skipping help

I have a 89 cherokee 4.0 4x4 aw4 that needs some help. Seems that is is running rough at idle. then as you get to around 35-55 mph the jeep starts to jump and buck around, only to be fixed by flooring it. the a couple of seconds later it starts doing it again. What could this be?

My '97 did that exact thing after I changed the plugs and wires. I never located the problem, but I wiggled just about every wire under the hood and it went away so I guess I knocked some wires loose. You're problem may not be that. It could be that your IAC Motor is cloged up. Check your throttle body for excesive carbon build up. If it is dirty try cleaning the throttle body and the IAC Motor. BE CAREFULL! If you dont remove the sensors off the throttle body they can and probably will get damaged by the cleaner. Take them off, and optimally take the whole throttle body off the intake manifold and clean it on a bench so you dont get crud in your intake/cylinders.
If your throttle body IS clean, then my next guess would be that your CPS Sensor is bad. This is located on the bell housing of the transmition and is fairly common to go out on Jeeps. It is a fairly simple replace, one plug and two screws, but it is in a hard to reach area so you may have to use a little ingenuity to get it off and back on. Also check the wires going from the CPS and make sure they aren't grounding out on the exhaust manifold. Some of the older Jeeps have extra wire from the sensor that touches the manifold, melts, and then grounds out. I think most of the replacement sensors have shorter wires to prevent this problem.

assuming the basics are ok, plugs, plug wires etc.,it sounds like the egr valve could be hanging up or there is a crossfire condition where you would have to have the distributor re-indexed if it hasnt been done by now. it was a common problem for the 87-89 4.0 xjs. there are several good post on this web site to assist you in that procedure if you are inclined to do so. if not a dealer can probably take care of it for about 1.5 labor hours.

Could a faluty o2 sensor be causing this?

..Before we start removing the engine, start simple..
Are the plugs good? look at the glass, not the contacts. make sure there aren't any cracks in them. If you are brave, you can take one plug wire off at a time, and see if it runs any worse - if it doesn't, it's that plug, wire, piece of the distributor cap, or whatever..

Good luck
-Nick