2000 XJ rpm issues

Hey everybody I have a problem with my 2000 XJ. Every once in a while I will be sitting at a stoplight and the idle will drop down to around 300 rpm and occasionally drop lower the car will stall and shut off to where I have to restart it. Then on to the opposite end of the spectrum I'll be driving it will feel like cruise control on with kicks and my rpm will hold about 3500 and we'll keep it until the right speed for that rpm. I just wondering if anybody had any ideas what could be causing this and what I can do to fix it thanks.

Also she only has about 81k miles

Sounds like a classic case of a mis-behaving Throttle Position Sensor (TPS). Second guess would be the Idle Air Controller (IAC). Search around this site, or Google the problem, using in the search, and you'll be amazed at what's there.

Have you ever seen a check engine light? If so, pull codes for clues. I agree that if it's an engine management sensor, it could be the throttle position sensor. They can be tested.

But to start with, (and especially if it has never been done) I'd clean the throttle body, idle air control and throttle mechanism. Carbon is not your friend and this procedure doesn't take all that long to perform. More on the "how-to" below.

Good luck and keep us posted!
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The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality

Thanks for both your inputs, I have never cleaned the throttle body, and there is a constant check engine light but it's just the rear o2 sensor. I'll give your suggestion a try this weekend

VashT1S said:

Thanks for both your inputs, I have never cleaned the throttle body, and there is a constant check engine light but it's just the rear o2 sensor. I'll give your suggestion a try this weekend

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Tolerate no check engine lights. It gets to be very confusing if you let a check engine light to exist and you then encounter another problem. Troubleshooting one CEL at a time is challenging enough.

Is it P0138? If so, replace the rear sensor. Use NTK as that is the OEM sensor for your XJ. Widely available and not that expensive.