2000-2001 XJs, Dual Cats Under The Hood, and the Dreaded P0303.

Warning: long preamble - jump to the "Questions" flag below if you want to skip down to the meat of my questions.

At this point it is probably safe to say that the P0303 errors that MY00-MY01 XJs with dual cats under the hood generate are not going to be fixed properly any time soon by Chrysler as a recall or a TSB or whatever. Typical Chrysler, where quality is spelt with a "K".

For those not aware, basically in hot weather once your MY00-MY01 Jeep is heat soaked and you shut it off but return to the Jeep a few minutes later and start it up it may vapour lock and throw codes for mis-fires on cylinder 3 and sometimes 4. The heat from the catalytic converters under the intake manifold causes the vapour lock.

There is a Chrysler/Jeep TSB that adds cheesy little heat insulators to the bodies of the injectors. It helps a little but doesn't really cure the problem. I've done this to both of my Jeeps with this system and I still get the issue occurring in hot whether (ambient temperature above say 25ºC/75ºF and the Jeep being worked hard crawling, towing and cruising at high speeds)

There's been some flash updates as well and I'm mostly up to date unless there has been new flashes put out since 2008 or so.

What to do about it?

I've seen others have done the "turbo timer" mod to run the electric fans for a few minutes after the Jeep has been shut down. I haven't heard of an instance of this modification not working but if you have to restart the Jeep before the cooling is complete then it must throw a code again? In practical sense it is likely not a big deal.

It's a good solution, especially if you want to retain the Jeep as close to stock as possible in a strict emissions testing area. I'm not keen on having the Jeep fans running after leaving the vehicle but that is probably me just being a git.

I'm not in a strict emissions testing area and I want to start working towards building performance into my engine to compensate for that extra ~1200 lbs of steel I've added to the Jeep over the last few years while building it up.

The Jeep has to pass the local (ON, Canada) emissions testing system every two years. It needs to pass an idle test and a load/cruise test. There is no cold idle test that would necessitate the under hood cats. I don't expect this to change any time soon. If it does I'll revert to the turbo timer modification mentioned above.

Questions

I fully intend to keep the "normal" catalytic converter and oxygen sensor under the floor just in front of the muffler.

There are two oxygen sensors after the cats under the hood.

Do these under hood, post cat sensors have heating elements in them? Do I need to retain the oxygen sensors if they have heating elements?

I want to remove the heat source that causes vapour lock (i.e. the under hood cats) and

• if I need to retain the stock oxygen sensors for the heating element portion and
• if I replace these under hood cats with straight pipe with oxygen sensor bungs and
• then intercept the oxygen sensor signal with oxygen sensor simulators

that should work as far as the electronics are concerned? I don't want to throw codes.

I fully intend to take the Jeep to a test centre once this project is complete in order to see if the last catalytic converter under the floor still cleans up the flow enough to be legal.

As part of this project I will be adding a new header, deleting the under hood cats and running a new exhaust and high flow cat under the floor.

Should I stick with the 00-01 header from someone like Banks or use an earlier header? For tubing diameter from the header back should I go 2.5" ID? I'm likely going to add a cam as well a little later.

Still with me?

Thanks!

FYI there is a computer that doesn't have the pre cat O2s. At least for 2000 anyway. As far as your plan to eliminate heat soak. You're on a path, but I don't think you'll eliminate it completely. My 2000 would still heat soak on warm weather, but I never recalled it being bad enough to throw any codes. It didn't have the pre cats.

EDIT: As far as the header unless you change the head I'm pretty sure you'll have to use a 00/01 header. IIRC there are differences in the exhaust ports between the 0113 and the earlier style heads.

scoobyxj said:

FYI there is a computer that doesn't have the pre cat O2s. At least for 2000 anyway. As far as your plan to eliminate heat soak. You're on a path, but I don't think you'll eliminate it completely. My 2000 would still heat soak on warm weather, but I never recalled it being bad enough to throw any codes. It didn't have the pre cats.

EDIT: As far as the header unless you change the head I'm pretty sure you'll have to use a 00/01 header. IIRC there are differences in the exhaust ports between the 0113 and the earlier style heads.

Click to expand...

Yeah, the weird thing about the heat soak issue is that it started happening suddenly in 2006 or so (I owned the Jeep starting in 2003). Nothing changed during that time. At first I thought it was an injector or ignition problem and replaced the required parts. Problem persisted. Maybe it's bum cats... not likely but you never know.

I don't want to swap the PCM if I can avoid it. I'm not 100% certain but I think all Canadian XJs of this vintage got the Cali emissions package so getting a used PCM locally is not possible.

My thing is I can deal with the hot start hesitation thing if I don't have to pull the OBD scanner out of the glove box and reset the codes. I'm lazy that way.

I wasn't sure if the 00-01 heads had a different flange pattern than 97-99. Anyone know for certain?

2000-2001 have a different head than the previous years, they have smaller exhaust ports and the older header will not work.

Search, this is similar to the earlier head swap on the 00-01s.

1) use an earlier manifold/ header and have the precat O2 bungs welded in, but the manifold has to be a tri Y style so the bungs can be welded so the front O2 sensor monitors the front 3 cylinders and the rear O2 sensor monitors the read 3 cylinders. Then you would have to cut your stock down pipe off with the cats and have a custom down pipe bent up. Once that is done you need to do something about the after cat sensors. Sim them out in some fashion or put them after the main cat. More of a performance mod and a bit more involved than you need.

2) Probably best in your case is to just have a new Y/down pipe welded up. The common misconception is that the precats are part of the manifold, they are not, but part of the down pipe. Replace down stream O2 sensors with sims.

3) You can swap the PCM to a Fed emissions, but you would have to do something about the wiring harness as it is a bit different for the O2 sensors.

4) Ditch the 0331 head and go with #1. Kill two birds with one stone. Unless you have already replaced it with a reinforced or TUPY head.

FWIW removing the precats is illegal.

Some info in this thread: http://www.naxja.org/forum/showthread.php?t=1072688

jeepboy381 said:

2000-2001 have a different head than the previous years, they have smaller exhaust ports and the older header will not work.

Click to expand...

You have that backwards. 00-01 stock manifolds won't work on earlier heads. An earlier header will cover the later head ports fine.

What i do: I kick the e-fan on for 30 or so seconds before I turn off the jeep. Then once it's off, I let the e-fan run for 30 or so seconds.

It helps that I have the hood shimmed (in the summer).