Jeep 4.0 experts, I have questions!


My ‘97 XJ has a 4.0 transplanted from a ‘95 XJ. The engine’s been with me since high school and the XJ’s been with me for roughly 8 years. There was nothing wrong with the ‘97 engine, I just wanted to keep as much of my original Jeep as I could. It has barely-in-spec oil pressure, but it’s in spec, it has 150-155 PSI of compression in each hole, it has no relevant MIL codes (it has an EVAP code that refuses to go away), the ignition system is tuned up and in good nick, the exhaust is free flowing (Borla header, Magnaflow cat, Dynomax cat back exhaust), the air filter is clean...

but it is absolutely, positively GUTLESS. It has been for years. When it had 3.55 gears and an automatic, 4-low was the only way to get through anything, and the only real difference I noticed going to 4.56 gears was more RPM and I could use OD with the air conditioning on going up hills. The AX-15 swap certainly helped, but it’s still a gutless wonder. It’s not crazy heavy, having barely more than a winch and a spare tire to weigh it down over stock, the gearing is appropriate for the size of tires- what gives? I remember in high school thinking it really ripped and could walk all over my friends lowly 3.1 liter Monte Carlos and Grand Prixs.


It seems to run fine, if a bit rattly. Could I have flattened cam lobes without lots of missing/rough running? Could the timing chain be stretched enough to affect power? I’ve lived with it this way for years because I gave up trying to get it back to how it used to be- but now my interest in fixing it is ramping back up again. Anyone have a clue?

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