High and erratic idle finally fixed
 

I've been trying since I bought the 560SL ('86) a couple of years ago to track down the cause of and fix a high idle condition when the engine warms up. It always started off at about 1000, then as it warmed up, about the time when the air/smog pump kicks out, the rpms would drop to about 700. Then, after driving and getting the engine temp up to normal operating level, the idle would revert to 900 - 1000 on restart, never returning to 700 or so until it had a chance to cool down again.
I cleaned the IACV twice, bought a replacement control module off EBay, chased down and repaired several small air leaks, and one major one in the large rubber air intake shroud (silicone), and closely followed several threads here regarding the possibility of a faulty temperature sender (maybe the control module was getting a false signal, thinks the engine wasn't warmed up?).
Well, I got lucky and found a NEW IACV on EBay, that had been bought at a dealer, installed briefly on the car, then removed when it was found not to be the problem. The dealer wouldn't take it back, apparantly because the guy (stupidly IMO) mentioned it had been installed. Not a mark on it. I snagged it in mid-winter for $35.00 plus postage (lots of bidders asleep at the switch that day!).
Any-who, it went on the engine yesterday, a 15 minute job, and the idle is now PERFECT every time I start and restart. I now really think that any electro-mechanical and sensitive pair of devices more than 20 years old, such as the control module and IACV, should just be replaced, period, and be done with the frigging of cleaning this valve with carb cleaner or whatever, and resoldering module joints. The driving season here is way too short to be spoiled with idle aggrevation.

agree if you can find the parts reasonable like you did!