So, 2001 XJ. The thing cranks great and runs like an absolute sewing machine, smooth, quiet, powerful, but after 10 to 15 minutes it picks up a miss that quickly and progressively becomes quite bad, quite loud and causes a loss of power with shaking, surging and stuttering upon attempted acceleration and such.

I replaced the coil rail and spark plugs last year and cleaned the throttle body recently. I checked temperature at the inlet and outlet of the cat and it didn't indicate a clog but damn it sounds bad when I'm down under there, rattling and ticking all inside the muffler area but I assume that is just the noise from the miss? It's not slinging a code either… Assuming the damn OBD even works… Lol.

Advice?

Thanks!

Sounds like it is not happy when going from open loop to closed loop. Not throwing a code is a surprise, as my 2000 throws a code frustratingly quick on the rough idle from hot restarts.

This could get expensive quickly if you are not getting a code. Hopefully someone will chime in with a "I had that before" post.

How long have you let it run when it runs crappy? Maybe let it run longer and hopefully you will get a code to appear.

*Have a shop take a measurement of "backpressure" of your exhaust to help identify a clogged cat.

Good luck, keep us posted!

I have a 2001 XJ also and it does similar things. It was throwing 455 now 121 and 161. I replaced the o2's upper and lower and no change except I now thing the check light burned out but I still can read OBDII. It goes nice and smooth and then it goes to rough or smooth and when I let off on the gas it pounds fast like I got hit in the rear. I get out and adjust the trans cable and ground out the cables off the battery and all is good for a few days or under 50 MPH. I changed the throttle body and all sensors on it to new and again it was good for a few weeks and then all over again. Changed those sensors again and same thing.