92 XJ Laredo, 4.0, 4x4 AX15 267k. Stumbles above 2000 rpm. Most noticeable when on the highway driving 75ish, especially noticeable when using cruise but it does it regardless. Jeep had good power, still maintains speed well, just seems like it sputters. Pulled the trouble codes and got lean fuel mixture (Trouble code 51). The PO pulled the stock air box and added a "cold air intake" with the k&n cone filter. Not sure if perhaps its getting too much air? He also added a throttle body spacer. I also have a strange idle. When started it idles exactly where it should. But after prolonged operation at a higher rpm (such as when driving on the highway at 2200 rpm or so) it will want to maintain that rpm even after slowing the rpm down considerably. The rpm will jump right back up to 2k when the clutch is pressed.. I've looked for vacuum leaks and found none. Any idea would be appreciated.

 

Update: Pulled the throttle body spacer and fixed the high idle. I think it was a linkage bind due to the throttle body spacer. Took if out on the interstate after posting the first post and it takes it a bit to start doing it. maybe 10 minutes of driving, and once the stumble starts it gets progressively worse.

 

Bad plug wires (also cap and plugs) can cause a stumble under load. Check the fuel pressure too.

 

Plugged Cat? My 91 had very similar symptoms. Easy check, pull the o2 sensor and take her for a drive, see if it still does it.

 

Had some plugs laying around for a 350 that had never been installed, so I took them back to the friendly neighborhood parts guy and traded them for plugs for my cherokee. Actually made money for changing plugs out! Anyways, Took her for a drive today and seems like that fixed the problem. The plugs I pulled were Bosch Platinums that seemed like they've been there for quite awhile. They weren't fouled but they've certainly been used. I'll keep an eye on it, and I'll also probably get new wires and a cap soon, but so far the Jeep is running much better.

 

If the problem persists check fuel pressure as suggested. If the plugs were that bad ypu may want to do wires and cap/rotor right away just because.

 

I Also changed the plugs. Smoothed out the idle a bit more and just generally seems to run better. I took a look at the cap and decided it could wait a bit (mostly due to funds). This seems to have taken care of the problem, but I have not had a good opportunity to drive it on the highway for any period of time. I am definitely keeping an eye on it. Thanks for all of the suggestions.

 

Engine is running lean, check vacuum and fuel pressure

 

When you checked codes what were they?

 

Only code was 51, lean fuel mix