2000 APN header swap – O2 questions

This weekend I plan on replacing my 0331 head with a reman’d 630 head and an APN header. This a very common and effective swap, but now after I have everything bought and setting in the garage, I’m having cold feet.

Last night as I was spraying everything down with PB Blaster in preparation for this weekend, I was looking for the O2 sensor on the exhaust knowing I would have to move it after the header is installed. I have a 2000 Limited, and as bad luck would have it, my Cherokee has California emissions on it (I live in Illinois). This setup has the downpipe with two smaller catalytic converters that “Y” off of the exhaust manifold prior to the main converter (three cats total). This setup also has four O2 sensors. I’ve been researching this head and header swap for a month now, and was confident I was good -to -go. Now I’m not sure what I need to do with FOUR freak’n O2 sensors!. Everything I’ve been reading on the forums talked about welding an O2 bung down on the downpipe for the sensor, but apparently everyone on these boards that are doing these swaps have not done it on the heeps with the Cali emissions.

Please – someone help me with this! What do I do with FOUR O2 sensors??? Do I:
1) Remove the two smaller (pre?) cats and weld FOUR bungs to the new downpipe and put all four sensors there before the main cat?
2) Somehow plumb the two pre-cats into the new system and weld the bungs before and after them as they currently are?
3) Remove certain sensors. Which ones? I DON”T WANT a check engine light on ALL THE TIME.
4) None of the above. Now what?

I really want to do this swap, but don’t want the CEL on all the time – it’s there for a reason after all.

(For those that are unfamiliar with this swap, the 2000 and 2001 heads (casting # 0331) are prone to cracking, thereby allowing coolant to enter your block. The exhaust manifold also has a history of cracking as well. To take care of these problems, a head from earlier 4.0’s with the casting numbers of 7120 or 630 are used with an aftermarket header that fits that era of head: 1991 – 1999. The 2000+ emissions are not built in with this header, so you typically move the O2 sensor to the downpipe.) I apparently have FOUR to contend with and I’m not sure how to approach this.)

Thanks in advance!

If you bolt a 0630 head on the block made for a 0331, there's no where to mount the ignition coils.
Somehow you have to secure the coil pack to the head without compromising the thing with extra holes.

The 4 sensors and precats have to stay, if the ECM is expected to behave itself. You might find a federal emissions computer, but you still have 4 sensors to deal with.

The swap from a "coil rail" head to a "non-coil rail" head has been done here - you might want to search for it (I don't have the link handy offhand.) It's possible to make brackets that attach to valve cover screws to retain the coil rail - and the writeup I'm thinking of also had pix.

As far as the HEGO sensors, I think you're dealing with a "split header," where the header exits into two banks of three - then each bank has a HEGO sensors and a pre-cat (Bank 1 and Bank 2.) If you don't want a CEL/MIL, the best way to fix that is to get a header with two exits - and I'm not sure APN makes one (I should probably check - they may be able to make one for you, they might have measurements, but don't keep one on the shelf.) I'm also not sure about pre-cat function offhand - have you also a "main" catalytic converter with a HEGO sensor in it? Or are the "downstream" HEGO sensors after the pre-cats?

You can probably mount two HEGO sensors in the header collector or the downpipe, which should solve your upstream HEGO problem - but if you get a "rich" or "lean" condition, it will be more difficult to isolate (since it will trip both HEGO sensors at the same time.)

And 92Drip is correct - even the Federal computers in that era expect four sensors if it's going to behave - it's an OBD-II thing. The two upstream sensors monitor fuel mix and combustion, the two downstream sensors monitor catalyst function.

Anyhow, pillage around for the 0630 conversion - I think using the search term "coil rail brackets" may prove useful, but I'm not sure...

for reference this useful little chart shows the various PN#'s over the model years. Not sure if it's gospel though.

http://remanufactured.com/AMC_Jeep_Engines.htm

0331 castings continued until 04, why not buy a late model reman 0331 head?
I'd throw a new water pump and radiator on a reman 0331, and keep the OEM emissions junk intact.

92DripCherokee said:

If you bolt a 0630 head on the block made for a 0331, there's no where to mount the ignition coils.

Click to expand...

See post 19 in this link, uber easy. http://www.naxja.org/forum/showthread.php?t=98206&page=2&highlight=aluminum

rstarch345 said:

See post 19 in this link, uber easy. http://www.naxja.org/forum/showthread.php?t=98206&page=2&highlight=aluminum

Click to expand...

Somehow, I thought that might have been you (after that PM I'd just gotten.) If you don't mind, could you do a more detailed writeup - I'll either submit it for the FAQ/Tech section here, or put it up on my site in the Tech section - I'm sure other people would like to know what you did and how you did it... Since all my rigs are RENIX, I don't have the means to hand to duplicate your work myself...

Quoting 5-90

"As far as the HEGO sensors, I think you're dealing with a "split header," where the header exits into two banks of three - then each bank has a HEGO sensors and a pre-cat (Bank 1 and Bank 2.) "

Exactly.

"I'm also not sure about pre-cat function offhand - have you also a "main" catalytic converter with a HEGO sensor in it? Or are the "downstream" HEGO sensors after the pre-cats?"

Downstream... after the pre-cats. (no sensors in the main cat).

"You can probably mount two HEGO sensors in the header collector or the downpipe, which should solve your upstream HEGO problem - but if you get a "rich" or "lean" condition, it will be more difficult to isolate (since it will trip both HEGO sensors at the same time.)"

OK -- for welding in the bungs, do they make (do I need) SS ones for the SS APN header, or can regular metal ones be welded in (on SS).

"And 92Drip is correct - even the Federal computers in that era expect four sensors if it's going to behave - it's an OBD-II thing. The two upstream sensors monitor fuel mix and combustion, the two downstream sensors monitor catalyst function."

OK -- you are telling me to somehow plumb in the two pre-cats: weld two bungs on the header (upstream), then two bungs after the pre-cats on the downpipe (downstream)?

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I know it may not help you but I have a early production 2000 XJ. It has the 0331 head and split exhaust manifold, but no pre-cats! Basicaly the exhaust manifold is two units cast as one. I then have two pipes that converge to one near the bottom of the bell housing. This Y-pipe ends in a collector bolted to the crossover pipe leading to a older looking XJ exhaust system. I have one large cat and two O2 sensors, again much like older systems. I have seen other 2000's that had the two pre-cats so mine might be special.:clap:

Angus

The threads in the bung are standard, and I think most of them are CRES anyhow - so it should be no trouble getting a competent exhaust shop to weld them in for you (they'll probably TIG them in anyhow, and should have a drawerful for custom jobs, since they only cost a few bucks to begin with.) So, a competent exhaust shop can probably get you fixed up in a half-hour or so from your walking in the door, if they're not too busy.

Since I'm not certain how the "pre-cat/catalytic converter" setup works, I can't really give you any technical advice on it. If you didn't have the precats, I could probably tell you offhand where to weld in what, and you'd be solid with that info - but I don't like to guess, because I don't like what happens if I'm wrong. So, I'd have to do some more digging before I can answer that (I'd like to see if it can be consolidated into a single converter, for instance - but then it's both a technical and a legal question - because your Vehicle Code would also need to be referenced. I don't recall where you are - didn't look before typing this. You may be somewhere that won't allow that sort of modification.)

hey thanks for your continued help!

On the way home tonight, I too was thinking of using just one pre-cat (I live with no emissions testing). So, it sounds like we're thinking along the same line: adding two bungs on the APN header for the two "upstream" O2 sensors, then custom bending a downpipe with only one pre-cat (eliminating the other pre-cat), then putting the other two bungs after the pre-cat (well before the main cat).

thanks again, man! :yelclap:

Times like this, I really miss living in Indiana...