I have a 1998 XJ Classic, 4.0, AW4, federal emissions, 108K miles that I just recently purchased. It was idling very high at about 2K RPM or more in park and 1.5K or so in gear when I purchased it. My thoughts were that it might have a bad TPS sensor or IAC valve. I replaced both of these with factory Mopar parts and still had the same trouble. I borrowed a PCM/ECU from a friend's 1999 XJ Sport, 4.0, AW4, federal emissions, 145K miles and my XJ ran perfectly. The only trouble I saw was that the oil pressure gauge was not working with the loaner PCM/ECU. It read "0", but reads around 40PSI with the original unit. Figuring that I had a bad PCM/ECU I purchased a used one off of the internet that was reprogrammed with my vehicle information. It has the exact same part number (56041534AF) and gives me the EXACT same trouble. This unit is supposed to be the correct one for my vehicle, so far as my research shows. I do not have the part number from my friends unit at the moment, but I will post it tomorrow if it makes a difference. I'm beginning to wonder if the wiring harness was replaced at some point or something like that.
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