High idle on the M116/M117 engines is usually caused by vacuum leaks. On my M116 (essentially the same) I had to take apart the whole top of the motor and replace EVERY piece of rubber, especially the rubber rings that seal the two halves of the intake. Once everything is done and replaced, it will idle smooooooth. Mine idles at 500 in drive and you can't even feel it, at idle in park its around 700 unloaded, 600 loaded (a/c or electrical loads)

If your engine has never been torn apart to replace all the rubber parts, its long overdue, they will all be hard as rock and leaking vacuum.

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