Where to start! Car broke down on highway (92 300E 3.0L). Checked for spark and fuel- Had a weak spark (found distributor cap button had worn out and was missing). Replaced the dist. cap and the car started for two minutes then died and would not restart. Checked fuel pressure- 80 psi with normal pressure drop. Vacuum reading when cranking was 3 to 4 inches. No visable vacuum leaks. Tested EHA valve connector 12V with key on 10.25V when cranking (seems to have cis brain awake and alive). NO fuel leak at valve. Fuel distributor plunger moves freely and no fuel has leaked out underneath. Removed egr valve thinking cat. convertor was clogged- made no difference still would not start. Sprayed ether into throttle housing- there was not so much as a hickup at all. Engine cranks normal and has good compression on all six cylinders. I tryed replacing the temp sensor and over protection relay which did nothing. Can hear both fuel pumps running!!! The car has had good spark the entire time after replacing the dist. cap. I am totally out of Ideas!! Would the MAS or CIS brain cause this problem??? HELP!!!

Here is some more info. Got it started plugs were fouled out. Ran then died it is get to much fuel at times and too little fuel other times. Disconnected MAS unit and ran car off of 2+2 all day!!! I going to replaced the fuel distributor and injrctor since all the fuel pressures are normal!

Replaced the fuel distributor and all six injectors, and as you guessed it, still will not start. Checked the spark plugs they were fouled out. Replaced the plugs and she fired up. Now that the car is running I noticed that any time I dipped into the throttle she bogged down or died. I hook a meter up to the FHA signal and found that o2 sensor was not switching. I replaced the o2 sensor and now she runs great. However it will not start cold unless I mash the gas pedal to the floor. I finally was able to set the co2 mixture and car starts all the time without pressing on the gas pedal now. I drove the car ten miles without a problem then all of the sudden the car hesitates (WHEN FULLY HOT) from a stands still (almost like a dead pedal). The engine sometimes back fires now. What in the world could be the problem now ??? I am soooooo frustrated. I hate cis injection!!!! Can somebody, anybody give me a clue to what is going on??? I am a ASE Master Tech with 12 years experience and I can't figuere this one out.

Checked the potentiometer today and it seems fine. Adjusted main flap and readjusted co2. Car stumbles and has hard start problem now! Backfires and is not going into closed loop. No one in here has been any help. I am aboutto stick some explosives in the tailpipe, light it, and call it a day! Seriously I am going to farm this one to the Benz dealer!!! Let them have the headaches!!! I am tired of this car!!!! At this point I think it needs a computer for the fuel injection!!! Told the owner to buy a american car since they a bit cheaper to repair and have half the electrical problems that german cars have!!

How many miles? Big difference between 100k and 200k (regarding where to look first). If its high miles 200K+, I would replace the CPS crank position sensor. It needs one anyway if its high milage. Mine recently went and gave me a rainbow of symptons. Everything from hard cold starts to shut downs after warmed up.

The CPS gets weak and tired as they get older. Im not sure but I believe weakening magnets may effecting the sensitivity of the pickup. Oil leaking on them from the filter/sending unit also wears them thin and may effect resistances. I believe the replacements are improved (mine was a little different). Well worth the $140.

The fuel pressure regulator is vacuum controled and either increases or decreases flow when told to do so. Perhaps it is stuck at increased flow(?) position and needs to be replaced (flooding). If the accumulator is bad, I think it would starve for fuel.

You may want to put a cap and wires on it also if they look tired.

More and more I stand by the rule that whenever a high mileage 124 is purchased, the buyer should replace the below original parts with new...

OVP
MAS (fuel pump relay)
Both pumps
Accumulator
Fuel filter
Primary fuel line (2' long). They get rusty at the bottoim curve.
Tank Vent
Crank position sensor
Oxygen sensor
Fuel pressure regulator
Oil presure sending unit
Plugs, wires, cap
Coil

If you do the work yourself it will all cost around $900 low, $1200 high,. When you are done, you can drive that car coast to coast for another 200K.

here here. sounds like possible ovp relay,elongated metal housing with fuse on top. new one will come with 2 fuses usually. also 2nd the crank sensor but favor the ovp. they get corroded easily and cause myriad symptoms

Already replaced the OVP relay. No change.

The fix was a faulty coil breaking down only under load. I had good spark when cranking. This was a really weird failure- never saw a coil intermittently go bad. Car runs 100% now. Thanks for all the help everyone!

Did you find the problem yourself or the dealer?

Simply amazing.

Fellow DIYERS. Don't feel too badly next time you take the "shotgun" approach.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

I simply disclosed it as a ignition related problem due to the fact that every time I physically check it for spark it had a great blue colored spark! But it must fail under load producing a weaker and/or no spark condition! The car has 220 K on it so I don't feel the injectors were a waste of money!!!! I have to say that the cis injection system is a difficult system to diagnose. Many of the parts that go bad give the same symptoms!!!! I found the problem myself and was quite happy that this car is out of my life now!!! Thanks for everyones help!

I have 4 1985 300D Turbodiesels. They all start easily.
I have 1 1991 300SE and it does not start. It is amazing how many things can go wrong to make it not start or run properly. All of those things do not exist in the 1985 Turbodiesels.

Working on the 300SE makes me love the simplicity of the older 300D Turbodiesels.

The spark being blue was indicator number 1 that is was weak, it should be a bright white and it will jump about an inch to ground form the coil wire end. MB’s test procedure is fallible, if you follow the test steps that are written you will never suspect the coil as the cause.

Performing the available voltage test will always nail a weak and or dying ignition coil, however you do have to have a way of testing the coils secondary voltage. I believe you can obtain a thumb screw type device at the local auto parts store which tests secondary voltage via the gap that you manually set. Not a bad investment for the DIYer.