Update on mystery problem changed....

Also, the fuel filter was changed, even though he (mechanic) said it was clear. As for the fuel pump, his computers showed that it had the proper fuel pressure and volume. He did mention that some people have that done every 100,000 miles as preventative maintenance, but I don't want to put another of those in if I don't have to.

Thanks again for all the help (I'm stumped), you guys are great, Phil

Yes. You need a new mechanic.

Phil,

I agree with Bill. The PCM can fail. I've replaced mine twice since I've owned my 91YJ 4.0L. Since the PCM is rather expensive ($600-800) depending on the dealership (cheaper at a junk yard if you match the VIN), you would do well to take your Jeep to a Jeep dealer and let them do full diagnostic on the PCM rather than let a local mech do it. Most folks don't want to hear that, but you have to do what you have to do.

My PCM failures were different than your symptoms. All 91YJ 4.0L functions are controlled by a myriad of sensors that feed the PCM. So either one of those sensors has failed or its sensor circuit, or the PCM itself. The only other items that could possibly cause your problems are:

Bad sparkplugs Bad rotor Bad/cracked distributor cap Bad plug wires Bad/faulty fuel injector Bad engine ground Fuel filter plugged/dirty fuel lines Plugged/faulty/wornout/dirty catalytic converter (If it rattles, it's toast)

BTW, the 91YJ PCM does not have a "safe mode" like later TJs. I do not suspect the fuel pump since the pressure check showed normal. However, while at the dealer, have them check pressure at the fuel rail and do a leakdown test. The 4.0L are increditably reliable motors and will last a very long time if properly maintained, but they do have they're quirks which are mostly the sensors and PCM.

Scott