Anyone got a fix for a high idle speed of 1500 rpm on a 92 LSC? My rich mechanic says the air intake plenum gaskets are leaking and messing up the fuel/air mixuture and messing with the computer. Is there a cheaper effective way to fix this? My mechanic wants $300.

Is it always idling at 1500rpm, or only in the cold, or only when the engine is cold? Does it surge, or does it simply idle at 1500rpm and no less?

Pull codes, see what that tells you.

If it is a steady 1500rpm at idle no matter what, you can adjust the idle screw on the back side of the throttle body to a lower rpm.

If it is surging, you probably need to clean or replace your IAC and/or EGR. This can be done easily with carb cleaner.