I posted on this a couple months ago and found the replies very informative, however, I never could get the idle below 2000 revs. I screwed that air control plug in and out and no change, even took it out and cleaned it up.
Any help??
Dick C.

 

a fast idle on a gas engine means you are getting to much air into the inlet system,

just think about where the excess air is getting in, then start checking, the internet can only help so much.

its up to you and your diagnosis,in the final results you will find your hands are smarter than your head, i'm sure they will fix it!!

 

Would help to know the model year and engine...

 

1989 xjs v12 fi

 

The idle speed screw ("plug") goes into the Aux Air Valve (AAV).

The AAV itself could be faulty. There's a piston in there that sticks. Common issue. Some have luck disassembling and repairing the AAV. Others bypass or remove it. Others replace it with a new one.

But, as mentioned, extra air could be entering from any number of other places. One place to check is the air filter housing-to-throttle body bolts.

Cheers
DD

 

check under the intakes for a vacuum leak,there is an annoying hose.that caused my car to idle really high when I first rebuilt it. actually just check the motor with a vacuum gauge.

 

Easiest way to check the AAV is to remove the LH air filter cover (makes getting at everything easier) and remove the big pipe from the rear just infront of the throttlebody (this is the AAV intake) start the car let it warm up and put your hand over the pipe if the rpm drops your AAV is faulty. If not you need to look for a leak.