milmat1

My 2000 XJ Cherokee Sport 4.0 Would not Idle. It would stay running if I kept my foot on the gas and held the rpm's up. But if I let off the gas it would idle down instantly and die. (Tight Throttle) The Problem Started all at once with no warning or cause. Here's what I have done to get her running again.
Of course the first thing was to scan the OBD for trouble codes. I did find a code but it was the P0432 (Catylist) that has been there for a while...So No Related codes!
I Suspected the Battery / charging system because I had recent trouble with this battery. And with a low battery voltage these engines do all sorts of crazy things. (Like Forgetting its idle settings.) So checking the battery voltage with engine off and again with engine running showed 11.6V and 13.9V respectively. So all good there...Hmm

Then I began thinking the Idle Air Control motor may have failed or become clogged. Since it ran fine "Off Idle" I didn't really think the TPS sensor was bad. ( Throttle Position Sensor)
I removed the intake hose etc and gave the throttle body a serious cleaning with carb/choke cleaner. Focusing on the idle air slot towards the rear of throttle body. Where the IAC motor is. Also I disconnected all the electrical connectors for the IAC and TPS etc. Cleaned them and gave them a shot of WD40.
This process seems to have solved the idle problem as she is running fine again now.

So when you have these strange problems it is always worth trying a good cleaning first, and Clean all electrical connections. I give any/all elect connections a shot of WD40 as part of my regular maintenance stuff. (Dispels any moisture that gets in them)....

FYI: What I call a "Tight Throttle" or "Loose Throttle"
When the motor is very rich the throttle response will seem "TIGHT" It will be very fast and No Overrun when you tap the gas peddle, and return to idle almost instantly.
When the engine is very lean (like an intake or vacuum leak) the throttle response will tend to be "Loose" And Will overrun when you tap the gas peddle, RPM's Will Float up and it will idle back down slowly....