Can ANyone help me CAN NOT FIGURE IT OUT!!!!

1998 jeep cherokee 4.0 aw4, 140,000

Starts up perfect, but it has idelling problems, it will be running just fine and then it will run like complete crap, it will drop to like 500 rpms, shake the whole jeep, giving it gas dosnt help but if you shut it off and restart it it will run fine again untill it does it again, seems to only do it once its warmed up.

Problem Number 2- On acceleration at about 15 mph it will studder and hesitate and pop from the intake, does this to 30 and it runs fine, does it the worst when the engine is cold or when its low on gas, but also does it at operating temp and with a full tank of gas.

It throws 02 sensor code, P138 Running rich

Iv replaced both o2 sensors, cps, tbs, computer, map,

Also performed Fuel pressure test and it was it spec,
Did compression test and it was also in spec
Vacume while ideling good is in spec but when it idels bad the vacume drops severly

Im getting like 9mpg in town and 12 highway
I have 32s and stock gears

Its been doing this for almost a year and i can not figure it out, im ready to push it off a cliff and stat over...:dunno:

you've replaced the o2 sensors...that was going to be my first guess. Have you tested the cat. for flow?? Also, pull your spark plugs and take a good look at them. You can tell a lot about a motor by the burn patterns on your plugs.....

Your year has a recall from DCC. Have you checked the cat?
The recall covers some 98 cherokees, you may have catalyst failure, and the dealer will replace it under this recall.

http://www.jeepforum.com/forum/showthread.php?t=312129

http://www.epa.gov/compliance/resources/decrees/civil/caa/daimlerchrysler-cd.pdf

If you replace the cat yourself, you can get reimbursed

http://www.jeepforum.com/forum/showthread.php?t=342035

check ALL of your vacuum lines

One more to consider, does your 4.0L have an EGR? EGR's can clog or stick open and mess up idle and mileage on the motor. You can clean up the tubes and passages, even clean the valve a bit, but you may have to replace the valve if its stuck open.

The clogged CAT sounds like a real possibility as well, I'd look into that. If its under a recall, have it fixed by the recall, if NOT, you might want to try a test with the CAT removed.

Sounds like your cat.

i forgot to put it in my post, the cat was clogged so i put a straight pipe in and for the o2 sensor after the cat i got one of those things you splice into the wiring and it tells the computer that the cat is working properly even though the cats not there.

Welcome to my world............my 98 does exactly the same thing.......and I've replaced all the same sensors and stuff you have and it still does it.

Hopefully someone will have an idea.

My 98 did pretty much the same as well, was a bad upstream O2 sensor, but I see you've replace that.

When it is acting up, try going WOT and see if it is still running bad. If it's not, that would usually indicate a sensor/electrical problem because many sensors are ignored at WOT.