my 500 SEC did the same when the throttle position switch was left unplugged when I reinstalled the engine.

It turned out that if the TPS failed open, the engine goes high idle as well.
I ended up finding this by shorting the tps harness where it connects at the fender, and the car dropped back down to 600-700 rpm.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...