2000 XJ with a new rebuilt engine installed almost a year ago. Since engine installed, jeep idles really hard, sometimes shaking. Mechanic has been very good: adjusting, resetting computers, checking everything....but still a year on, Jeep isnt right. Now says I must replace the catalytic converter. -the jeep drives great on highways, but in stop and go traffic, its starts to stall and idle really hard. Mechanic says the Check Engine code is about the catalytic converter. Before the engine was replaced, the catalytic converter was totally ok. Any thoughts? Thank you.
( I have an '89 XJ with way over 200k original miles that drives totally fine )

What was the exact engine code?

Something to do with the way the new engine is working with the older catalytic converter. I dont exactly know. Thats what the mechanic is telling me.

If he can't/won't give you the actual OBD-II codes (example of an OBD-II code: P0420) I call foul. Get a new mechanic.

Having an actual code(s) will help the other folks here give you some solutions

x2

Also, places like O'Reilly and AutoZone will read your OBD codes for free… they won't/can't tell you what they mean, but that's what forums are for, and especially for such an "old" vehicle like a jeep. I mean, I wouldn't go to a forum for codes on a Merc/Bimmer …nor would I own one for that matter. lol

X4 on the code. Codes are not always straight forward to interpret. With OBDII codes, the devil is in the details.

Ask your wrench for the code and post the exact code here for comment. Don't feel shy about asking for the code. Your XJ, your money.

Without the code, it's just pure guesswork on our part.

Something doesn't pass the sniff test here...

OP, you've had it for nearly a year and it's been doing this all along? What has your fuel economy been like all this time? Do you ever smell fuel? Does it exhibit the symptoms no matter what the ambient temp?

I'm with the others, a specific code is needed. Otherwise, it's speculation.

I've bought my 2000XJ new in 2000. And yes, I've had the headers replaced. I learned about that defect. this issue started last August, while driving I was hearing metal hitting metal in the engine, turned out a cylinder blew. The Jeep is mint condition, like my '89, so the mechanic said the best thing to do would be just replace the engine. I was told that by other mechanics too, since I keep my Jeeps in showroom condition. After replacing the engine, the vehicle has never been the same. Now Im in the hole $6000 with a Jeep that the mechanic who builds race cars cant figure out why its having these performance issues. I will post the codes as soon as I get them. Thank you everyone. If you look at my posted pictures, you'll see the Jeep. Its the Green one. Im using my '89 now as my daily ride. Thanks again. I'll post codes by this weekend when I talk to the guy in person

If you havent already, buy a code reader, fuel pressure gauge, a multimeter and a test light. You can do alot with just those four. A factory service manual (not Haynes) goes a long way as well be it a hard copy, PDF or cd. Even we use service manuals from time to time.

BTW. Once you have the code, dont replace whatever sensor the code may be for right off the bat as it could be the wiring.