95 C280 intermittent rev limit problem

[periap]

I need help diagnosing this one!

1995 C280 with 79,000 miles - intermittent problem.
From a stop - after car is warmed up - the system is limiting RPM to 1500 RPM. When you push the gas pedal - in any gear position including park or neutral - the rpm increased to 1500 RPM - then dies as if a rev limiter kicked in - and it will "bounce" between idle speed and 1500 if you keep on the throttle. You can hold it at any speed up to 1500 and the engine is smooth and fine.


OK - here is the situation:

• This never happens in the morning going to work - 20+ miles mixed hwy and side roads - no long delays - abour 45 minutes (in Atlanta traffic)
• Ussually happend on very hot days after driving on hwy for some 5-10 miles and/or in stop&go traffic for 30 minutes or so.
• Only seems to occur after stopping at a stop light or stop sign - then when you try to accelerate - you cannot go - only 1500 rpm does not let you get to 2nd gear - so you are "limping" along.
• 1st question - Is this what we are calling limp home mode? I had experienced bad plugs and the car would run on reduced numbers of cylinders, but would allow higher RPM's - are these two different modes?
• This has occured only about 10 times in the last 2 years, but appears to be getting worse - and has started at lower outside temperatures - first few time temp was in the high 90's so I thought it was temp related - now it has happened with temp in the 80's.
• Turning the car off and back on does not reset the problem
• Letting the car sit for 15-20 minutes sometimes clears the problem - whether the car is left idling or off. Sometimes it clears itself after about 2-3 minutes of idling.
• One odd thing - If I am on a hill and get the car rolling down hill fast enough to shift into 2nd gear at lower than 1500 RPM - the problem seems to clear itself.
• There are no ODBII codes showing whether with engine off or while running with the condition active. No misfire codes - nothing. Sometimes the Check engine light will come on.
• Any ideas?

Other info - Plugs, wires and coils were replaced a few months back and have not corrected the problem.
Any help would be appreciated!

Tony P.

[stevebfl]

I would check for harness problems on both the harness and the throttle actuator. You should also view codes on the EA/CC/ISC controller or CC/ISC if it doesn't have ASR.

[periap]

Thanks for the info - I checked the wiring harness in a few places where accessible and it seems OK. Have not checked anything related to the throttle-Accelorator assembly - was hoping to look into other possibilities before going that far.

I will check with the local MB shop I deal with and see if they can read any codes as you suggest.

Does anyone else have any suggestions?
I cleaned the Purge Valve today and will see if that helps, but it could be weeks/months before it comes back - wishful thinking!

Thanks,

Tony P.

[stevebfl]

If your shop can't read those codes it is NOT a Mercedes repair shop. I would suggest that all real MB specialty shops will have the factory MB diagnostic computer. I tell my customers when seeking a shop out of town of if they move that having the MB SDS is the easy first step to finding a shop.

If a shop doesn't have the commitment to own such equipment it likely doesn't have the training or experience to be a value when you have a need. Having the proper tools shows commitment to the technology.

[periap]

Thanks for the advice - I had had such little trouble from our two MB's that I have not tasked there ability.

Tony P.