All things related to SEC high idle are detailed in the mbcoupes.com archive. I would start by looking for vacuum leaks related to the idle control valve--expect there is a feedback circuit to purposely increase the idle when the tranny is in-gear, and maybe that tubing is open somewhere and over-compensating when that feedback is activated.

My printed manuals came from a friend, though occaisonally they will show up on E-bay; rare and expensive, though worth it in comparison to the scanned images on the CD. Cheers, Andrew Seidel.

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86 560SEC
85 500SEC Lorinser
83 300D Turbo
75 240D