O2 bypass question

99ncxj

99ncxj

Well-Known Member
Joined
Sep 23, 2006
Location
Aliso Viejo, CA
I've searched all over the web and can't find what i'm looking for.

I'm building my exhaust in the jeep and will be eliminating the cat. It will be just 2.25" pipe all the way from the header to the muffler. I don't mind drilling holes and welding in some bungs for the (2) O2 sensors but I need to find a way to "trick" the computer into thinking that there is a cat in there and its working right. does anyone know the resistance values that the down stream sensor needs to read to keep from throwing a code and having the check engine light on? I'm sure plenty of people have used these newer 4.0L's in buggys and didn't run a cat, but I can't seem to run any of them down to ask.

Thanks.
 
Actually it changes. That's how the computer 'sees' that it's working..

First if the computer doesn't see any reading or an out of range/unexpected reading it throws the O2 code.
The computer verifies the up stream pre-cat O2 by purposely creating a very slight rich/lean oscillation. Creating a condition where it should see an expected pattern.
It verifies the down stream by doing a similar rich/lean swing but for a longer and higher period, more than the cat would be expected to scrub out. IF it doesn't see the expected swing/reading on the down steam it usually throws the CAT code. OR it can be an old/lazy O2 sensor that can't keep pace.

I went through this with my car recently. Code was CAT but I wasn't replacing 30k cat(s) so I changed the 170k O2s.. Problem solved...

In a buggy they are probably having the PCM reworked to ignore/skip the CAT/O2 tests. Or in many cases ignoring it.
 
I think it's funny how every one wants to get rid of the cat and goes to the ends of the earth to find a way to "trick the computer" etc. Get yourself a cheap cat at vato zone and be done with it. They really DON'T rob all the horsepower everyone thinks they do. It's probably cheaper in the long run and a helluva lot easier. (And your Junk doesn't stink to high heavens)
 
I'm not removing the cat to help with performance, i've removed it and reworked my exhaust down the drivers side since its now a pass side drop rig. i just don't have very much room between my skid plate and the body for a cat/muffler combo. I just built the exhaust this morning with just 2.25" pipe and a flow master muffler. I welded in a bung for the upstream sensor but havn't decided what to do with the down stream yet.
 
A guy on another forum has an 01 Crewcab S10 and used 2 anti-fouler's together and did the test pipe on the 4.3. Said its been about 10k and it is still reading fine. Ill be doing it to my 96 s10 when i get my exhaust reworked. It just sounds so much better with the cat gone/gutted.
 
I have a 99 xj with 4.0

no cat here just precat. light stays off once in a blue moon i need to reset but thats about every 3 months with dd
 
"precat"???

I don't have anything like that on my 99. Just headers, upstream sensor, cat and down stream sensor.
 
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