Hangar1

First time poster, so here goes. I have been slowly upgrading 1998 Cherokee Sport 4.0 that has an issue where it bogs down and sometimes hesitates under load (half throttle or more) when rpm's cross 2800-3000. Jeep runs great above and below that range. No codes showing. The following items have been repaired/replaced in the 7 months i have owned it. Inspected all rockers and push rods (good), air filter replaced, cap, rotor, plugs (champion), wires, CPS (mopar), TPS (mopar), fuel pump (bosch, pressure good), upstream 02 (ntk), 4 hole injectors (ebay), stainless header (JBA), High flow cat (flowmaster). Jeep cables upgrade, all grounds cleaned up. Also new mopar coil.
I am starting to think it may be mechanical like slack in the timing chain causing ignition and valve timing to vary under load? I did find a bit of play in the distributor shaft last night going through stuff to trace this down but i don't believe it is enough to cause this. Has anyone had this issue or something similar before? If so what ended up being the problem? Any help would be greatly appreciated! Thanks.

Hangar1

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4.3L XJ

If you have play in the distributor shaft, then the place to start is there. Fix that. Might be affecting the cam position sensor.

Hangar1

Thanks 4.3, I put one on order yesterday, and hopefully that does it. Replacing most of the other items has cured a lot of issues but this problem has been happening since the beginning.

4.3L XJ

It might still be something else, but this is a problem that needs to be addressed anyway. If it persists, you can go from there

Hangar1

Update. I replaced the distributor with a Crown unit and Mopar pickup and still have the bog skip jerk between 2800-3000rpm under heavy load. The old dizzy was pretty sloppy and replacing it did stop a tapping noise that i had for the first few minutes after cold start. So if you think you have piston slap, you might want to check the distributor. Guess i will keep searching.

Cordless telephone

Does your 98 have a ballast resistor? Idk when they did away with it. It's usually on the driver side fender under the hood. Looks like a ceramic rectangle with two connectors sticking out the front. My 93 was having the same problem and I bypassed it a few months back and has ran fine ever since.

Hangar1

Cordless, I do not think i have a ballast resistor, but the issue does seem electrical. Thanks

Cordless telephone

good luck in your endeavors lol. You're welcome

4.3L XJ

I know you replaced your tps, but I would suggest checking your voltage coming from it. It won't cost you anything. My thinking that the flat spot comes from a certain position on the tps

awg

Alternatively, hopefully the OP did not throw away his original working TPS, but kept it as a spare..re-install it...if the problem is identical, probably isnt the TPS

Hangar1

Ok, so on the TPS i had the same issue with what looked to be the original sensor, i bought a BWD at the parts store and it did not help the problem and messed up the idle so i took it back. I then replaced it with a new Mopar unit but still have the problem. Also my code reader will graph certain sensor outputs and i am not seeing any dropout or change in the TPS, o2,fuel trims, rpm, map, or iac when this happens! This is the frustrating part because issues like this are usually fairly easy to identify. When i bought the jeep it was stock and this "dead spot" was there but it became much more noticeable after lift and 33"s causing more load and moving through this rpm range more slowly. Thanks again 4.3 and awg! I am going to check compression and timing chain slack next, just to try to eliminate something mechanical.

Hangar1

Quick follow up on this issue. I pulled the pan and found #5 piston rocking from side to side when turning the engine through, distributor drive gear on the cam was half way worn through, and excessive slack in the timing chain. No telling what else i will find when i start tearing it down, but it was definitely time to freshen up.

Tom Jackowski Jr.

Had this same problem on my 92. Replacing the O2 sensor with an OEM NTK one solved the issue for me. There was a bosch in there before