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Hypothetical Cat Question

Old 02-24-2019, 04:52 PM
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Hi guys,
I own a 1998 Jeep Cherokee XJ with a 4.0 with 300000 KMs on it. Ever since Iíve bought it, the Catalytic Covertor has rattled at idle and when Iím taking off. This has led me to believe its bad. Right now, I am weighing my options. Because Iím a broke student, gas mileage is the most important factor to me instead of power. My hypothetical question is what would hollowing out the cat do in terms of gas mileage? This entails cutting out a segment of the cat, taking out all the innards, and welding back on the cut section. This would leave the shell of the cat along with the downstream o2 sensor. I have read that the down stream o2 sensor has no play on fuel trimming on late nineties models like mine, however I would like a definitive answer for my situation. My thinking is that relieving some of the back pressure that the cat is creating will increase the power of the engine making it more efficient and at the very least keeping gas mileage the same. What do you guys think? Also, I know this is illegal and this is all hypothetical. Please donít leave things like, here is a cheap $40 cat that works, I know. Curiosity is the only driving force behind this question. I am more than likely going to order a high flowing cat as cats are federally mandated where I live. Anyways, thanks so much for any constructive answers you guys may have and have a good day!

Last edited by Lweller; 02-24-2019 at 04:55 PM.
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Old 02-24-2019, 05:29 PM
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If your cat went hollow you would start getting a code for a bad cat and in some places you can get a fine if they see that the cat was cut open , I don't know where you live but maybe you can get a good used pipe and cat .
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Old 02-24-2019, 05:30 PM
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Originally Posted by tech View Post
If your cat went hollow you would start getting a code for a bad cat and in some places you can get a fine if they see that the cat was cut open , I don't know where you live but maybe you can get a good used pipe and cat .
Sure, I understand that much but thanks for your help! Iím wondering more on the lines of what would hypothetically happen if I did hollow out my cat given the circumstances (98 Cherokee with 300000 klics)
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Old 02-24-2019, 05:40 PM
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In most cases you fuel mileage and power would would stay the same as when the cat was good , the only way it hurts power and mileage is when the cat plugs up and does not let the exhaust flow .
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Old 02-24-2019, 05:42 PM
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Originally Posted by tech View Post
In most cases you fuel mileage and power would would stay the same as when the cat was good , the only way it hurts power and mileage is when the cat plugs up and does not let the exhaust flow .
Ah thanks! So would the cat rattling be a good indicator for it being bad? Clogged? And would the downstream o2 sensor keep it open loop mode?
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Old 02-24-2019, 05:58 PM
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Ya the cat is going bad and it mite plug up , the down stream o2s are only there to check how good the cats work
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Old 02-24-2019, 06:23 PM
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As getting rid of the cat if it's plugged will help with power and fuel mileage, with no negative effects other than it may trip the check engine light, my 97 doesn't have a cat and I don't get a check engine light from it.
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Old 02-24-2019, 06:25 PM
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Originally Posted by 00t444e View Post
As getting rid of the cat if it's plugged will help with power and fuel mileage, with no negative effects other than it may trip the check engine light, my 97 doesn't have a cat and I don't get a check engine light from it.
Thanks! How did you verify this? Namely mileage. And did you get rid of the whole casing or just the innards? And does your Jeep have one or two o2 sensors?

Last edited by Lweller; 02-24-2019 at 06:28 PM.
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Old 02-24-2019, 09:47 PM
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A o2 spacer eliminated my cat efficiency code after I hollowed my cat. I cut it like a clam shell and took the 2 bricks out. One of them had turned sideways inside the shell and everytime I would go WOT it would cut out and stall the engine.
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Old 02-24-2019, 10:14 PM
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Originally Posted by Lweller View Post


Thanks! How did you verify this? Namely mileage. And did you get rid of the whole casing or just the innards? And does your Jeep have one or two o2 sensors?
It's just a straight pipe with no cat, it was like that when I got it, it has 2 o2 sensors.
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Old 02-24-2019, 11:47 PM
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So I emptied my cat, here are the results:

1. A huge increase in power and torque, so much that I can do burnouts
2. Better fuel economy, especially on the highway
3. My tail pipe emits visible steam (itís sub zero here). This is something every car does but mine never did
4. No cel
5. Wicked sound and it sounds like the motor is somewhat relieved.

Thanks you all for your help! I was really worried but it is definitely noticeable that this helped my car.
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