I have a '99 TJ, stock 6 cylinder, stock manual transmission. (I know, I know, but anyway...) my mechanic who is supposed to be the best on Jeeps in my town can't find a bad engine hesitation problem. Already had new plugs, wires. It's definitely engine, not transmission or clutch. Any of you Upper East Tennesseans out there know of a really good mechanic in the Tri-Cities? I am not going to try to fix this problem myself. I don't have the know how on this one, and like I said, the "best" mechanic in my town can't figure it out mainly because it is intermittent and lasts for about 30 sec to a minute and it only did it on him once. He can't get the bloody thing to do it again, but it does it with me and my son a lot.

throw out some details* - maybe someone can add ideas.


*Its a 4.0L, sometimes it (describe) - around town, hiway, hot, cold, first start, doing errands ??
Jeep is STOCK, no aftermarket BS, the codes are, the live data shows, we tried?


PS Tri cities - Kingsport, Johnson City, and Bristol, I spent a year there... I digress - memories

The hesitation seems to be random. Usually does not have to do with hot or cold engine, although it usually does not hesitate the first time it is started in the morning, it will wait until maybe running 15-20 min, but then later in the day, it might have a hesitation very soon after starting or immediately after starting. This is a really bad hesitation with what seems to be backfiring on occasion, not every time. Loses power when hesitating (that probably is obvious), so much so that I've had difficulty getting off to the side of the road at times. Most of the time it corrects itself in 30 sec to a minute. If I turn it off and start it again, many times it won't have the hesitation again for a while. Sometimes it will, but turning it off and on again a couple of times fixes it. I don't drive long distances anymore with the Jeep, only around town in case I have to pull it over and shut it off. I've had it at the mechanic's shop twice for it. The second time, he kept it for about 3 weeks, and drove it himself, he says almost daily. It didn't happen when he drove it, of course. Again, new battery, alternator, plugs, wires, harness. Had new O2 sensors about a year ago. Haven't changed the fuel pump.

Normally the fuel pump will blow fuses when it is going out. Maybe injectors, if they are original 22 years is a long time. When I had my TJ you could get 4 hole injectors which worked much better then my tired original ones.

X2
Jeep is STOCK, no aftermarket BS, the codes are, the live data shows, we tried?

For starters, have you tried some injector cleaner in the tank?

Is the Check Engine Light functioning and self-testing upon start-up ? Is or was the Check Engine Light ON ? Have you checked for stored or pending Trouble Codes ?

Any hesitation, bucking, stalling or backfiring should make one suspect O2 sensor issues. The lack of symptoms at the days first start, and onset of symptoms after warm up with the PCM entering Closed Loop operation are a clue. Once any and all O2 sensor troubles are ruled out, the next most likely suspect is a faulty Crankshaft Position Sensor. After inspecting and testing the CPS, and wire plug, and wiring the Throttle Position Sensor, wire plug, and wiring should be inspected and tested.

Are the O2 sensor NTK and NOT Bosch or auto parts store generics ? It is very common for the Jeep 4.0L to have driveability issues when Bosch or non-name O2 sensors are installed.

The O2 sensor wire harness should be inspected for chafed or melted wire insulation, especially after a replacement is followed by what seem to be O2 sensor symptoms. The O2 sensor wire plugs should be inspected for bent, pushed back, or corroded wire pins. The O2 sensor function should be tested. The O2 sensor fuses should be tested.

Fuel injectors should only require replacement if they are faulty when tested, or physically damaged. A faulty fuel injector will almost always trigger the Check Engine Light and stroe a trouble code. My 1998 XJ 4.0 fuel injectors have +284,000 miles on them and are fine, except for the one that was cracked and replaced with a junkyard part.

I would also start looking for a more skilled mechanic.