too many rpms when warm

[robbieyukon]

why is my 4.0 idling so high (2,000 rpms) when the motor has warmed up? anyone else have this problem?

[old_man]

Its normally caused by a vacuum leak.

[98JeepXJ]

Its normally caused by a vacuum leak.

Manifold to cylinder head bolts would be suspect. Check for a dry-rotted, cracked, or disconnected vacuum line. Open the hood when it is idling high and listen for a whistling/sucking/hissing noise, a distinct sign of a vacuum leak.

It could also be a faulty IAC (idle air controller) that is sticking open when warm.

[4xBob]

Mine (1990 with 4.0 engine, auto tranny) has done that twice, and both times it was the Throttle Position Sensor (TPS). I also noticed a little weirdness in the shifting, since the TPS sends a signal to the TCU also. Replace and adjust on XJs up to 1990, just replace for 1991 on (no adjustment necessary). What year XJ? Some years you can pull an error code to help narrow it down.

[robbieyukon]

mine is a 1987. so maybe i need to check the TPS?

[old_man]

It can't hurt to check the TPS, have you washed your engine lately? That seems to cause most of the TPS failures, but don't rule out vacuum. My experiences are 10 to 1 on a vacuum leak. Nobody ever seems to use a vacuum gauge any more. It is a real handy tool.

[robbieyukon]

i have not cleaned it, but need too. i will get the gauge out this weekend and start digging around.

[Only In A Jeep XJ]

Another way to check for vacume leaks is with a can of ether if the line is leaking it wil cause a rise in rpm for just a few seconds.