring on the solenoid body. I tapered the washer to the shape of the solenoid port, and painstakingly filed it down until it held the new o-ring in place, but didn’t squeeze the o-ring too tightly when fitted to the CSV body.

Cleaning the jet/nozzle:


This was a bugger of a job also. Tried to clean with “5star carbie cleaner” and soaked it for a week. I got one side cleaned, but the other jet was still blocked. In the end, I used pool acid (HCl acid) and a small dental syringe to blast it clean (helps if your wife is a dentist). The syringe had a small plastic tube, about 0.5mm dia which fitted inside the jet whole.

I wouldn’t really recommend this – but here’s what I did. I squirted a small amount of HCl inside the blocked jet, and let it bubble away for a minute or so. Then flushed with water from the hose. I suspect that the acid would eat away the metal if left. Still blocked, so I repeated the process – and this time success!

The completed CSV assembly

Following tools help 17mm, 19mm Open Ended Spanners 19mm Crows foot Spanner 5mm Allen Key (cut short)

Access is more difficult on RHD cars due to Brake Servo and Resevoirs. Removal of solenoid may be required to gain access to Air Pipe connection

Old Yahoo content

The following is the content from the old Yahoo documents on the site. It needs to be structured and edited in the correct sections of the entire document. After moving particular content to its correct place in the manual, please delete it here.

We’re talking about two different things. The cold start valve is on the intake manifold and the cold start thermostat is on top of the injection pump. The cold start valve is on the far right side of the intake manifold (facing forward), has 1 wire and two metal pipes attached to it. The constant speed solenoid should be a solenoid mounted on top of the manifold near the linkage, its job is to keep the idle speed up on auto transmission cars when they are in gear by moving the linkage a little. You should be able to check it by watching it when you put the car in gear.

Can richness be caused by a leaking cold start valve?

Richness can indeed be caused by a leaking cold start valve. Some have said the valve can be lapped to restore the seal.

Dan Caron says: the cold start valve is not unlike a needle and seat in a carburator. In this case the needle is moved by an electric impulse to a solenoid. The reason why they're so expensive is because they are made by Bosch and one else.

I recently had my cold start valve rebuilt by H&R Fuel Injection for $125.

I'm not sure of the model you refer to here and I don't know if this will work on the later 113 injection systems, but on my 220 SE coupe, I disassembled the cold start thermostat myself and cleaned the muck out - which fixed the problem!

My cold start valve was simply stuck shut. Had to knock the movable piece out with a few taps of a hammer and punch, then lightly reamed out the bore with a roll of emery cloth soaked with WD-40. Pretty simple device, really, as Dan said it's like a carb needle and seat.

Can I perform some kind of bench test on the cold start valve to ensure that this item is working, if so, how? I'm a keen amateur so keep it simple, please.

There is something called a leak test that can be performed on the cold start valve. I requires a graduated tube to accomplish. The entire procedure is described in the MB Service manual that covers cars form 1959 on. It's pretty involved to describe here. You're right the prices for new are out of sight. If yours is bad, try to pick up a used one off a wrecked 250 SE, 280 SE or 300 SE. The part was used on a number of vehicles other than the 113s. I've seen these at auto jumbles for next to nothing. You take your luck as to how good they may be but for the prices you pay you may just get lucky and find one better than the one you have.

There is a simple way to test the operation of the cold start valve (yes, the one on the intake manifold with the tubes and electric wires, small device). With the engine cold, take it out of the manifold but leave it hooked up. Start the car and watch a fine mist spray out of the valve. It should shut off within seconds or less I believe - not sure about the actual time but it is very short. I suppose if (1) it does not spray fuel with a cold engine then it or the wiring can be defective and (2) when it does not stop spraying (at least some) fuel then it does not close properly and this would cause a rich condition. I checked mine that way and, fortunately, it was fine. There is a similar procedure to test the operation of the cold start thermostat (located on top of the FI pump) and it involves taking off the small air filter on the side. Start the car with the filter off and engine cold, and you should hear a loud hissing noise. As the engine warms up (this takes much longer than the cold start valve) the hissing should dissapear, as the thermostat closes. If it does not operate properly you could try to clean the inside out or, worst case, have to replace the thermostat itself which, on 230 SL models, is very expensive, not so on the 280 and I believe 250. Underneath the thermostat tower there are shims that can be removed or added to correct fuel mixture issues, just like underneath the barometric pressure device on the FI pump. The constant speed solenoid is very often not functioning properly. They, too, are quite expensive when new, but there's a lot you can do to correct most faults. These do not influence fuel mixture at all.

I removed my cold valve switch to get rebuilt. It works, but could be leaking. Is there an easier way to get the bottom allen screw off (there are three---the one on the bottom is hard to get to. Only a one-inch clearance - a socket won't fit). I used an allen wrench, but it took a long time. I dread screwing it back on. Also, are they supposed to be torqued? If they are, how in the heck do you do it?

They make allen wrenches that have a cut out (looks like a ball) close to the end that allows for it to be inserted into the head of the bolt at an angle. I'm not sure if this will work in you case, however I've used with great success on my Mustangs. They can be found at hardware stores, auto parts houses, and in tool magazines.

The ball-ended allen key worked for me. 5mm!

My engine will not start unless I apply 12 volts momentarily to the electro-magnetic starting valve, prior to engaging the starter. I've looked at the wiring diagram, however that doesn’t explain how the valve works. It seems like it should get power when the key is moved to start and remove power when the key is released. When the car is warm it starts ok, so I'm sure there is something wrong with the cold start circuit. Thanks for continue to indulge me and my lack of knowledge on these fabulous cars.

I assume you are referring to the cold start valve on the intake manifold. The circuit to energize the cold start valve is something like this: a relay is energized when the key is in the crank position, but the relay is grounded through the thermo-time switch which is mounted on the head or block, depending on the year of your car. If the thermo-time switch is closed (as it should be when the coolant is cold), the relay is grounded and the cold start valve is energized. If either the relay or thermo-time switch are bad, the cold start valve will not be energized. The circuits are pretty easy to check out if you have a wiring diagram. Beware of the thermo-time switch, though, it has two circuits; one for the relay ground and one for an internal heating element.

Sorry about that, my car is a 1966 230 SL. I am talking about the cold start valve. The wiring diagram I have would be much better, if there was some correlation to where the components are located on the car. Working on early Mustangs was a snap compared to this.

My car is a '69 280 SL, but I'd guess the arrangement is the same. The relays are in the engine compartment, driver's side, mounted on a bracket on the inside fender. There are two, they look the same. The forward-most relay is for the cold start valve, the rear-most for the solenoid on the injection pump. The thermo-time switch is screwed into a casting that is bolted to the side of the head, above the injection pump, has two wires running to it.

First let me apologise for this post being so long. This is happening on a 1966 230 SL with a 250 SL engine. I’m still trying to figure out the starting system, which includes the time switch, relay, solenoid switch (injection pump), starting valve, and thermo time switch. I disconnected the wire from the starter switch to the starter, so I could hold the ignition switch in the start position, without the motor turning over, so I could monitor the actions of the various relays and switches. When I turn the ignition switch to start, power is applies to the time switch, which passes power to the relay causing it to close and allow power to pass through to the starting valve and solenoid switch. As soon as I release the start switch, power is removed from the time switch and this causes the starting valve and solenoid switch to de-energize. Since this happens this way, I can see no purpose for a time switch. I would think, since this is a time switch, power should be applied and it remain energized until the correct amount of time has elapsed, therefore keeping power applied to the relay, which would kept the starting valve and solenoid switch energized. I haven’t even started to think what part the thermo time switch plays in all this. On the wiring diagram it looks like the wire coming from the starter switch becomes three wires, one to the time switch, one to the relay and one to the thermo time switch, which would cause power to be applied to all three at the same time. I’m hoping there is a logical explanation for all this and someone out there can help me see the light. Many thanks in advance.

I can tell you how it works on my 280 SL. I think it's all the same except for the time switch, which my car doesn't have. There are 3 systems involved in a cold start: the thermostatic cold start and warm up system on the injection pump, this is not electric, it's mechanical controlled by coolant temp; the cold start valve (CSV) on the intake manifold; and the cold start solenoid (CSS) on the injection pump. During a cold start, the CSV and CSS are activated only while cranking; once you release the key, they are both deactivated and only the thermostatic system keeps working until coolant reaches the cut off temp. There are two relays on the driver's side of the engine compartment. The CSS is controlled by the rear-most of these two relays. It is activated when the key is in the crank position. The CSV is controlled by the front-most of these relays, plus the thermo-time switch. The thermo-time switch deactivates the relay if coolant is above the cut off temp, or if you crank for more than about 12 seconds (it uses an internal electric heating element to do that). Last, the time switch (as I understand it) causes the CSV to be activated for 1 second even if coolant temp is above the thermo-time switch cut off temp. The CSS seems to be fairly robust, and if it clicks when the key is in the crank position, it's probably ok. The CSV should also click when activated, but even if the solenoid is working, the valve itself can still be stuck shut. If the CSV doesn't click, it could be the relay or the thermo-time switch or other wiring problem, or the solenoid can be stuck. The thermo-time switch can be bypassed easily. It's job is to ground the relay; switch open means no ground and relay doesn't activate. If the thermo-time switch is bad, you can jumper the wire to the thermo-time switch to ground to check the relay and CSV operation once you figure out which wire is which. Don't ground the wrong wire, though, because the other wire is power for the internal heating element and you'll create a short to ground.

I'm having a problem and can't seem to figure it out. I have four relays on a panel mounted to the left fender well. I'm not sure what they do and can't tell by looking at the wiring diagram. The one closest to the firewall is round and the other three are rectangular. The one closest to the front of the car has four wires connect via screws, the other three have plugs, and two of those wires are not connected to anything. One is about 2 feet long and has nothing on the end. The other is about a foot long and has a round connector on the end, and I looked everywhere and can't see where it connects. Also when trying to start the car, I have to jump the cold start valve to get it started and when it warms up it dies. I can jump the valve on the end of the injection pump and it will start again and run for about a minute. Seems like it is not getting fuel.

Dan Caron says: I think you have a 230 SL from what you describe. The two smaller rectangular relays are for the cold start valve and the start solenoid on the back of injection pump. The round one was used on early cars and is a time switch. The bigger relay that has the screwed-on wires is your wiper motor relay. The cold start relay always has pink coloured wires going to it.

I am sure this subject has come up before but I did a search and did not find anything specific. When my 68-250SL is warm it idles great right at 900 rpm and smooth as can be. When I first start it up when cold it runs rich. I just drive it right away and everything is fine but if I let it sit and idle it loads up and starts missing. The cold start valve is not leaking and I cannot seem to find anything about how this system is adjusted.

Cees: your car behaves exactly like mine - when I start it up cold it idles fine for maybe about 15 seconds, then I have to drive off or else it will start missing. After less than a minute or so of regular driving, it will run just fine. I checked my cold start valve operation as well as the cold start solenoid on the intake manifold, and both operate fine. Next I was going to try the following suggestion from Frank Mallory/Tom Collitt, in a post dated April 19th (called "rich fuel"): "I may be a little to early in saying that I fixed my overly rich condition on my 280sl, but following Frank Mallory's advice I removed some adjustment shims from the barometric compensator diaphragm. This unit sits on top of the fuel injection pump and is about 7-8cm in diameter. It pushes down on a lever inside which adjusts fuel mixture over the whole range. With age these compensators wear out and the pin does not push in as far as required. That is why you have to remove some shims to get things back to normal. Unless your car is very obviously running very rich you should first adjust all linkages, throttle baffle, etc. Be sure not to create an overly lean mixture (i.e., check the spark plug reading for typical lean condition signs, also back firing, pinging, etc.)"

I will look into that. My only concern is that I may make it too lean as it runs quite lean when it is warm. If I pull a plug after running warm the tip looks like it just came out of the box, no discoloration at all and theses plugs have been in there for over 3000 miles.

The cold start valve on the intake manifold is a one shot device, when you first hit the starter. It sounds like it's working fine. It has no adjustment. I think your problem is with the thermostatic valve on the governor housing. The one with the two water lines attached. It is adjusted by adding or removing shims beneath the two bolt flange attaching it to the governor housing. Sounds like you need to remove a shim.

It sounds like your cold start valve could be leaking. I had a similar problem with mine. It did not leak allof the time. When I checked the drip test it always passed. The problem got worse and I took the valve apart and found the brass valve was deformed. The original valve was brass. I understand that they are ususally rebuilt with a stronger steel valve. If you have not had it rebuilt you might consider it.

Sorry guys, but you both said you car runs fine when hot. This sounds like a cold start problem, not an altitude or barometric compensator problem. If this were the culprit it would run rich all of the time, hot or cold. I still think you need to remove a shim under the bottom of the thermostatic valve. It connects to the same linkage inside of the governor housing, but is used solely for warm-up. I will not have a chance to try the barometric compensator diaphragm adjustment for a few weeks, but I will post the results when I do.

The thermostat shims will change the opening/closing temperature of the valve. It doesn't sound like this is your problem; it's simply opening the fuel rack too much when cold. My problem was: after starting, the car ran pretty well but was subject to stalling at about 100 Degrees. I.e. it was closing too soon. Adjusting the thermostat setting is time consuming. Actually mine was not like the one pictured, and had no internal shims, but had an adjustable button on the end of the valve.It took me several hours of standing over the stove with a candy thermometer. then partially reassembling the valve to see if the air valve was closed. After I got this set, I had to add a shim under the assembly, next to the housing, as I frequently had to start it 2 or 3 times, when cold. I think I have it adjusted near perfect, and don't plan to touch it for a long time.

After reading what you did Cees I just had to try removing a shim. My injection pump only had 2 shims both about .015 of an inch thick. I removed one and it helped the excessive richness at idle when cold. I will live with this for a while before I consider removing the other one. Dick, did you mention that this only effects the mixture when the engine is cold and after warm up this system does not have any bearing on the mixture? I have also noticed that the throttle response is better with this adjustment. I can also reduce the warm idle down as low as 600 RPM and it is still very smooth. Thanks for your help gentlemen!

At what temp. should the thermo valve on the injection pump close? Mine seams to operate but I am not sure if it is opening up too early. As a recap I am having trouble with warm starts.

With all this FI stuff, I read through some fault diagnosis in the MB manual. One of the reasons for 'Difficulty with warm starting' is given as: ‘Incorrect operating method. Remedy: Full throttle must always be engaged before and while starting the warm engine is attempted’. Maybe that will help?

Here is some more info on the warm start problem. MB warns that the following must be in order before anything else: Spark plugs, ignition timing & dwell, distributor, coil, valve clearances, oil pressure. Here are the mentioned causes for hard starting when warm:

  1. Incorrect operating procedure (As yesterday) Must always have full throttle during starting the warm or hot engine.
  2. Non functioning starting valve (cold start valve) and/or starting solenoid on pump. This valve must not drip fuel but must also feed fuel for about 1 second every time the car is started. The solenoid on the pump must pull in together with the starting valve. If not, relay might be defect. Note: The above operation depends on the revision of the electrical wiring which changed sometime in 1963 from an earlier version with 2 relays and a thermo switch. The earlier version should be changed to the above later version if starting troubles occur.
  3. Starting solenoid and mechanism on injector pump not operating correctly. The solenoid must pull the rack in the pump into the 'start' position and release it from there when it switches off. If the rack does not return from the 'start' position, warm start troubles will follow. This can be observed by removing the lid on the side of the pump and watching the rack move.
  4. Slightly leaky washers on the elements of the injector pump just under where the pipes connect. On motors with a high mileage, these may go leaky. They do not mention how to inspect this but if this is the case, the washers should be replaced.

I checked my Thermo Valve with the setting shown in my Haynes manual, which states it should switch off @ 149-158 Degree F. (Later {280 SL's?} North American versions were set at 122-131 deg F.) I did this on the kitchen stove in an "old" pan filled halfway with water, and a candy thermometer. Raise the water slowly, all the while stirring the water with the thermo valve, held with pliers or tongs. Make sure the valve itself has stabilized to the water temperature. Do not rest the thermo valve on the bottom of the pan, as this will give a wrong reading. By slipping the valve into the base, the sliding air piston, you can see, (or using a piece of short clean hose, pull a slight vacuum with your mouth) exactly when it closes. If you're brave enough, you can adjust the setting on the stem. I was successful at adjusting mine, improving my startability measurably. My final phase was adding a shim under the assembly, and now it starts great, both summer and winter.

How can you check your cold start thermostat? I am very new to this type of injection system. And I am having the same worm start problems. My car will start great when cold and will restart great as long as I don't let it set more than 2 or 3 min. But if I wait 15 to 40 min I have a very hard time starting the car I can only get it to start by holding the gas peddle all the way to the floor and it is then very reluctant to do so. After this it dose not want to idle for the first 30 to 45 seconds. Does anyone have any ideas. P.S. The car runs great and I have no other problems, with idle or performance any other time.

All of these cars tend to have trouble starting when hot. There is a 1 second time time switch that MBZ introduced that essentially activates certain elements of the cold start system (for 1 second) even when the temperature is warm that solves the problem. Others wire a switch that activates the cold start valve from inside the car. Less elegant but works. Part number for time swithch is 001 545 16 24; Part number for harness is 108 540 10 09.

Tom Hanson: put a hot-start relay and harness on it. Common cure for a common problem.

On the FI pump thermostat/switch. This is contained in the housing that has the two small water coolant hoses attached. The thermostat is brass. Heads up on two items, assuming the 230sl and 280sl thermostat is similar:

  1. Don't pull the pin from the thermostat!. The thermostat will come with a shipping wire/spring that secures the pin to the body of the thermostat. You will want to remove that spring and discard, but be careful not to allow the pin to come out of the body.
  2. If you install this thermostat, compare the new one to the old one. You will find that the new one is slightly larger, which will require that you drill out the large silver washer that it fits into in order for the thermostat to fit. If you try and replace the old one with the new one without adjusting the washer, you *may* break the FI pump housing.

Is the little air filter for the cold start valve on the injection pump cleanable, or will soaking it in degreaser ruin it?

I have been told by several sources that they should be replaced, not cleaned.

I took mine off for inspection. I found some black soot which I was able to remove by simply tapping it against a hard object. I then blew into it and found no resistance. I believe it is a very simple filter which is functional as long as air can easily pass through it.

Removed the cold start thermostat from the injection pump because the air slide stays open even after the car is warm. Decided to test the heat sensor in some hot water, and found that the plunger doesn't fully extend until 200F or so. The plunger travel from room temp. to 160F is only 2mm., and to max extension is only 3mm. Does all that sound right, or is there a new one in my future?

Can't answer the question as to the correctness of the extension, but for $30, you can replace it since you have it out. If you go new, you will need to find a replacement washer or drill out the silver washer to allow the new thermostat to fit. The new thermostat has a slightly larger brass base-if you don't adjust the washer in some way-it will be impossible to re-attach the housing without damaging the housing-I know first hand.

FYI: The 230SL injection pump uses a different t/stat than the 250/280SL, and it's around $600 !!!

What should the spray pattern be on the cold start injector on the inlet manifold? I have three squirts (not spray mist) pointing in the direction of the three pairs of inlet tubes.

I seem to remember someone writing about adding shims to adjust the warm-up regulator? Can someone explain where this thing is and how it works?

The warm-up regulator is on top of the injection pump, it has two coolant hoses connected to it, plus a round air filter that can be screwed off with a thin 22 mm wrench. It operates like a thermostat in reverse, open when cold (allowing more air and fuel into the engine) then it gradually closes as the coolant warms up. Yours may simply be stuck closed right now, so disassembly and cleaning may fix it. I believe the thermostatic element is replacable, too. You can check it by unscrewing the air filter and starting the car from cold. You should hear air hissing where the filter screws in, with the hiss decreasing to zero as the car warms up. No hiss, or very little hiss, means it's not working right.

Yes, the hissing when cold and no hissing when warm will tell you if there is movement in the thermostat. It moves vertically in the housing on top of the F.I. pump. To hear the hissing better, place a length of hose from where the filter was (after removed) to your ear. The sound carries through that very well. Anyway, just because it's moving does not mean it's adjusted properly. For doing that, others with experience will be able to support you in those efforts.

Usually it just needs a new thermostat. The t'stat is a brass capsule with a (fixed-length) rod sticking out. To test it, remove the pump's air filter, then start the cold engine. Hold your finger over the hole where the air filter goes- the vacuum should pull hard on your finger and the engine slow down. Wait while it warms up- when at 180 or so, holding your finger over the hole should produce slight or no vacuum, and NO change in engine speed. If your finger still slows the engine when warm, the thermostat is bad, and not shutting off additional air and fuel enrichment. This interferes with proper (warm) engine mixture.

What is the procedure for removing the themostatic valve from the injection pump. Do I need to drain my cooling system? I am having the valve rebuilt. My 71 sl pops when first started for about 2 min. After sitting for about 2 hrs, It cranks about 5-6 times before starting.

Did you do all the other checks first, to make sure the cold start valve is the culprit? Lots of stuff in the archives.... Yes, you'll have to drain the cooling system. I just loosened the cap on the expansion tank, and removed the radiator drain plug. That seemed to drain the system enough for the start valve removal. Then remove the 2 screws holding the start valve to the injection pump body. If they are hard to remove, put a big screw driver in the screw slot, and hit the screwdriver handle with a hammer to free them up.

Cees: contrary to Will's experience, I managed to disassemble and check the cold start thermostat without draining the whole coolant system. Some coolant did leak out when I opened up the housing tower, but it was very little.

Rodd: This part of the cooling system is about the highest point. It pretty much all drains downhill from the F.I. pump thermostat. Cees' experience makes logical sense.

Albert: my "injection specialist" mechanic adjusted also the cold start thermostat of my 230 SL without draining the coolant system. He simply opened up the housing tower.

Bob Smith: TESTING of COLD START SYSTEMS on CARS with NEUTRAL SAFETY SWITCHES - NOTE : The Neutral Safety Switch MUST WORK PROPERLY. This is a way to test the system without the need to disconnect any wires on the Starter Motor.

  1. remove the fuse for the FUEL PUMP…..mine is NUMBER 4
  2. place the gear lever in any DRIVE POSITION (REVERSE OR 4,3,2). Doing this will Disabled the Starter Motor but it will leave the Electrical Circuits for the RELAYS AND SOLENOIDS available for testing. You can switch the ignition to the "Start" position and listen for the operation of the FUEL ENRICHMENT RELAYS and if you hook up a 12Volt test lamp to the Cold Start Solenoid (manifold) you can watch the THERMO TIME SWITCH working. Each relay can be tested individually just by unplugging the other relay. To make my life even easier when working on my cark I have installed one of those +VE battery terminals with the unscrewable link. This makes it easier when I want to remove or replace a fuse with out the worry of blowing even more fuses. This HINT might be obvious to the experienced mechanic. I hope it is useful to somebody besides myself.

My FI pump does not have any solenoids. This is apparently normal for 280SL's, as listed in the BBB. So I don't have a cold start solenoid (which means my cold start injection valve on the intake manifold should be larger-capacity - I need to check this because my cold starts always take some time) and apparently I also don't have a cut-off solenoid - I wonder whether this is only for European cars. Time to hit the books again I suppose, or does anyone know: if the FI pump does not have any solenoids, is there an alternative fuel cut-off mechanism? Or is there no such mechanism and this is because in Europe this was not required?

I believe the fuel cut-off was not required in Europe; it was only put on later US market 280SLs ('70 and '71???) to reduce emissions during coast-down because of more stringent requirements in the US. The BBB has a nice couple of pages in the data pages of the fuel injection section that describes the various injection pumps used in Europe and the US, that should confirm it.

Pete Lesler: this contraption was mandated by the EPA on US cars only (perhaps all North American cars as well). It was mandated to prevent raw fuel from being injected into the combustion chamber during cost down periods. It worked to well and most owners have disconnected it in the interest of drivability and to mitigate stalling problems.

I have recently been working to solve my cold and hard start problems. I seam to get one fixed and then I have trouble with the other, I know that their has to be a middle ground. Maybe one of you can help. A quick history. First I had a very bad hot start problem. some time the car almost wouldn't start at all if it was hot. Finally I learn that I was not using the correct starting procedure. All was good for a short while but I Had a persistent problem with my first start of the day. After investigation I discovered that my cold start injector was not operating. I discovered that the time relay was only allowing the injector to open for about 1/2 a second. I did find that you could adjust the relay, and so I did but I am not sure that I adjusted it to open for the correct amount of time. Because now my cold start is excellent yet I am back to having a very difficult time when the engine is hot. Even using the correct starting procedure. If any one knows the specific time duration for the time relay I would appreciate it.

The time delay switch (round relay next wiper washer bottle) should give a 1second injection as soon as the starter is engaged. It should activate the cold start valve as well as the injector pump start solenoid. However, on water temperatures below 35 centigrade, the thermo-timeswitch will override the time delay switch and produce a longer time, depending on the water temperature and going to about 17 seconds at minus 20 degrees. On a normal cold motor, I measured around 4 - 5 seconds.

Some of you may recall that I’ve been fighting a fast idle problem with my 280SL since I first bought it March 2002. With your help, I correctly identified the problem as being related to the cold temperature thermostat on the injection pump. It was never shutting off as evidenced by air sucking noises at the thermostat housing air filter. Such noise is clearly heard when using a piece of hose as a “stethoscope”. Anyway, after three thermostats and about $800 thrown at various “mechanics”, I finally found someone who knows something (another story) and together (no charge for sharing his shade tree) we went over the same old ground again. But this time, his experience led him to the realization that the thermostat housing was not getting nearly as hot as it should. The problem turned out to be a clogged water line. The clog was at the banjo fitting where the water returns from the throttle heater to the engine thermostat housing.

The clog looked like a black oily grease. It didn't appear to be rust. The first thing I did was flush the system using a cooling system flush additive, draining, filling with water, running, draining again. The drained fluids did not seem at all dirty, but the clog was un-affected. I'm hoping the clog was the result of some gunk that was introduced by the previous owners mechanic (the engine was supposedly rebuilt 5000 miles ago). The thermostat that was replaced three times was the one that controls the fast idle, not the engine temperature thermostat. I replaced it once. One of the mechanics I took it to replaced it with an incorrect one. (He threw out my good one).

I want to check over and adjust the mechanisms for cold starting and running on my 230SL, but before doing so, I’d like to make sure I understand what’s supposed to happen. Here’s the way I think it works; the numbers refer to the wiring diagram: the starter valve (31) on the manifold and the mixture control magneto (33) on the fuel injection pump are directly connected. When the ignition key is in starting position (and only then), the automatic starting aid relay (21) sends current to both the valve and the magneto as long as either the time switch (20) or the thermo time switch (32) is closed. The time switch stays closed for a second or so after the key is turned on; the thermo time switch stays closed for a couple of seconds or until the engine is warm, whichever comes first. So on a cold start, the starter valve and the mixture control magneto operate for a couple of seconds. On a warm start, they operate for about a second. The other thing that affects cold running as well as starting is the thermostat on top of the fuel injection pump. The rod on the thermostat pushes down on a valve inside the pump, and as the engine warms up, the rod pushes down farther, gradually leaning the mixture. Hence to get a richer mixture when cold, one adds shims below the thermostat, and to get a leaner mixture when cold, one removes shims. Have I got it right?

Cold starting has been a problem for my 230SL, so I checked the cold start solenoid at the manifold during cold start and I got only a momentary pulse at the solenoid. Is it fair to assume that the cold start time switch is no longer functioning? Also be interested in knowing where in the engine compartment it is located - is it the round switch adjacent to the brake reservoir??

Is the warm up thermostat supposed to be sealed to the fuel injection pump assy as well as to the coolant jacket fitting? I am concerned that water could enter the FI pump by running down the gap between the thermostat and the FI pump assy.

Joe± theoretically it could happen. However have never seen this occur in the last 35 years. I think that compression fit makes a very nice seal even without sealant.

Joe, thanks for the reply. I remember there was a square groove in the top of the FI pump fitting and I wasn't sure if this might have held an o-ring(mine was apart when I bought it so I didn't have a reference).
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