Well since it's a renix (like mine) you need a multi meter to test it cheaply. I would check the throttle position sensor as mine was off and many others I know. You are looking for close to five volts coming in and 82% coming out. I went to the local dollar store to get sewing needles to backprobe the system, they work great. My expensive multi-meter had too big probes to get this done otherwise. Excerpt from a fellow xj-er:
There are two connectors coming off the TPS (87 - 90 4.0L). One with a flat connection and one with a square connector. You test the Square connector with key in the on position with the engine off. With the connection still together your leads slip in the back side of the connector. The connector is labeled AB and CD. First test positive in A and neg. in D and see what the reading is, this is the input side, should be approx 5v. The test Pos in B neg still in D and set this output voltage to 82% of the fist reading. To adjust loosen the two screws that hold the TPS in place, and rotate the TPS and tighten the screws with the meter still connected to make sure you don't [FONT="]rotate the TPS out of adjustment.
Having typed all this, I hope you have already checked the precat o2 as well as the air charge sensor which is relatively inexpensive from most parts stores. Let me know how you make out.
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