THE CAR IS A 1988 560SL
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Upon startup, the car idles at about 1500 rpm. It goes down to about 1000 - 1100 in 'D'. Sounded like the idle control, but it wasn't. Here's what I did:

I replaced the temperature sensor, the ignition control, the air slide, the idle control box, and the K-Jetronic box. These parts were switched out from an '89 560SL. The '89 560SL ran fine with those parts in place, so they were good.

What else should I check? What else would cause this problem? Is the wiring messed up somewhere?