One more thing, your test of the idle air valve might be meaningless. If the computer is getting bad sensor readings, it might not even be trying to adjust the valve so unplugging it may not be revealing of anything. To test the valve, I take them off and apply 12v directly to the terminals and see with my eyes if it is working or not. Often a 9v battery will work too, and is super easy to test with since terminals line up. It's on the low side of the needed voltage though, so while I might declare a valve OK with a 9v test, I wouldn't declare it BAD until I tried it with 12v.

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition