I have a 1988 W124 TE with about 200k miles on it.
One thing that I have tried to figure out for a while now is the rpm at idle.

It only happens sometimes. Other times RPM sits normal at about 600.

Here is the situation. Sometimes when I start the car, the RPM will go up to 1500 in P and stay there. When I change gears to R or D, the RPM will immediately go down to 600-800.

It happens when I am ready to park after driving too. If I switch to P, the RPM will go up to 1500.

I noticed that it usually happens on my way back home, meaning not the first start in the morning.

Any ideas? I will add if I think of anything else.
I may post a video too.

Many of the idle speed controllers recognize the P & N positions, and are programed to provide an increased idle speed in those positions. The increase, however, is on the order of 50-100 RPM, depending on coolant temperature. When warm, the controller may be cutting off power to the idle air valve (IAV), rather than slightly reducing it, which will result in a wide open IAV, and the attendant high idle speed.

Thanks. This would happen sometimes even after a whole day parked in the parking lot.

89 300E, W124, M103 High idle... SOLVED

I might try what was done in this thread too

Did adding a resistor to the temp sensor work on your high idle ??

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