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  #1  
Old 09-22-2011, 05:59 AM
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Location: Perth, Australia
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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.

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  #2  
Old 09-22-2011, 08:25 AM
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Possibly a problem with the overvoltage relay not providing power to the ECU.
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  #3  
Old 09-22-2011, 11:52 AM
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Hi, where is the overvoltage relay and what pins should have voltage etc.
Many thanks, Brian.
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  #4  
Old 09-23-2011, 06:05 AM
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I would suspect a faulty Oxygen sensor.
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  #5  
Old 09-23-2011, 09:20 AM
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Location: Tucker, Ga USA
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O2 sensor has NOTHING to do with starting!

Overvoltage relay is near the brake booster & has a red 10A ATC fuse in it.
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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator
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  #6  
Old 09-23-2011, 11:44 AM
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From my experience with that engine a faulty Oxygen sensor results in hesitation/stumbling when accelerating.
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  #7  
Old 09-24-2011, 02:07 AM
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Distributor rotor arm ...

Quote:
Originally Posted by Digtyler41 View Post
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.
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
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  #8  
Old 09-24-2011, 04:30 AM
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From my experience best to use Bosch distributor cap & rotor and Beru ignition wires.
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  #9  
Old 09-24-2011, 10:04 PM
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Thanks guys, I will have a look at what has been suggested and report back.
Cheers, Brian.
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  #10  
Old 09-25-2011, 02:51 PM
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Funny you should mention it ...

Quote:
Originally Posted by Ferdman View Post
From my experience best to use Bosch distributor cap & rotor and Beru ignition wires.
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
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  #11  
Old 09-26-2011, 04:45 AM
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Location: Perth, Australia
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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.
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  #12  
Old 09-26-2011, 07:51 PM
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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
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  #13  
Old 09-27-2011, 02:26 AM
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Rotor arm ...

Quote:
Originally Posted by S-Class Guru View Post
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
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
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  #14  
Old 09-27-2011, 07:42 PM
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Location: Dallas
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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
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  #15  
Old 09-29-2011, 02:54 AM
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Location: Perth, Australia
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Hi all, dissy cap and rotor arrived today. Fitted and no more problems.
Cheers, BrianD.

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