Verry nice post! I want to mention that I have similar problems with my 86 420 SEL. Short history, for over 1 year had rough idle problem, and the the idle was high too aprox. 900-1000rpm, I changed every thing (ruber part) in the intake including fuel injectors, all the ignition parts and stil rough idle, until finely replais fuel distributor and now is smooth idle. But stil 1000 rpm. When engine is cold, idle seems to be normal~750 rpm, after few minutes goes high 1000rpm even higher. I checked throtle position switch, engine temperature sensor, if I disconect them idle speed goes higher. I chenged idle speed solenoid with one from my 88 560 sel, and stil did not fix the problem. I assume the solenoid is ok because the 560 sel runs smooth. I chenged the idle speed control box from 560 sel to 420 sel (they look similar but part no. are different) and still high rpm. Anyone can tell me if this control boxes are intercengeble from this two cars? Finely, I have this idle speed solenoid from an 84 380 se, is smaller then 420, but I"ve tryed it anyway. For some reason the idle speed is ~750 rpm and smooth! I'm puzzled and don't know what could be wrong! I appreciate for any help!