Re: TJ 2000 engine RPMs doesn,t return to idle
Flaky IAT (Intake Air Temp) sensor, maybe??
I don't know much about TJ's, but since it's a 2000-model vehicle I am going to assume that the injection system uses a mass air meter (MAF). Most mass air meters are hot-wire type sensors that measure the amount of current needed to keep a piece of wire at a certain temperature; as air flows across the wire, the wire is cooled, and more current is required to heat the wire. The more air, the greater the cooling, and the greater the current.
If there is something flaky in the MAF, the computer may think that the motor is drawing more air than it really is (cold air temps would obviously increase the cooling effect of the air flowing across the hot wire in the MAF) and use the IAC to adjust the throttle position to where everything makes sense (to the computer, anyhow).
As someone else suggested, perhaps the ticket is to make sure that the service tech sees the Jeep when it is stone cold. What I have seen people do before is park a car right outside the dealer's service drive at night, then get a friend to take 'em home... and get a ride to the dealer in the morning (where their car has sat for several hours since they dropped it off). Walk into the service area, grab a service writer by his ear, and drag him out to the Jeep, start it, show him the problem, and tell him to fix it.