124 300TE 91 Rough Warm Idle Problem

[joe_]

Hello everyone,
I have a problem with my estate I bought a few weeks ago. It starts, runs and idles fine until the engine is warmed to normal operating temperature. Once warm it runs roughly at idle, dipping down briefly every couple of seconds. It continues running in neutral but eventually stalls if in drive. Any ideas? I've had a browse through several MB forums and it seems like there could be hundreds of causes! I've recently changed the spark plugs, HT leads and distributor cap with no difference. There doesn't sound like any vacuum leaks although I'm no expert, and the economy dial in the dashboard is behaving usually.
My next guesses are the ignition coil, fuel distributor, idle control valve or OVP relay?
Any help is much appreciated.
Thanks,
Joe

 

[GEORGEROV]

Hi Joe, Next cheapest solution I would go for would be the coolant temperature sensor. If it is stuck with a high resistance reading then it will send the right info to the ecu when cold but when the engine heats up the ecu still thinks its cold and will result in rough running, if you have the means to check it it should read about 6kohm when cold and about 680ohm at 80C.

 

[joe_]

Thanks Georgerov,
Thats a new one for the list. Can I measure with a standard electrical multi meter? Many Thanks,
Joe

 

[GEORGEROV]

Yes Joe you can. If its the continuos injection system, KE- jetronic then the coolant temp sensor will have two terminals, one going to the ignition ecu and one for the fueling ecu. Measure each terminal to earth with the engine hot and cold.

 

[joe_]

ignition control unit

Hi, I've taken my car into a mercedes specialist who thinks the problem is a failing ignition control unit. They have put one of their units in and its runs fine apparently. Does anybody know where I might be able to get one of these (cheap as poss)
Many thanks
Joe

 

[Smudge]

Hi, I've taken my car into a mercedes specialist who thinks the problem is a failing ignition control unit. They have put one of their units in and its runs fine apparently. Does anybody know where I might be able to get one of these (cheap as poss)
Many thanks
Joe

I am breaking a 300te at the moment and also have several ignition control units (EZL units) available along with all other ignition parts
I have PM'd you too!

 

[joe_]

An update on this issue. Changed the ignition ECU but still no success. Its still unresolved and the problem has worsened. Its now almost impossible to start as its not run for a few weeks. When it was last run it was idling even more roughly and starting to run rough earlier than before.
Could this be faulty injectors?
Thanks
Joe

 

[crammy69]

Ive had a very similar problem in that my car starts fine but once hot appears to misfire. However whilst mine feels like its going to stall it never does. Im assuming yours is the 24valve (M104 engine) like mine, if not then whatever I say may well be irrelevant.

One thing to perhaps look at is the EHA ( Electro Hydraulic actuator AKA Fuel pressure regulator) see if there is any fuel weeping out of it, if so then change the 'O' rings in it ( their cheap) and see if that helps - remember to depresureise the fuel system first!.

Assuming your garage has looked at it already, did they not tell you what fault codes (if any ) came up? have you had the ECU looked at (not the EZL ignition one) ? I understand the people in the IOW mentioned on this forum can examine it and normally repair it for somewhere between £100 -£350.

Cheers
Simon

 

[joe_]

Hi this is another update. Thanks for the info crammy, I dont think mine is a 24V engine, nothings appears to be weping fuel. Its seems there can be so many things that can cause the same symptoms..(I wish I'd stuck to a diesel) , a few months later and 3 more garages and its got to the stage where it wont even start now.....Apparently it must be ignition system as theres no spark now...Could be 2 problems??
I've taken it to the Merc Main dealership for diagnosis, and awaiting news. Just found out that labour cost at Mercedes has dropped from around £90 per hour to around £40 per hour as the vehicle is over 10 years old!

 

[Bolide]

Ive had a very similar problem in that my car starts fine but once hot appears to misfire. However whilst mine feels like its going to stall it never does. Im assuming yours is the 24valve (M104 engine) like mine, if not then whatever I say may well be irrelevant.

A 1991 300 will have an M103, not a twin-cam M104, unless it is a 300-24 valve. In which case it's an M104 but with M103-style injection

Nick Froome
www.w124.co.uk