Runs fine for 3 minutes, bad for 3 minutes, then fine. Like clockork....

[ralphiecat]

Here's a good one for you. It's a 99 Cherokee Sport, 4L straight 6, 5 speed trans. Start it up and it'll run and drive fine for about 3 minutes. About that time, power fades at 1200 rpm. If I try to push for more rpm (speed), it's as though the engine just goes flat, no power, no acceleration, in fact rpms will drop to around 1000. The engine will then accelerate to 1200rpm again, and the whole process starts over again. This continues for another 3 minutes or so and results in a bucking sort of ride. If I simply crush the gas peddle, she'll fart a couple of times, and THEN accelerate beyond the problem rpm range. Often doing that cures the problem, more often it just takes time to work itself out. Once it runs normally again, it will do so until I shut it down. When it's off for anything longer than 10 minutes, it will go through the entire problem routine again. Possible a thermal, warming problem? It will also do this standing still while idling for warm up if I manipulate the accelerator by hand. What say you? Throws no codes....

 

[69jeepcj]

My car does something similar to what you describe except its not at any certain rpm. I am 100% positive that someone will chime in soon and give you an answer and probably a simple cure for it, so keep checking in.

 

[69jeepcj]

that's messed up! I say im 100% sure someone will help and nobody showed up Im sure they will soon, maybe the right people haven't read your post yet. Their coming!!

 

[rockfangd]

Have you tried replacing the Throttle position sensor. It may be going bad but not throwing a code

 

[greg92jeepxj]

Ralphiecat , check the post I wrote for rockfangd , it may contain something for you to check out on your jeep too . Sounds like you possiblbly have a stuck evap purge control .
But the fact that your engine is back firing is not good either . Rather than jump the gun and start suggestions like skipped timing , carbon tracking on distributor cap , bad ignition coil , etc. , check these items to see if any help . It can't hurt to check your plugs for fouling and look for carbon tracking or bad plug wires and even make certain that timing mark and distributor rotor are all aligned either. Process of elimination will find what's causing it . Loosing rpms usually suggests bad spark but must be certain engine is not flooding with a rich mixture.
69jeepcj was right , we missed your post . Screwed the pooch !

yes , check TPS as suggested , check MAP too .

 

[greg92jeepxj]

Here's a good one for you. It's a 99 Cherokee Sport, 4L straight 6, 5 speed trans. Start it up and it'll run and drive fine for about 3 minutes. About that time, power fades at 1200 rpm. If I try to push for more rpm (speed), it's as though the engine just goes flat, no power, no acceleration, in fact rpms will drop to around 1000. The engine will then accelerate to 1200rpm again, and the whole process starts over again. This continues for another 3 minutes or so and results in a bucking sort of ride. If I simply crush the gas peddle, she'll fart a couple of times, and THEN accelerate beyond the problem rpm range. Often doing that cures the problem, more often it just takes time to work itself out. Once it runs normally again, it will do so until I shut it down. When it's off for anything longer than 10 minutes, it will go through the entire problem routine again. Possible a thermal, warming problem? It will also do this standing still while idling for warm up if I manipulate the accelerator by hand. What say you? Throws no codes....

Dont know if the '99 has any thermal vacuum switches or CTO , but if so , should be checked as your problem begins when PCM goes to closed loop .

 

[greg92jeepxj]

I forgot something very important on ralphiecat's '99 cherokee . I hope that this does not apply in his case , and I would hate for this to be why his jeep is troubling him. The 4.0's produced from , I believe it was 1999 to 2001 had a factory defect in the cylinder head requiring cylinder head replacement due to a bad cylinder head casting causing a crack between two cylinders .
Hopefully this is not the cause , but it was even documented on Wikipedia. To be safe , a compression test should be performed . If it winds up being a bad cylinder head , it is better to know now and plan for replacement rather then when disaster strikes . I hope this is not the case.

 

[ralphiecat]

Thanks fellas. I will post solution as quickly as I find it. TPS sounds like a good place to start, as there is a dead range of movement with throttle as the trouble occurs. Thanks again....

 

[greg92jeepxj]

Your more than welcome . Hope swapping TPS helps out , but the more proper working parts there are makes it easier to diagnose. Best of luck . Greg

 

[Catoctin Jeep]

That bad head design found its way to my WJ. They crack between number 3 and 4. When this happens you will start to loose coolant.

 

[greg92jeepxj]

Drats and double drats ! Those bad cylinder head castings are still out there ! As far as I know , yes , it was from '99 to '01 but I don't have the casting number anymore . It better not be on a '98 because I salvaged one and hope to use it real soon . I think I may have seen the number googling , maybe on Wikipedia ?