Hi guys,
I own a 1998 Jeep Cherokee XJ with a 4.0 with 300000 KMs on it. Ever since I’ve bought it, the Catalytic Covertor has rattled at idle and when I’m taking off. This has led me to believe its bad. Right now, I am weighing my options. Because I’m 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 don’t 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!

Sure, I understand that much but thanks for your help! I’m 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)

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?

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?

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 (it’s 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.