1991 300 E (M103.983) has suddenly started idling at approx. 1500-1800 rpm at startup, regardless of engine coolant temperature. Disconnecting the power harness to the idle speed valve results in a drop of RPM to approx. 750 in park; reconnect the harness and the engine idles fine under all conditions. With harness left disconnected, engine idle speed falls below specs when drive &/or A/C is engaged.

Installed a new idle speed valve, with same results: high idle speed at start-up and then drop to normal when harness is pulled and reinstalled. Fuse on overvoltage protection relay is intact. Any ideas as to where to look next? TIA.

You should check the idle speed contact on the lower part of the throttle plate.
Also check to make sure that there is no binding in the throttle cable or related hardware.

The other possibility is a fault in the fuel mixture controller (air plate housing)> If any problems are seen, the system will go into limp.

A lambda tester will show a fixed percentage and can be traced this way if the fault is occurring at that time.

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Benzmac:
Donnie Drummonds
300E
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM
OWNER OF MB AUTOWERKS .COM

had similar problem before. my mechanic replaced idle control valve, temperature sensor, and overvoltage relay. i believed the real culprit, in my case was the overvoltage relay. good luck.

I have a '89 190e, 2.6 that does the exact same thing. I checked all the same things, idle air control, idle switch, etc. If I turn off the engine and then turn it back on, it usually corrects the idle. I suspect the Overvoltage Protection Relay, but have not yet replaced it.

Replacing the throttle linkage microswitch (002 545 68 14) solved the problem.

Thanks for the assistance!