300SE 1991 will not start when hot.
 

Hello everyone, its great to find people online who love Mercedes.
I have owned many over the years, starting with a 1969 280SE.
I now have a 1991 300SE which has run very well until a few days ago. I need any help that you can give me.
My problem is as follows. The car will start and run very well to the shops about 2 miles away. After I come out and try to start car it will not start. I did eventually get the car to run by putting the accelerator peddle to the floor and holding it there. It reluctantly started after about 20/30 sec cranking. On the way home it would misfire/stumble it I put the accelerator peddle down to hard.

This sequence will repeat if I let engine get cold. I have tried pulling of plugs from Temp sender and what I think is the idle control when engine is running and it makes no difference to the engine running. I am at a loss as to what could be causing this. I need desperately need any help that can be given.
Cheers, Brian.

Possibly a problem with the overvoltage relay not providing power to the ECU.

Hi, where is the overvoltage relay and what pins should have voltage etc.
Many thanks, Brian.

I would suspect a faulty Oxygen sensor.

O2 sensor has NOTHING to do with starting!

Overvoltage relay is near the brake booster & has a red 10A ATC fuse in it.

From my experience with that engine a faulty Oxygen sensor results in hesitation/stumbling when accelerating.

Distributor rotor arm ...
 

I had something similar and it was due to the Bremi rotor arm that was only 18 months old. Replaced it with the original MB part (250K Kms old) and it was fine. I now have a new MB rotor and cap.

Bon courage.

RayH

From my experience best to use Bosch distributor cap & rotor and Beru ignition wires.

Thanks guys, I will have a look at what has been suggested and report back.
Cheers, Brian.

Funny you should mention it ...
 

I now have MB original (maybe Bosch but I didn't look) cap and rotor teamed up with Beru wires and plugs. Seems to work.

RayH

Hello to all, took dissy cap of today and what a mess. The rotor also very burnt. I do not know how it ran so well for so long. Have ordered and will arrive in two days. Will report back then.
Cheers, Brian.

Anyone got a theory on why heat would affect the distributor cap and rotor?
Coil heat soak?
Expansion causing larger gap? Naw.
More spark energy needed in hot environment?
Mixture lean when warm, more spark needed to ignite?

Let us know how it works out. I have a heat-soak stumble that i can't find, so I'm always looking for solutions.

DG

Rotor arm ...
 

In my case it was merely that the rotor arm was going high-resistance. Normally it has a 1K ohm resistance for interference suppression purposes. Mine was breaking down under heat as things get pretty hostile at the top of an M104 engine.

RayH

Veery interesting. I never thought about how the distributor created the internal resistance it is specified to have, but one can assume it's some kind of thin-film resistive trace. These type traces can certainly change resistance with age, temp, stress cracking, etc. I dug out an old used rotor/cap and measured resistance.
I found consistent 1.1K ohms in the rotor just as you said. But, I also found 1K in the cap from the internal cylinder 1 contact to the external #1 plug wire connector? if that is the actual case, then individual cylinders could see misfires if the internal traces in the cap opened up and became more more resistive?

Then again, maybe not; the resistance to jump the plug gap and the rotor-cap contact gap is orders-of-magnitude greater than would be a crack in a trace in the rotor or cap. But..... I've seen auto ignitions systems seemingly defy all the laws of electricity.

Haven't had a new cap/rotor in many years, think I'll try it.

DG

Hi all, dissy cap and rotor arrived today. Fitted and no more problems.
Cheers, BrianD.