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So recently my '92 4.0L XJ has started having an idling issue. I only notice it after I've been driving for a little bit. Basically it idles very low and runs poorly. As soon as I give it gas, the issue disappears. It has not completely stalled yet, but I want to try and fix it because the XJ is my only functional vehicle. I'm new to the Jeep world so I'm not very familiar with the 4.0L yet. Is there anything I should check first or any common things to check/replace?
 

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CRUISER'S MOSTLY RENIX TIPS
THROTTLE BODY AND IAC CLEANING
OCTOBER 30, 2015 SALAD 24 COMMENTS EDIT

Originally by TJWalker of CherokeeForum & JeepForum

dirty IAC
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 retracted, the air control passage flows more air which raises the idle speed. When the valve plunger is extended, 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. This should be part of a normal tune-up procedure and whenever idle/stalling issues are present.
CLEANING THE JEEP 4.0 IDLE AIR CONTROL
dirty renix throttle body
  1. 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). On ’91 and later, it may be easier to just remove the whole throttle body. Be sure to use a new throttle body to manifold gasket when reinstalling.
  2. “Gently” wiggle out the IAC from the throttle body. Gasket/O-ring on the IAC can be re-used if it is not damaged
  3. 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 oxygen sensors. 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.
  4. Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner.
  5. It is also a good idea to clean the entire throttle body bore itself, the butterfly valve inside of the throttle body and it’s edges, and all associated linkage as long as you have things disassembled.
dirty chrysler throttle body
Revised 1-31-2016
 

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I had a similar problem and it turned out to be 2 bad fuel injectors. Keep in mind I have a 2000 jeep xj, so my engine and fuel system is different. So cruiser 54 already covered throttle body and IAC. I don't know if your xj has all the same sensors mine does but I literally changed EVERYTHING before realizing I couldve saved a ton of money by just doing a systematic diagnosis. Also would have saved time because doing your throttle body, IAC, CPS, etc, takes time. Start with your fuel system and check its pressure - way simpler than replacing sensors or disassembling throttle body.

rent or buy a fuel pressure indicator from autozone. Hook it up. Turn your key all the way without starting the car. Do it three times. Get out and watch the guage; do you immediatley lose pressure? Depress the schrader valve at the rail and have paper towels there! Does fuel spit out or is there 0 pressure? If you have 0 pressure, you know that you have a leak somewhere: fuel injectors, something else on your rail, fuel lines, filter (maybe not leak, but clog?). Or it could be a fault at your fuel pump - or pump-related stuff. Congrats - you just narrowed it down without spending a dime or taking crap apart. If your fuel system is all good, then you can move on to sensors, etc.

I didn't take my own advice and replaced all the sensors, the fuel pump, etc, only to find out I had 2 bad injectors. DOH!!!! At least I have a new fuel system? lol
 
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