I have a XJ with 140K on it, three owner including me, with service history. Generally well taken care of. I use the 87 octane gas from various gas stations too. My idle is rough. The RPM gauge doesn't move too much when it's idling, but you can feel the whole truck stumble. It's not bad enough to cause the motor to climb in RPM's or anything, but is very annoying.

I did clean my IAC valve (which wasn't really THAT bad) and that was way over 100 start cycles ago. No change.

What else can affect it?

Motor mounts are perfect and have no abnormal movement.

 

Clean the throttle body. How old are your plugs, wires, cap, rotor, MAP sensor? All can contribute

 

Look for a vacuum leak.
Spray starting fluid around the injectors, and the base of the throttle body listening for change in idle, if it does you found a vacuum leak.

 

Two very good checks. I'm going to check both this weekend. I didn't even think the vacuum leak could do that. I had to replace a 4 foot section that was dry-rotted by the battery. I bet there are more lines to look at.

As far as how old the "plugs, wires, cap, rotor, MAP sensor" are, I'm not sure. I bought this 6-8 months ago, and looking through the service receipts, I didn't see those being replaced....ever. Which is weird. The guy got oil changes, 4x4 transfer case servicing, batteries, a whole other list of stuff done, and I didn't see plugs or wires. They are red wires that say Mopar on them. I don't know if that is original or what. I'm going to replace them with the Accel tune up kit to be safe. Where is the MAP sensor?

Sorry it took so long to responds. Thank you both.

 

Start with easy stuff, clean throttle body, new plugs and wires (nothing fancy 4.0s like the champion copper ones), distributor cap, air filter, and run some fuel injector cleaner through it.

 

Thought they liked ngk....

 

I have heard good things about ngk and champion. Our engines like cheap plugs (thankfully haha). I have champion copper and they work great. I've also heard people loving ngk copper on the 4.0L. Either will work well. Not so sure about the Accel plugs though. Never heard of anybody using those before. And I saw that that tune up kit is pretty dang cheap on quadratec so if I needed to change those four things, I'd definitely give it a shot

 

I'll take a look at it this weekend. Thanks!

 

Is the accel "tune up" kit worth the money? I think it's $69.00 for the '99' XJ.