I have been having a erractic idle issue with my 400E. the car idles at approx. 1500rpm in park and idle bounces every few seconds from 1700rpm down to 1200rpm and everywhere in between(very high). I replaced the wiring harness because it was all to pieces, still had problem. I replaced module 1245453632 the cruise control/ idle module with a used part, this took care of the idle jumping around but now that I have a smooth idle it is still high it stays at a constant 1500rpm in park, drops down to the 700rpm range in drive which that is fine with me. My question is, is there any way to adjust idle, or what parts controls this function on this car. Thanks

status of cap, rotors, plugs, plug wires?
vacuum leaks?
there is no idle control valve on the m119s im pretty sure its controlled by the ecu..

Did the "visible" vacuum lines on top of the motor, and the (irrelevant) trans lines, so it must be something else - either one of the 'buried" vac lines, or another component. Ideas?

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL

Caps and rotors were replaced around 5k ago. The idle is very smooth just high, I would suspect a wire or plug being a problem with how smooth the idle is. I can't seem to find any vacuum leaks. Everything Im seeing looks fine. What ECU controls idle function the one I replaced or is there another one? I guess the used part I used could be faulty, but it sure cleared up the surging idle I was having so you would figure it would be good.

sorry meant to say I WOULDN'T suspect a plug or wire being the problem with how smooth idle is.

Did you maybe by accident mess up the calibration of the cable at the throttle body when you changed the harness!!!