Okay, so after a long hard battle with my cooling system on my 89 Renix 4.0, ive finally gotten rid of my overheating problem. (New radiator,180deg t-stat,water pump,volvo cap) Anyway the thing is running like a champ around 195-200. The only problem im having is one that doesn't seem to go away. If my jeep gets all warmed up and after driving I turn it off and turn it back on, it will misfire pretty bad. But it only misfires until I rev it up in neutral or drive it hard a little, then it runs fine. Ive heard of other xj's doing this after warming up and restarting. Could anyone help me out?

 

Ever clean the throttle body and IAC?

How old are the tune up parts? Plugs, wires, cap and rotor?

Have you ever clicked on the link in my signature and done Post #1 at a bare minimum?

 

Honestly it needs a tune up, I never thought about that. But no I've never cleaned the throttle body or IAC. I'm sure they could use a cleaning It also idles around 1100rpm

 

Here's what I would do.

Replace the spark plugs with NGK 7252s
Premium cap and rotor with brass contacts.
Belden plug wires from Napa.

Clean throttle body and IAC:

Courtesy of TJWalker:
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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



Click on the pink link in my signature and do posts 1,3 and 4 for starters.

Check all the intake manifold bolts and make sure they're not loose.