idle control valve
Had similar problems with my 85 380 SE, but solved them by:
1. starting warm car which idled at 1600 rpm, spraying brake cleaner around manifold, injectors, etc. to see if idle would change. It did not, so good chance there are no air leaks.
2. Checked tiny bypass hose on idle control valve, the pulled valve and cleaned it with contact cleaner (amazing what black goop came out).
3. Reassembled it, but still no change in high idle.
4. Stopped car, checked voltage with ignition switched on (but car not running) at plug to idle control valve, 11.47 volts, but no "click" when I snapped connector onto valve. hmmm, is valve no good?
5. Yanked idle control computer box, jumped contacts 1 and as well as 4 and 5 at same time (as per MB troubleshooting instructions) with ignition on but car not running. Voila - an audible click from the valve. I guessed it was the computer, pulled the cover with the idle control computer installed (w/o cover - don't touch any metal with it) and twisted it the board. Presto! - smooth idle at 500 rpm.
6. Bought remanufactured board for CDN$350 with warranty.
7. Car is now good.
The valve is simple, but I guess anything can fail (usually the computer though). My tech also suggested checking the TPS sensor on the throttle, it has "stops" for idle and full throttle, if it does not read, the idle control computer will not function properly.