Need help before school next week!

[mattl31]

Hey everyone, I searched and came up with a few results....but I feel that my situation doesn't really fit some of the criteria in other diagnosis threads. Hopefully we can weed this out. Anyway i was driving yesterday at about 45 mph with max a/c on when my xj started making some violent thunks. at first i thought it was driveline related but i then realized it was actually the engine "bucking" if you will. i continued to drive or about another mile, and with any movement of the throttle it would start bucking again. i decided to floor it, and when i did it stumbled up to 5,000 rpms, but i didnt accelerate! anyway i decided it was best that i pull over, so i did. it idled fine, but every 5 minutes or so it would die....but fired back up every time i tried to restart it! any helps appreciated

1996 Cherokee Country I6 Auto
New Coil, Wires, Cap, Rotor, Plugs, Crank Position Sensor in last 3000 miles.
Seemingly poor fuel mileage (if that helps diagnose problem)
Newer Cat <10000 miles
Downstream o2 sensor replaced with cat

Checked for Codes....NONE
ALSO...I can't get it to do it anymore....but it NEEDS to be fixed!

[Weasel]

check the O2. bad mpg usualy points to a cat. Mine was bucking pretty bad or surging but it had something to due with the tranny, doubt that you have the same problem.

[Menzenski]

Have you checked your motor mounts?

[mattl31]

i havent checked either...but i figured that a bad o2 would turn on the CEL....and i dont understand how a bad motor mount would cause it to die at idle?

[langer1]

Water in fuel? Add a can of Drygas

[Weasel]

good point on the codes, mine doesn't have them so I don't know. Vaccum leaks could do they same type of thing. Not sure how motor mounts would effect it either.

[Matthew Currie]

Check among other things for some wiring problem, perhaps a dangling wire that is shorting intermittently. I would expect most such things to set a code, but don't count on it. Look especially at the wire for the CPS, which can short against the exhaust manifold. Double-check the ignition system for a bad cap or rotor, or crossfiring wires. If the problem seems worst under heavy load or low vacuum, and it isn't ignition, it might also be MAP sensor.

Also might be time to check fuel pressure, and perhaps get the injectors tested. Since it doesn't set a code, you have to try to figure out what sorts of problems will not set a code. A mechanical problem in a fuel injector will not, but an electrical one will. An ignition misfire should, but might not. (My son kindly referred to his 96 FSM for this, and according to that, it will set a code for misfire based on speed fluctuations read at the crank position sensor. That's a kind of roundabout way of reading, and I would not count on it.) A vacuum problem probably will not, and poor fuel pressure, low from a bad filter or pump, high from a bad pressure regulator, will not.