Is the idle actually high, or is the tach just off? What I mean is - does the engine actually sound like it's running fast when the RPM is elevated?

On some of the later cars there is a circuit built into the speedometer that can potentially cause problems with properly returning to idle. Member "Stretch" has quite a posting somewhere on this forum chasing that issue down in his W201. I don't remember where I read his saga or I'd post the link. It may be in his epic saga in the Long-Term Build forum (worth reading just because!)

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]