Up and down idle

[js81]

I've got an issue where my car (96 C220, 141k) varies its idle from about 500-900 rpm. It does this in gear, in park, in neutral, AC on, AC off, etc... in other words, all the time. :-) However, the weird thing is that it only does it when both the engine and outside air temperatures are warm (operating temp for engine, outside air above 55-60F) - it idles perfectly when the engine is cold and/or the outside air temp is sub 50F. It has only completely died a couple of times, and both of those were warm engine restarts while running errands. Suggestions? MAF sensor? Dirty EGR valve? I just had my head gasket replaced (it was doing it before, too, so that's not it) and new plugs and wires (OEM, if that matters). Thanks. :-)

[RemoLexi]

Possibly slight vacuum leak, should probably throw a lean mixture code.

When the engine is cold it runs a little richer on the fuel to warm up quicker, a small vacuum leak won't be detected, but when hot, the fuel mixture is normal and cannot accommodate for an extra air leak, causing idle to be lean or hunt up and down.

Also could be a dirty throttle body.

Also could be a failing maf. (Don't clean it, replace it)

[js81]

Thanks, RemoLexi. I don't have a CEL... I'd thought about swapping the MAF just for good measure, anyway - it's 18 years old, after all, with 141k. :-) Should I pull the EGR and clean it out, too, while I'm at it?

[RemoLexi]

EGR flow malfunction will throw a CEL, if not I'd leave it alone

[js81]

Thanks, RemoLexi. I'm planning to do some work next weekend (brakes, tire rotation, etc.) - guess I'll add cleaning the throttle body and replacing the MAF to that list. :-)

[jeff-abalone]

Mostly MAF problem.

I just bought a 95 C220 2 weeks ago. After the car warm up, it will stall when press the gas pedal. End up replaced a MAF, no more stall.