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Rough 4.0l Idle


New member
Hi all. I'm a new owner of a 2000 XJ, 4.0l with 45k miles on it. The Jeep runs great except at idle. I've replace the plugs and air filter but it still has this irregular skip at idle (the tach bounces around). Can't really tell if it's happening at higher speed. My dealer did a diagnostic check and said that the ignition timing was off a few degrees. He made a slight adjustment (no effect that I can tell) but also claimed that this engine has a "natural lope" due to factory specs that make it environmentally acceptable. Don't think that I buy that explanation. Every other vehicle that I've owned (including the 91 Explorer w/220k miles on it that I sold to buy this Jeep) ran smoothly or had an identifiable malfunction.

Is this really something that I am supposed to get used to or might there be a correctable problem here? I have no clue about what may or may not have been done, maintenance-wise in its first 40k miles. Would dirty injectors be a likely suspect? Thanks for any suggestions.
derinleis said:
...said that the ignition timing was off a few degrees. He made a slight adjustment (no effect that I can tell) but also claimed that this engine has a "natural lope" due to factory specs that make it environmentally acceptable.


The last time I checked, timing was controlled by the ECU and is not adjustable.

~Clean the Throttle body
~run some injector cleaner through
~make sure all vacuum lines are secure

These things will most likely clean up the rough idle.

Next time check in here or JU or something before going to the dealer. How much did they charge you to adjust something that is not adjustable? :mad:
Last edited:
Thanks Pawk.

Actually, there was no charge for the timing "adjustment" since I was still within the 3k mi. used car dealer warranty. I'll try cleaning the injectors, etc. and ask for an explanation from the dealer next time I'm in the area. Appreciate the advice.

I hope you got the owner's manual when your purchased your XJ. There is alot of good infromation in there. If not then go buy one from the dealer or at Techauthority on line.

I got my XJ with 102,000 on it and began to catch up on the maintenance that had never been done on it. I found I had alot of vacuum leaks. It still had the factory plug wires with teh AMC logo on them.

The 4.0 engine has a Continuous Crankcase Ventelation System (CCV) system and the large hose and small hose connected to the were to be changed at 50,000 miles along with the gas cap. They are listed in my owner's manual as an EPA requirement.

On my 88 the replacement large tube harness came with small vacuum lines for the fuel regulator, EGR, etc.

Once I replaced the vacuum lines the XJ had a better idle.

For cleaning the fuel injectors I have had very good luck with Chevron's Techron. It not cheap, like $10 a bottle, but did good things for my cars I used it in and now live close to a chevron station and buy their gas since they add it to every gallon. You should be able to get it at any parts store, and make sure you get a bottle that will treat 20 gallons.

You may want to take it to your local Jeep dealer and have them do a throttle body cleaning. They will remove the throttle body to clean it, usually an hour's job. Also when my tech reinstalled the throttle body he did a full system check with the electrical test set, it checked out okay. It worth it to find out, also you still should have an emissions warranty so if they find something wrong it should be fixed for free.

When you replaced the plugs did you use the same brand as it came with or did you put in another brand of exotic metal plugs? I recommend you go install of the OEM plugs. My 88 came with champions and don't be affraid to go to the dealer and buy parts, as an XJ owner knowing the dealer parts guy is an important relationship.


Thanks for the feedback. I did get an owner's manual with the XJ but, unfortunately, there was no indication of what, if any, maintenance items had been taken care of. Assuming the worst case I replaced all of the fluids (most w/synthetics) along with the plugs and air filter. My manual makes no mention of the CCV (or PCV) or gas cap replacement requirement.

The Techron suggestion sounds like a good one. In fact. I added 20 ozs. when I filled up today. I don't recall exactly what plugs I used when I replaced them - Champion, I think. Nothing exotic in any case.

Thanks again.

I am thinking you have a dirty Idle air controller, that will be cleaned as part of a throttle body cleaning, that should solve your idle problem.