Jeep Bucking at High Speeds

Hello, I have a 99 Cherokee with a 4.6 stroker with about 6000 miles on it now. The stroker has all the typical mods, comp cam, ported head, ported intake, bored throttle body, ford injectors, etc... The only things that haven't been done yet are a 3in exhaust, currently its all stock, and a professional tune. While I love the power it makes and for the most part it runs great, the jeep has developed a problem that has me absolutely stumped. I used to drive it 40+ miles a day to school and after a couple thousand miles it began to buck or jerk randomly while driving down the highway. At first it was only very slight and occurred once or twice a drive. Now I have moved to be closer to school so I don't have to drive as far but the jerking continues to get worse. It only happens after the jeep is warm and at speeds of 50mph and above. Yesterday I drove 10 miles across town on the highway and by the time I got to my destination the jeep was doing this every 30 seconds or so, but when I parked it for a while then drove back through town it didn't do it all. Although the it gets me to school and work just fine I don't feel comfortable driving long distances to go off roading or camping, which basically defeats the whole purpose of having a jeep. I'm thinking its something more minor by the way it runs just fine around the city and when not warmed up, but the only things i can think of are a shorted wire in the ignition ( no clue where it would be ), the catalytic converter, or some dumb efi sensor. Has anyone had a problem like this before or can y'all think of what this could possibly be? All help is appreciated, but I do have an extensive mechanical background so please no newbie responses.

Thanks,
Nathan

I had something similar on my 92 4.7, at abouuuut the same mileage, but once mine went, it never restarted.

I had cutting out at highway rpms, allmost like a battery connection was cutting in and out.... turns out the "new, not remanufactured" distributor i had installed was a POS, the top plate that drives the rotor inside the cap was spinning indepedantly of the shaft....

Maybe try pulling the cap and rotor, then try to spin this plate by hand. Good luck

It may do that if it is running out of fuel, either because of fuel pressure/volume, *O2 sensor* or whatever. The motor runs way lean and detonates.

My be your issue, I haven't seen this on late models, but it is fairly common in early models. The early XJ programming had trouble dealing with faulty O2 sensor inputs and could run the motor too lean. Usually happens between 2000 and 2500 RPM with a steady pedal (cruising).

Just a guess, but it may be your programming (pulse width) is having trouble with the injectors, a miss match.

8Mud said:

It may do that if it is running out of fuel, either because of fuel pressure/volume, *O2 sensor* or whatever. The motor runs way lean and detonates.



My be your issue, I haven't seen this on late models, but it is fairly common in early models. The early XJ programming had trouble dealing with faulty O2 sensor inputs and could run the motor too lean. Usually happens between 2000 and 2500 RPM with a steady pedal (cruising).



Just a guess, but it may be your programming (pulse width) is having trouble with the injectors, a miss match.

Click to expand...

Sorta hijack/similar problem. How would you go about fixing that? I oddly have a 97' 4.0l and at about 2000 rpm I get a bucking, like you described.

I wish I could give you a definitive answer. When mine was doing that, I swapped out the O2 sensor and fixed a TPS ground issue at the same time. The bucking disappeared. Could be either, or even a combination of both.

Other XJ's with the same issue have had some success just swapping out the O2 sensor.

There may be more than one failure scenario that can cause this.

I'm not usually big on swapping parts and hoping for the best. But if all else fails I occasionally go with my best guess.

I'd ohm test the TPS ground and if that is OK, swap out the O2 sensor and hope. Maybe do a sweep test on the TPS and make sure there are no dead spots or obvious low resistance issues. The needle on an analog ohm meter should move smoothly through the resistance range of the TPS, with no dead spots or low resistance spikes.