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?

 

Yes!
Clean your IAC (Idle Air Control) and Throttle body as a first thing and see how that goes. It's free and good chance it might improve or even solve this issue. There is plenty of info here on the forum on cleaning these. After you put them back, adjust your throttle cable again while you're there.

Let us know how you go

 

CRUISER'S MOSTLY RENIX TIPS

THROTTLE BODY AND IAC CLEANING

OCTOBER 30, 2015 SALAD 24 COMMENTS EDIT
Originally by TJWalker of CherokeeForum & JeepForum



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

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.

 

Thanks, I will probably be doing this tomorrow and I'll definitely let you know if it helps.

 

Let us know either way!

 

Grab a TB gasket if you can, in case the old one can't be reused. Otherwise you might end up with a high idle 😉

Good luck and keen to hear how you go

 

So I am working on cleaning the throttle body and IAC. I removed the piece that the IAC mounts in from the throttle body and the gasket between them kinda fell apart. It's the squareish gasket in the picture. I'm not sure if I can get another one without replacing the throttle body. Any advice?

 

I suppose you could use a THIN film of RTV on there.

 

On occasion, when I can't find the right one, I make my own.

Get some suitable gasket material (comes in sheets). Smear some grease on the part that needs the gasket (in your case, the IAC housing) and "stamp" the gasket on the sheet. Don't smear too much grease, just enough to "ink" the part. Then use sharp scissors, razor knife, hole punch, etc., to cut the gasket out.

The oring looks bad too.

 

I'll probably do that as I don't know how to get this gasket without purchasing a new throttle body. Right now it has nothing on there and no new issues.
I drove the XJ a little and it idles fine. We'll see how it does this week. I have a new IAC coming from rock auto ($15 shipped to my house) so I'll get a new oring with that.

 

Good to hear it sorted the idle!
$15 shipped to your door....with the XJ I often wish I lived in the US when it comes to getting parts cheap.

Happy travels

 

Are the parts themselves more or is the shipping from the US crazy? Also, Australia is a pretty cool place. 4wd Action on youtube has some great videos of overlanding/offroading in Australia.

Back to my idle: now it seems to idle high when I let off the gas but am still rolling. Maybe the new IAC O ring will fix that or maybe it's the square gasket . . . It's better than the low idle, so we'll call it a win lol

 

Yeah most likely
See my post #6 albeit a different gasket/o-ring 😉

 

Mine did this recently. High idle when starting sometimes, but remain at high idle when backing off the gas. Mine was the o-ring.

 

Alright, I finally got around to replacing the iac with the new one from rock auto. Apparently I got a wrong part, because my Jeep was idling at 3k rpm! I changed the new part out for the original one but used the new o ring. I need to drive it more to see if it makes a difference.
The original iac has a longer thing that sticks into the throttle body than the new one, maybe that's why the idle was way off?