Take them off if you want the Jeep as a trail rig only. If not I would repair them. If bypassed in some shape or form you will have codes in the ECM. Most states will not pass a vehicle with modded emissions.

Here look at number 2:
http://www.mainelegislature.org/legis/statutes/29-A/title29-Asec1756.html

 

sounds like more trouble than its worth. if you decide to go with headers, do it then. I don't think the pulling the cat will give you any thing noticable as far as power goes. just my .02

 

Another thread about removing the cats. :brickwall

 

Dont. It will run like crap, smell pretty bad and have potentially noxious fumes when on low speed trails, obdII will hate you and throw codes, and it can cause even worse gas mileage.

Just removing the mini cats is way different than completely removing the cats. It won't run like crap and smell bad because you still have the main cat, and as long as you don't remove the o2 sensors it won't throw codes. Then again I don't know how the engine reads the data, I know I ran without a downstream o2 sensor for many many years and it ran fine. Threw a code only because the sensor was completely gone. Eventually got around to drilling a new hole for it, did notice a slight MPG increase, but thats it.

Here in Colorado they do a sniff test, but it would be impossible to fail by not having the mini cats, even if you brought your Jeep in cold because if you fail the first time they do it again as a "second chance" run, and by then everything is warmed up and its all the main cat.

It's not like it's any different then my Jeep, which only has a the one cat.

 

Dont. It will run like crap, smell pretty bad and have potentially noxious fumes when on low speed trails, obdII will hate you and throw codes, and it can cause even worse gas mileage.

 

Now I do not suggest this, Im not a jeep mechanic only a VW mechanic so I dont know which one would work.

Find the value and how the ECM reads the O2 sensors after the cat. Use one of the two:
A) A resistor that works but doesnt do voltage drop
B) A rectifier diode that drops voltage so that its in spec of the ECM readings.

Figure out which one you need and you MAY not get codes there for no full inspection.


I have seen this on many VWs, Audis people using the rectifier diodes its really hit or miss. I have seen the resistors on Hondas also.

Oh and if this is against the rules...just delete my post....

 

IIRC the cats in the manifold do more than the auxilary cat in the newer style tjs. There are pre and post o2 sensors in each bank of the 2piece header. It will run bad, trust me been there done that on an 01 XJ bought without cats.

 

some guys put spark plug non fouler to cheat on the downstream O2 sensors thus avoiding codes to be thrown ...:nono:
DON'T do that!

 

There's no benefit to removing the cats. There's also no benefit to tricking the computer. In fact, both of those things can cause lots of problems. Why waste your time and money on a mod that doesn't give you a single shred of performance, and also happens to be illegal?

 

I would also like to point out I in no way ever said the OP to remove the cats, I was merely thinking about what might happen in you possibly remove them based on personal experience. But if you guys who actually have the mini cats say it will run bad, I believe you.

 

bad idea on multiple levels, don't do it. if the cats are bad, replace them with OEM equivalent parts. if they're not bad, then don't change them.

 

Lots of misinformed individuals in this thread. The rear 02 sensors have no effect on the A/F ratio or engine performance and any of you that are claiming such must be repeating what you heard someone else say.

 

Check your FSM:

Downstream Sensor (Non-California Emissions):
The downstream heated oxygen sensor (1/2
sensor) is located near the outlet end of the catalytic
convertor. The downstream sensor is also used to
determine the correct air fuel ratio. As the oxygen
content changes at the downstream the PCM calculates
how much air fuel ratio change is required. The
PCM then looks at the upstream oxygen sensor voltage
and changes fuel delivery until the upstream
sensor voltage changes enough to correct the downstream
sensor voltage (oxygen content).

The downstream oxygen sensor also provides an
input to determine catalyst efficiency.

Thread:
http://www.jeepforum.com/forum/f9/did-exaust-mod-now-poor-mpg-help-897940/index2.html#post8262052

 

Lots of misinformed individuals in this thread. The rear 02 sensors have no effect on the A/F ratio or engine performance and any of you that are claiming such must be repeating what you heard someone else say.

Check your FSM:

:laugh:

But for real my dads 01 XJ would barely run at lower rpms when he bought it without the oem manifold with the mini-cats. Assuming the XJ responds the same, removing them is a bad idea IMO. On a side note, when buying the XJ we talked the guy down almost a grand telling him it needed engine work :2thumbsup:

 

Does the OBDII system work differently from one vehicle to the next as far as 02 function?

 

I would be curious about this to, because it seems like the new TJs are a lot pickier about the sensor, whereas I've explained my experiences, in that not having the downstream sensor on my '97 didn't make a noticeable difference.

 

ok, quick question i have and 05 LJ and the o2 sensors codes come and go with mine, now i plan on getting them changed out but what isthe harm of running them with the bad codes besides worse milage and power. like i said the codes come and go but never notice difference in power etc.

 

It'll kill your cat a bit faster.

 

For the OP's main concern - exhaust backpressure:

I'ld really like to see some comparitive testing done. What's the exhaust pressure before the front cat? What's the pressure level before the rear/after the front cat? After the rear cat, before the muffler?

I would theorize that removal of the secondary cat might reduce a significant amount of back pressure without contending with the O2 functions of the front cats.

 

Why does everyone think cats restrict exhaust and add in a ton of backpressure? You have exotic car makers and even Chevy selling cars out of the dealership in excess of 500HP that are fully emissions legal with cats. The amount of misinformation on this thread would be amusing if it werent so damn sad.

 

ive heard of people just tucking there number 2 o2 sensor up and out of the way after the cats removed!

think about it ... its seeing more oxygen than number 1 no codes...

and i agree with both cat arguemants ... modern cats do not cause a substantial restriction to flow, the old pellet style cats are like a potatoe in the tail pipe....

as far as iam concerned if your cats working properly dont remove it because it will not adversly affect performance.. if youve got a restricted cat or a cat that just istn heating up any more take it out its not doing any good and if you have no emmissions testing why bother replacing it unless you do notice a drop in performance without it

my camaro came with 4 cats a resonator and a muffler with 2 pipes out the back to simulate the look of a dual exhaust.... guess how much better it is now with full length headers 2.25" pipe magnaflow mufflers and a nice mandrel bend up over the axel .........