Need some help here 1998 XJ 4.0
 

My jeep was having this problem the other day after idling for about 20 minutes. The rpms would go real low, the oil pressure went to about 20 and it sounded like it was misfiring. And then when i was driving, it hesitated while accelerating and if i let off the gas, it would continue at the same speed and not slow down. I cleaned the iac yesterday really well and now its not idling real bad, but it still hesitates while accelerating and when i come to a stop, the oil pressure goes down to about 25.

Is the check engine light on or have you seen it on? If so, you need to retrieve the codes from the computer for clues on the root cause.

I can't remember the code, but it said running too lean

Things that cause lean condition:

A variety of conditions can cause a lean engine code. At its root is the fact that the computer thinks it is not getting enough fuel in the air/fuel ratio that it is constantly trying to achieve. The computer will keep trimming the fuel "up" (basically holding the fuel injectors open longer and longer) and when the fuel trim gets too high, it will throw that code.

Things that can cause it:
1. MAP sensor reporting false high values of incoming air
2. Vacuum leak letting unmeasured air into the system (after the MAP sensor) such as a intake manifold leak.
3. Fuel pump not providing adequate pressure
4. Fuel injectors not delivering fuel at the expected rate
5. Upstream O2 sensors falsely reporting the results of combustion (basically telling the computer that there is too much air remaining after combustion).

Fuel pressure is probably the easiest things to begin checking, followed by vacuum leaks. Shops troubleshoot and repair this code routinely, so if you have a shop that you can trust, they can probably isolate the cause pretty efficiently.

how old is the o2 sensor ? the o2 could just be bad that will also give a lean code .

That is a start, but if you post the actual code number, we can help you pinpoint it much easier.

With codes, the devil is definitely in the details. A good technician will research and chase that code with a "testing based strategy" to determine the exact root cause. Jordan posted a few of the usual suspects.

Don't throw parts at it on a hunch. Terribly inefficient and gets to be expensive in a hurry.