what do you think the possibility is that my cat is going??

the basics:
-99 xj 4L
-done a very complete tune up... everything from battery terminals to the o2 sensors and everything in between, including sea foaming it a coupla times.
-good fluids and as far as we can tell, consistent fuel pressure.

the issue: when i start it up and take off (at any point day or night, if it was just running or sitting for a while...) it looses power, and starts to buck, chug and hesitate. it also makes a popping sound, like its backfiring, but it sounds like its coming from the intake manifold...
i have changed the o2's out twice now. both times ive used the good quality, 4-wire, pre-heated ones.

BUT after all this, im wondering if the cat isnt to blame. seems like it runs well for a little while with the new o2's, but quickly reverts to its crappy running mode... thoughts???

 

Mine backfires and misfires a little . My cat is KO. when i start it you can hear clank clank. Same when you get under and hit it.

 

Have you checked the timing? If it's too advanced it would run awful and backfire.

 

Have you checked or cleaned the tps sensor?

 

PCM adjusts the timing electronically.

This and or IAC.

 

What's that?

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throttle position sensor

Mine does it on occasion, only when pulling out at low revs, but I've replaced the TPS and it still does it. Mine doesn't backfire, but hesitates, and if I let off the throttle and then reapply throttle, it goes away. It never does it at larger throttle openings, and it will happily rev past 4500 rpm. I have no CELs and the cat was replaced under warranty a couple of years back when they relflashed the ECM at recall. I dunno what it is (yet), but I'm sure that someone here will find the solution.