There are a lot of unsolved high idle threads because the cause always seems to be different in every case!

I had various idle problems on my 300E, I am pretty sure it was due the very degraded wire harness on top of the engine, but I traded it before digging into it any further (rest of the car was not that great either).

My 420SEL had a high idle and it was due to a ton of vacuum leaks. Replaced ALL rubber parts on the entire engine(huge job), and problem solved. Idles 800 in park and 600 in drive/reverse.

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