1994 Corsica (6 Cyl)

Car is idling high and has high vacuum (18" according to mechanic). Pull PCV valve from rocker cover and plug, idle drops to near normal. Mechanic says he has “checked every vacuum line”. Scanner shows no faults. Any ideas?

I think you have a stuck idle air control valve. My experience with the GM 3.1 shows that the ECM can’t always detect an IAC fault. The vacuum is an effect of the car idling high, not the other way around. When you yank the PCV valve, you’re causing a vacuum leak and dropping the idle.