ok guys, i had the dreaded rough idle. it started with really horrible idle when first started for about 5 minutes,then it would smooth out and id be able to drive it great for the rest of the day. then it started getting rough only when in park and neutral and when started. at this point, no codes would show up. so one day the check engine light comes on and my car goes to ***. it started stalling,and would shut off sometimes when turning it on.

i started getting error codes,from maf voltage too high/low, to engine coolant temp sensor, to o2 sensor, to idle control valve. replacing the maf,engine coolant sensor,o2 sensor didnt do a thing,and i still received the codes.

so i replaced the wiring harness. it was in horrible condition.

started her up,and low and behold, it seemed that did it. the codes were cleared, check engine light was off,it was no longer idling rough. took it for a spin and came back, park and neutral she was smooth.

so today i turn it on,again smooth, drive for about 45 minutes with no problems, then when i put it in park, ITS BACK, OH NO. idling up and down up and down, but the car doesnt seem like its going to turn off like in the past. the base idle is normal but shoots up high and comes back down to normal.

so i turn it off and immediately turn it on, its gone. go for a spin for 5 minutes, come back and park, still smooth.

im going nuts. if i didn't replace the parts myself it would have cost me $4000 at the dealership(thats what i was quoted when i took it in for diagnostics).

dealership charged me $250 to tell me that, and another $250 total i was charged at other mechanic shops who also tried to diagnose the problem and came up with nothing, except try changing maf and wiring harness.

any ideas?

Idle surges are sometimes a symptom of a bad throttle actuator. The wires in the throttle actuator suffer from the same insulation decay problem as the engine wire harness. You might try and see if your cruise control works, it is sometimes rendered inop by a defective throttle actuator. Do a search--lots of info posted about this.

A bad vac leak can also cause idle problems......

J. M. van Swaay

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1994 E320 Wagon, 230,000 miles
1995 E320 Sedan, 106,000 miles
1994 E500 Sedan, 79,000 miles

Purge valve on the left fender might be clogged. I had these problem it was a easy fix but can't remember what it was remember changing the MAF with no luck it might have been the purge valve.

1)keep driving it and pull codes again .

2) the info in both posts #2 & #3 are correct.

the cheaper and easier thing to do , first rule out any vacuum leaks:

A) EGR valve(exhaust man.) : on the forum some people have reported that an egr valve can sometimes stick open, when it does you will experience a vacuum leak which will cause a rough idle. from reading up on the forum, I believe that mercedes updated the design of the egr valve to fix this problem.

B) purge valve(has MOT stamped on it, location between fuse box an ABS unit)
they tend to get gunked-up. a gunked-up purge valve will not close completely, and that will also cause a vacuum leak. you dont have to buy a new one, just clean it yourself.

C) since you recently replaced the wiring harness ,go back and retrace your steps, make sure you did not disturb any vacuum lines.

if none of the above works, then inspect the ETA wire harness.

i know its been 8 years, but i thought i would post what my fix was.

turned out my alternator was putting out too high a voltage, and was tripping the OVP.

with alternator replacement, still running great to this day.

Good to hear...how long have you owned the car? I miss my '93 300E 3.2, had over 360k on it.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

11 years. has around 100k miles
thanks

Motor mounts

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1993 e300
1995 e320
1994 e320
2006 s500 4matic
2004 Jeep wj overland
2001 Ducati 748
2004 Honda shadow aero