I have a 1991 300E that had a high idle problem. It would start OK and run fine; however, the idle would not get below 1200-1300RPM's. I disconnected the connector for the Idle Control Valve when the engine was running and at operating temp and the idle speed would drop to about 750 RPM's. It would stay that way until I restarted the car. I checked the Overload Protection Relay and it was the old style, I replaced it and the problem remained. I replaced the Water Temp Sender and still no change. Lastly, I took some time just to give the engine a good visual inspection and I noticed something that didn't look quite right on the Throttle Microswitch. I ended up removing it and sure enough the rubber boot that covered the switch button had cracked and jammed the cam arm and would not allow the switch to operate. I replaced the switch and idle returned to normal....problem solved.
This is an outstanding forum with tons of good info. Thanks to all and I hope this info helps someone else