Return of idle problems - '91 190E 2.3

[sbourg]

Idle problems saga continues - thought I'd taken care of all the problems, apparently not. Behavior had been normal for about 2 mo. since last time idle system worked on, before new problems appeared.

Symptom - for several days, engine will start and either go to 1500-2500 rpm and stay there, or start up at 500 rpm. When at the high idle, turning engine off then on may cause the same behavior, or a return to normal fast idle of 1000 rpm, dropping to 700 in a few seconds. Note no 'blipping' throttle or anything other than turning off then on affects the problem, even after driving off. Then, for several days, startup behavior may be normal.

Repairs - All idle circuit hoses replaced, as well as bottom of airflow meter plenum. All vacuum hoses inspected - no leaks found. Idle air valve cleaned - alcohol. OVP replaced. Fuses and holders burnished. Not sure where to turn next:

- Possibly the idle air valve is sticking and needs more potent cleaning? I would guess I could check voltage to the valve - if it doesn't drop low when the high idle occurs, I imagine the valve is not the problem.

- Bad NEW OVP? Anyone run into this?

- Bad ECU? I sure hope not!

- Bad air flow meter pot? I would think this could screw with the mixture, but isn't idle speed determined by engine rpm sensing in closed loop idle control?

- Bad rpm sensor? Wouldn't this likely cause problems at all speeds?

Any sage advice would be appreciated.

Steve