Any help appreciated guys, I've got an 88 xj 4.0 and it runs fine just seems to me like it has no power whatsoever. 65 down the highway is struggling for it and heaven forbid you hit a small hill. Also runs about 240 degrees at that speed while running 180-200 on the trails. 31" tires 3.55 gears and the axle disconnect is always locked. Any ideas? Am I missing something? My buddy's 99 4.0 shreds with 35" tires..

 

Check for clogged exhaust/catalytic converter.


CRUISER’S VACUUM TEST FOR EXHAUST RESTRICTION
Your vacuum gauge should come with an instruction booklet outlining the procedure. Hook the vacuum gauge up to a source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum. Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle. Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

 

That's what my dad was saying it could be. It has an aftermarket exhaust header back but it does have an aftermarket cat. I guess it's a good possibility it could be plugged.

 

One way to find out......

 

Yes sir. Had this same issue on my 04 silverado. Had to cut em off and guy em out

 

Let's not go there...
Test and see what you come up with.

 

alright, so I swapped out the aftermarket cat today for a piece of straight pipe (only for testing purposes of course ) and boy its like I added another cylinder to it! Wow what an improvement...she runs awesome now and is running cooler too...IDK when Ill have her on the highway again but after what Ive seen today Im almost sure that my overheating/poor power at highway speeds is cured...until then..thanks for the help cruiser

 

Glad to hear it.