I posted on the new members forum, but I thought I better post here too. I have a 1986 420SEL with a V8. The car has sat for a few months and whenI started it up it now idles at 2,000rpms. Any ideas what is wrong?........CR

 

High idle is normal when engine is not yet at operating temp. The idle should go down when you engage your gear to Drive. Ideal rpm is 750+/- 50.
If the idle persists to be high, one possible cause could be your Idle Control Valve, very expensive part, about $460.

 

if you still have the high idle problem I might be able to help, first of all, is the SRS or ABS light on when the engine is running, if sothen check the overload protection relay. it is located next to the fuel pump relay and klima control relay under the hood, near the left side of the brake booster,it has a red top with a clear lid, there is a 10 amp fuse on top of the relay, if it is blown it will cause the engine to idle high, if the fuse is not blown then the idle control relayunder the passenger side carpet foot rest area is at fault, remove carpet to gain access to the black panel, there is a 10mm nut that holds the panel in place, the idle control relay will be there,,,,,,or the idle control valve is at fault, located on top of engine dead center in front of the air cleaner, with engine running, you should be able to feel the valve working, it is round with 2 wires on the very end and 2 vac hoses, check with volt meter, you should also check for any vac leaks as well, my 1989 560SEL starts within 2 seconds, idles at 1,500rpm for about 5 seconds at cold then drops down to 700+/- in park, 650rpm in Drive and thats with 231K miles so yours should do somewhat the same as mine does,hope that helps you out. good luck