Surging problem - '93 300E W124
 

Hey everyone-

Trying to help my father diagnose a problem with his '93 300E. Rpm's are surging badly at idle and under moderate acceleration. Doesn't seem to occur when at W.O.T. He has checked for vacuum leaks and can't find any. What other steps can we take to diagnose?

Thanks in advance-

Vacuum leak (unmetered air), throttle actuator, engine wiring harness.

Thanks for the reply! Is there a way to test the throttle actuator? Can't seem to find any vacuum leaks so far. Regarding the wiring harness, that sounds like a pretty serious cause. Do the harnesses go bad in these cars? And if they do, does the whole engine harness need to be replaced? Sounds like a nightmare of a job... I mostly know BMWs; this Mercedes stuff is all new to me. :)

EDIT: Just watched a video on youtube of a harness replacement. Doesn't look too bad. Curious how you know if the harness is bad? Can that be tested as well?

Mine was the mass air flow sensor twice in 360K! 93 300E, 3.2L what a car!!! Occasionally a bad MAF will act up if you tap on it by hand. Unfortunately it usually takes a known good replacement to rule it out. As for the harness, remove some of the protective sheathing in various locations. If it is bad it will have crumbling insulation underneath.

There's a ton in the archives about the insulation on the wiring degrading in the cars on engine wiring harness and throttle actuators on M104 engines from 1993 to 1995.

Here is a good thread on engine wiring harnesses:

http://www.peachparts.com/shopforum/tech-help/346497-1994-e320-sedan-running-rough-idol-hunting-surging-plus-brake-issue.html