Pagoda SL Group
W113 Pagoda SL Group => Drive train, fuel, suspension, steering & brakes => Topic started by: Gregory on December 10, 2009, 17:02:43
-
I hope one of you top gurus can help me with a steady miss on my 1970 280sl at 2700 to 3200 RPMs..Low RPMS is good and above the 3000 RPM is great...only when the engine is at a constant speed RPMS does the engine miss or should I state buck around..I thought electrical but everything checked OK...If I am traveling @ 60 MPH and accelerate the miss disappears but once I let up on the pedal, it reappears...What is wrong?
-
Gregory,
Always a tough diagnosis...
Does it happen underway, or can you duplicate it in neutral/park while in your garage or driveway? If you can duplicate it there is a possibility that it is mixture (rich/lean) in your midrange.
There's a way to check it I think in tech manual, by disconnecting the linkage rod at the FI pump to separate the air and fuel linkages. You hold the two pieces together, press simultaneusly to mimic them being connected, to your trouble point of >2700 RPM. Then add air or fuel to determine if rich or lean.
Of course you might have an electrical issue too...I fought misfires for many years and finally got them all nailed down. What was it? Well, a combination of fuel and electrical issues, of course.
-
Greg
My car did this also? Does it happen after it is warmed up? If so I would look at the fuel situation if you know the timing is good. My filter in the fuel pump was clogged considerably and it ultimately caused it to miss at RPMs above 2500. This is easy and cheap to check. Let us know how it turns out.
Jesse
-
Adding to Jesse's comment about the fuel filter, that was one I solved a while ago, and it turned out to be very fine rust particles...I replaced the fuel tank and that solved that immediate problem.
-
I have some recent experience with air fuel ratio testers. See if you can borrow one and test your afr at all rpm ranges.
-
Thank you all for the tips..I will try the fuel filter in the pump first...I changed the larger one 1K ago and found no rust or debris in the filter or housing...Looked up the tech material on this wonderful site and read about the smaller filter in the pump...Also, do you have to empty the entire fuel tank to clean the tank filter? I will let you know what happens...Thanks again
-
A problem that I ran into a lot on 1970 USA models was that the TSZ electonic ignition requires the ignition points to be extremely clean or the system will not work properly. The fix was to rub a dollar bill through the closed contacts of the points.
-
.........talk about an abrasive US Dollar......
CHEERS !
WITT ! ;)
-
Greg
If you only have 5 gallons or less I would just drain the tank into a gas container. This will be the same line that is connected to the filter. I have included some pictures of the pump to clarify what may be dirty. You should be able to see through the filter like the picture. A[img][img][img]nything less and you may have a problem. Hope this helps
-
.........talk about an abrasive US Dollar......
CHEERS !
WITT ! ;)
Well, Witt, since there are no Canadian dollar bills, and you try to stick a loony through those points.... :D
-
Is it an actual miss, or do you feel a slight intermittent hesitation when traveling at 60mph? (when rpms above 2700) If the later, could be the dreaded speed switch and relay issue(2 black boxes on later US emissions versions) which control vacuum retard of ignition, and allow full mechanical advance. This intermittent on/off of the vacuum valve retards and advances timing on your distributor "simulating" a miss.
-
Witt...Thank you for the pics and info...I finally accomplished this task and not enough debris to be alarmed.....All clean and ready to go..
-
Markus....I think you may have this one.....your description is right on...the only problem is I do not know how to test the two boxes nor do I have the mental ability to check the advance on the distr....I think taking the distr. out and have it completely inspected and serviced by a shop....Any suggestions as to where...I live in Portland, Oregon but do not mind sending it out.....I checked the points and they are fine...
-
Witt...Thank you for the pics and info...I finally accomplished this task and not enough debris to be alarmed.....All clean and ready to go..
....thank you for the flowers, but the credit should go to Jesse.....
CHEERS !
WITT !
-
Before removing your distributor I would check the integrity of all the vacuum lines and rubber connector ends. No cracks or holes, should be a brown one from the intake throttle body to the vacuum valve, and a white one from the vacuum valve to the distributor. Can't believe how many of these lines get crushed by careless battery installation. If you want to isolate the speed switches and vacuum valve (diagnose if they are the problem) simply remove the vacuum line from the distributor secure the line, and drive the car. If the intermittent hesitation at 60mph (rpms above 2400) disappears you can now diagnose if the vacuum valve is the problem, or the speed switches may need to be rebuilt. If you still want to have your distributor rebuilt contact Will Samples at S&S Imports in Dallas, or Benz Dr I am sure they can help you. All of this, assumes of course, that you have the correct distributor installed for your year and model (061 aluminum housing) and speed switches (two black boxes) if it's a late US version.
-
Markus, I tried the disconnect and drive...MISTAKE...Not only did it idle very low cold but when warmed up at 2500RPM and above, it really did not like that disconnect method...Buck and POP...reconnected and she idled normal and continued the same above RPM dance....The distr. is the correct 061 alum. housing.. what would be my next step...you mentioned having the black boxes rebuilt...In special cases is it best to overhaul the distr. black boxes etc...start anew...
-
:o Well that's interesting, since the only thing that should happen is an increase in idle rpm because you have removed the vacuum source that is retarding your ignition timing below 2400rpms (these engines love advance) which means that something else is out of sync/calibration here. These speed switches my not be the problem at all. Before sending off for $145.00 a piece for rebuilds there is much else to check. 1) Check valve clearance. 2) Remove plugs, what colors, black, brown/tan, tan/grayish or yellow shiny glazed? 2a) Warm up engine, disconnect throttle linkage @ IP to cross over shaft, when disconnected push down on the separated IP linkage. Do rpms increase and run smoother or decrease immediately? Push down on the ball end of the crossover shaft, do the rpms drop immediately followed by snapping and popping or do they increase? Get back to us first, then if necessary,3) Follow the instructions of the linkage tour. It all starts here, the relationship between throttle position(butterfly) to IP linkage(fuel delivery) must be correct. 4) Back to basics, set points/ dwell, and ignition timing,(is timing rock steady, or wandering all over the place?) check integrity of cap, rotor, plugs, measure, resistance of each wire, and check coil resistance. Then we will know more. Best of luck, its not that hard ;D