M103 Engine starts fine and idles, then rough when warm

[Dzak66]

Hi, I am a newbie to this forum and thanks in advance for any help which is much appreciated.

I have a Feb 1989 built, 300 CE auto with the M103 3 litre engine.

The car starts on the button and idles and revs perfectly when cold. After about 5 to 10 mins as the car warms up it runs very rough as if running a very over rich mixture.

I thought perhaps it was the engine temperature sensor or engine coolant sensor but not really sure if its and where exactly they are (not on the thermostat cover) please see attached photos . Any ideas would be very much appreciated. Thank you.

 

[Will]

Have you checked the ignition side of things? M103s have an appetite for distributor caps and can also suffer from condensation in them causing them to misfire when warm.

 

[Dzak66]

Many thanks Will, I will check that side of things shortly.

 

[grober]

Other possibility is if you have an EZL ignition unit its overheating - this is due to the heatsink compound between the unit and the wing its mounted on drying out. lots of posts on this.

 

[Dzak66]

Have you checked the ignition side of things? M103s have an appetite for distributor caps and can also suffer from condensation in them causing them to misfire when warm.

This inside of the cap certainly doesn't look good and neither does the rotor arm - so I will change those for a start....I removed the plugs and they are very black and sooty which indicates an over rich mixture....

 

[Will]

Yeah that’s seen better days..!

Not sure what’s available these days but the quality of different aftermarket distributor scams varied quite a lot on M103s, so I would advise you to stick with genuine parts or the best OE ones you can find

 

[Dzak66]

Yeah that’s seen better days..!

Not sure what’s available these days but the quality of different aftermarket distributor scams varied quite a lot on M103s, so I would advise you to stick with genuine parts or the best OE ones you can find

Thanks, I was going to buy all Bosch replacement parts...

 

[ray_hennig]

Thanks, I was going to buy all Bosch replacement parts...

Word has it that Bosch quality is increasingly questionable. Some say Beru is better. I have Beru HT leads and non-resistor plugs and MB original distributor cap, arm and shield.

It's easy to spend your money but I would urge MB for cap and rotor and also the shield behind the rotor arm.

Good luck.

RayH

 

[Deadly Dave]

It is also worth checking the hoses at the idle valve, they could have perished.

 

[grober]

The injection system temperature sensor on the M103 is mounted at the rear of the cylinder head- is usually 4 pin. there are two more sensors one for the temp gauge [ single pin] and one switch for the aircon if fitted.
https://www.benzworld.org/forums/at...-hosting-supplement-14-etm_201_25035_230f.pdf
Here's a video in swedish? unfortunately bit a picture is worth a 1000 words- about 50 seconds in

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[optimusprime]

Temp sensor is the double plug on the back ---first picture . It may be stuck telling engine managment that the engine is cold and pushing in more rich fuel .In fact it should lean the mixture off a little/. Wont make a problem if the plug goes on left or right ..

 

[190dRC]

Not sure this will help the OP but for anyone reading this hopefully it will help you.
All electrics were already renewed and still had awful warm idle. Remember you need copper core plugs etc. so if you have done all of that and the dizzy too and still have bad WARM idle then read on…

The injectors are service items. They should be changed every 60-100k. If yours have not been done get them changed. You will need seals and insulators too. I’ve just done mine, all bought from the SL shop in Stratford Upon Avon. They are a knowledgeable bunch and will find the parts you need. Don’t skimp on the seals and insulators.
My 1991 300TE is now running perfect and almost feels like it has stalled at the traffic lights in D whereas previously it was rocking badly. It may need a tune too so take it to a good workshop than knows Ktronic to make some minor adjustments.

Can’t believe the difference now with mine.

 

[ray_hennig]

They should be changed every 60-100k.

I'd say that's a little excessive. But I agree with you in that hot start and idle issues improve greatly. Mine wasn't too bad but I was seeking perfection at around 350k Kms. A bit of churning followed by 20 seconds of misfiring after hot start. After replacing the injectors, guides and seals, it's a new car.

Then I replaced the fuel pressure regulator and both hot and cold starts are instantaneous.

And remember, only the best HT components on these engines.

All the best.

RayH

 

[optimusprime]

Yes and i think its a bit excessive ... My answer to this is, that electrical problems are 80% of all bad running engines .That and vacuum . So after the engine temperature sensor is replaced you will need plugs,, ht leads ,,distributor cap , and rotor.But only Bosch from a reputable seller ,or Beru . Also check this micro switch .

 

[optimusprime]

And this information .

 

[190dRC]

Not sure about being excessive as it’s a service item according to Merc at 60k. All my electrical components were changed, including the HT leads of course (missed that from above).
The difference is night and day now I have the new injectors insulators and seals in place. £16 a pop for the injectors (Bosch) and insulators £13 and the seals just a few £’s.

 

[optimusprime]

I knew a guy that worked for Mercedes back in the 1990 who told me that, in all the time he worked there , that he never had a reason to change any injector nozzles on the W124 when they came in for a service . They are bullet proof . Did replace mine after 90.00 and i never saw a change in its starting and running ..But i know now its good for another 100.00 miles

 

[ray_hennig]

£16 a pop for the injectors (Bosch)

Gosh, you did well. Mine are not the same (M104 engine) but they were about 55€ each!

R

 

[grober]

Ray remember the early JETRONIC systems weren't termed CIS [ continuous injection system] for nothing.

 

[ray_hennig]

continuous injection system

Ha!

R