OK, so i am going to be replacing the exhaust on my XJ. I drive it less than 5000 miles per year, so it's emissions "exempt". I've had a check engine light due to bad o2 sensors for quite some time. Never bothered to get it fixed cause it passes inspection regardless. Apparently it's Cali compliant, hence the three catalytic converters. I was considering replacing the 2 upstream cats as well and the o2 sensors, the downstream cat and get a cat-back system. I found out that if i get an ECU, manifold, and just a downstream cat from a non-cali XJ, i can build a non-cali compliant XJ and eliminate a lot of underhood heat as well as eliminate 2 o2 sensors. There are currently 4. Anyone ever done this swap?? I doubt i'm going to go that route. Most likely just going to replace the o2 sensors, the downstream cat with a magnaflow unit and get the cat-back and call it good. I am open to all advice as i think the upstream cats have been contributing to my heat soak misfire. Thanks in advance!

 

The pre-cats aren't a CA requirement - they're present in general on the 6-242 with the 0331 head (2000-2001XJ, 1999-up everything else.)

CA does have more stringent emissions level requirements, but the equipment is mechanically the same anymore (the biggest changes can be found in the ECU programming, most notably in the spark timing curves.)

What I'd do:

- Replace the 0331 head with a 0630 or 7120.
- Have a new exhaust made with the standing "three into one-wye" configuration, leaving off the precats. Reinstall the bungs for the HEGO sensors (the ECU will look for them, and they're necessary for metering fuel trim.)
- Have the header bent up so that it fits with the stock downpipe.
- Replace the undercar converter with a good aftermarket - like Random Technologies, say - and put the post-cat bung(s) in.

This should give you a system that is easier to service, just as effective, and cost much less in parts when you're done. Also, maintaining the HEGO sensors will help to keep the CEL/MIL off - therefore you won't be masking other faults.

NB: The HEGO sensors in the exhaust header are there to monitor fuel trim, the HEGO units after the cats are there to monitor catalyst efficiency. I see no reason why they can't be relocated to after an underbody cat with a wiring extension, if they're after the precats in the header, without any difficulty. Be sure to protect the wiring extensions from heat (natch!) or you can just use a "HEGO simulator" that will work with the ChryCo electronics to replace the downstream HEGO. However, the upstream sensors should not be simulated or eliminated, or fuel economy will suffer mightily.

 

Wow, that's way more complicated than i'm going to get on this project. I'm poor, so there's no way i'll be able to have someone build a custom header. I think at this point, i'm just going to leave it as it came from the factory and replace what needs replaced. Thanks for the guidance!