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Unread 07-17-2007, 09:42 PM   #1
radbay
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Running without catalytic converter (1998)?

My CAT needs replacing and I was thinking about removing it and running a 'test pipe' instead. Some vehicles don't run well without it, but I haven't heard anything about the I6 4.0 engines. Who has removed their cat, and did you still use the O2 sensor or remove it? Better gas mileage or worse?

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Unread 07-17-2007, 09:47 PM   #2
am3rican
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Quote:
Originally Posted by radbay
My CAT needs replacing and I was thinking about removing it and running a 'test pipe' instead. Some vehicles don't run well without it, but I haven't heard anything about the I6 4.0 engines. Who has removed their cat, and did you still use the O2 sensor or remove it? Better gas mileage or worse?
With any vehicle, if you remove the O2 sensor and the CAT, your gas mileage will go way down. For instance, I've seen 40-50% reductions in gas mileage. Plus your Jeep will throw codes and you will never pass inspection with a check engine light on
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Unread 07-18-2007, 05:39 AM   #3
Slithering_Joe
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Removing the cat will not affect gas mileage but removing the O2 sensor that is used for fuel control definitely will. There is an O2 sensor before (pre-) and after (post-) the cat. The pre-cat O2 sensor is used for feedback to control the amount of fuel needed. The post-cat O2 sensor is used to monitor the cat. The Powertrain Control Module compares the switching frequency of the pre-cat O2 sensor to the post-cat O2 sensor. If the frequency of the post-cat O2 sensor is below/slower and within preset limits it assumes the cat is working since it is burning off the remaining O2 and the fuel. If the frequency of the O2 sensors are equal then the cat is bad or missing and hasn't changed anything in the exhaust stream. A check engine light will result. It is also illegal to run without cats.
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Unread 07-18-2007, 06:34 AM   #4
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the only motors ive ever heard of running better without a cat are Diesels. and its not like the cat is doing much for a diesel motor anyway. all the guys around here rip them off and put high flow mufflers on. they floor it and let the black soot roll out
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Unread 07-18-2007, 08:47 PM   #5
radbay
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my truck runs much stronger without a cat, but i get about the same gas mileage that I did with it. Power gains could be because my old cat was stopped up, but most people I have talked to say they got more power from it. Does anybody have personal experience from their 4.0L Jeep motor?
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Unread 07-18-2007, 08:51 PM   #6
KKiowaTJ
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i took mine off my 97 sport,only differnce was it was a little louder.just drill a hole in the pipe and weld a nut and put the 02 sensor back in,and you wont have a CEL,but if the cat is bad,thats the only reason i would take mine off again,not hurting anything on there,but if it goes bad im not gonna waste good money for a simple fix as i said above.
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Unread 07-18-2007, 09:00 PM   #7
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thanks, I was just wondering if my Jeep fell into the 10% category of vehicles that don't run well without one. Muffler shop guys told me that late model chevrolet trucks are usually the only ones that have problems. But from what KKiowTJ wrote, I should be ok.....

anybody else have personal experience testimonials good/bad?
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Unread 07-18-2007, 09:41 PM   #8
will228
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i ran with a hollowed out cat for a while. jeep sounded like a tractor with a F40 muffler. pretty badazz but it got old and annoying.
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Unread 07-19-2007, 03:06 AM   #9
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Just to let you know, not only will you fail inspection.. but it is ILLEGAL to run without a cat.
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Unread 07-19-2007, 06:24 AM   #10
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not everyone has inspection, i dont
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Unread 07-19-2007, 08:43 AM   #11
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Quote:
Originally Posted by radbay
thanks, I was just wondering if my Jeep fell into the 10% category of vehicles that don't run well without one. Muffler shop guys told me that late model chevrolet trucks are usually the only ones that have problems. But from what KKiowTJ wrote, I should be ok.....

anybody else have personal experience testimonials good/bad?
Mine was inadvertantly ripped off in Hot Springs at Superlift ORV park. It was loud as hell. Ran like total crap and sounded even worse. Stalled out anytime I tried to crawl an obstacle. Then, my gas mileage dropped to about 9 or so MPG. Of course, the O2 sensor ripped off with the cat. Put the cat back on, got a new O2 sensor, and now I'm back up to 15 MPG and it runs MUCH better.

And for the record, AL doesn't have inspections.
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Unread 07-19-2007, 10:22 AM   #12
Slithering_Joe
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Removing a cat & installing a replacement staight-pipe is completely different compared to having the cat ripped off and running with an open exhaust. Yes, in both situations they are missing but the velocity of the exhaust gas flowing through the tubing helps to expel the exhaust from the cylinders. This velocity also helps the intake side of the cylinder during the intake-exhaust valve overlap. Having the cat ripped off probably didn't give the velocity that helps and that is why it ran like crap. If the pre-cat O2 sensor got ripped off along with the cat then no wonder the car ran like crap!

Those states that don't have inspections doesn't mean you can't get cited. If one gets pulled over and the police want to throw the book at you they can right then and there look under the car. Though, I have never heard of anyone getting cited for not having cats but that doesn't mean it can't happen.
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Unread 07-19-2007, 11:21 AM   #13
highrisetj39.5
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I took my cat off about 1 year ago, still dont have the o2 sensor in, ill be putting that back in soon when i have time. Its alot different in sound, i think alot louder i also have a flowmaster.

http://youtube.com/watch?v=C9e9nMZn5CY

last night also my buddy and I removed his cat on his 3000Gt and replaced it with a piece of pipe talk about a major difference in sound and power!
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Unread 07-19-2007, 11:28 AM   #14
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Your not going to get worse mileage, if anything you will get better. Some states or counties (such as mine) does'nt have emissions testing so if thats the case you have nothing to worry about. I ran a gutted cat on my '92 Dakota for years, never failed inspection(visual or sniffer) and that was when I was living in a county that had wanna be CA type testing. Depending on what the factory set up is on your year, you might not even get a code. If you have an 02 sensor behind the cat and you remove the cat, you will get a code. My '00 XJ has 3 cats (for now), one on each down pipe with an O2 sensor on each side of them and one in the Y-pipe once the two pipes have joined which has no O2 sensors around it. That one will be easily removed. If I can trick the computer (not that hard) I'll probably gut the two in the down tubes and remove the one in the y-pipe. Im not the original owner, it must have been like that when I bought it.
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Unread 07-19-2007, 11:47 AM   #15
tomsv700
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with no cat theres less backpressure so you might lose a little bottom end power but pick it up on top. with the o2 sensor removed though the computer runs open loop, it doesn't know how much fuel to use so it dumps in a constant large amount to keep from running lean and causing damage. thats why it runs like crap with no o2 sensors. now i don't know about for jeeps but they make o2 simulaters to replace after cat o2 sensors for performance engines that tells the computer its constantly running good, that way you can ditch the power robber.
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