Idle on my 500sec was low - ~500rpm - so I adjusted the throttle cable adjuster ( up to ~750-800) as a temp until I can look into it "someday". Mine is a euro so I don't know if others had this cable adjustment. It's a plastic threaded piece with nut near the end of the cable under the hood.

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85 300 SD ~ 115K
82 500 SEC Euro ??K
78 450SL 164K