How important is the catalytic converter for gas mileage?

Peppergat

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The 2006 I have with the 4.0 I recently noticed has the converter rodded out. I’m assuming that the 2 others are still intact inside. The question that I have is, how important is the converter in the fuel mileage game? I have no codes and, no issues other than a kind of cough thing that the engine does occasionally. I’m thinking that it may have the incorrect spark plugs installed. I’ve already purchased the right ones.
This thing just gets ridiculously bad mileage. Like really bad. I don’t really want to spend money on a new system with all new converters but, I will if it’s all for the greater good.
 
The one cat rodded out is not hurting the gas mileage- its going to be something else-

Has the mileage gotten worse? I see you have some mods so a number of inputs that are working against you. Change those plugs let us know how that goes? Basically start with the basics- proper tune up.
 
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The two small upper cats each have an oxygen sensor before and after, which is what the computer uses to control fuel, timing, etc. Without those, it would run bad, if at all. The lower main cat does not have any sensors, so it gives no info to the computer.

Define "really bad" mileage. Compared to other TJs, or compared to most other cars of that age? I thought all TJs got really bad mileage. :p
 
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Not sure what you mean by rodded out. If they aren't plugged, and you aren't getting efficiency DTC's, keep trucking. If there's a hole in them, change them. This will give false lean readings to O2's.

Whatever your issue, I'd be willing to bet that long term fuel trims are high. Start with new O2's per this forum. They get lazy in their ability to switch from lean to rich like they're supposed to and the PCM struggles to command the correct injector pulse. If it doesn't fix it, they're a maintenance item anyways so it's not a waste of money.

Without a scan tool and live data, you're just guessing. But there's my best guess.
 
The one cat rodded out is not hurting the gas mileage- its going to be something else-

Has the mileage gotten worse? I see you have some mods so a number of inputs that are working against you. Change those plugs let us know how that goes? Basically start with the basics- proper tune up.

I didn’t know if the cat being rodded out affected back pressure so the 02 sensors read inaccurately.
I don’t know that it’s any worse because I haven’t driven it enough to really get a mpg figure. I have 140 miles on my new gear break in and it’s already below 3/4 tank. Now I am used to the mileage my Subaru gets so, it’s a bummer that the Tj is so terrible. I am going to go ahead and do the plugs and make sure everything else is in order.
 
The two small upper cats each have an oxygen sensor before and after, which is what the computer uses to control fuel, timing, etc. Without those, it would run bad, if at all. The lower main cat does not have any sensors, so it gives no info to the computer.

Define "really bad" mileage. Compared to other TJs, or compared to most other cars of that age? I thought all TJs got really bad mileage. :p

Lol , it’s bad to me. I’m getting ready to fill up so I can continue my new gear break in so I’ll see what I’ve gotten with this tank.
 
Not sure what you mean by rodded out. If they aren't plugged, and you aren't getting efficiency DTC's, keep trucking. If there's a hole in them, change them. This will give false lean readings to O2's.

Whatever your issue, I'd be willing to bet that long term fuel trims are high. Start with new O2's per this forum. They get lazy in their ability to switch from lean to rich like they're supposed to and the PCM struggles to command the correct injector pulse. If it doesn't fix it, they're a maintenance item anyways so it's not a waste of money.

Without a scan tool and live data, you're just guessing. But there's my best guess.

Someone knocked all of the media from inside the big converter. It’s just hollow. I actually also bought all 4 02 sensors recommended here just in case. I also thought about it today at work that I can use our think tool and connect to see what’s happening.
 
I didn’t know if the cat being rodded out affected back pressure so the 02 sensors read inaccurately.
I don’t know that it’s any worse because I haven’t driven it enough to really get a mpg figure. I have 140 miles on my new gear break in and it’s already below 3/4 tank. Now I am used to the mileage my Subaru gets so, it’s a bummer that the Tj is so terrible. I am going to go ahead and do the plugs and make sure everything else is in order.

I'd expect it to be around 1/4 tank at 140 miles - mines goes around 200-220 on a tank - about 15/16 MPG.
 
Someone knocked all of the media from inside the big converter. It’s just hollow. I actually also bought all 4 02 sensors recommended here just in case. I also thought about it today at work that I can use our think tool and connect to see what’s happening.
The big converter doesn't matter.

I have 140 miles on my new gear break in and it’s already below 3/4 tank.
If you got 140 miles on 1/4 tank of gas, you'd have the world's best gas mileage out of a TJ.
 
I get about 200 per tank but it's not empty at 200. I estimate 12-15mpg.

Is your odometer accurateish after the regear?
 
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I get about 200 per tank but it's not empty at 200. I estimate 12-15mpg.

Is your odometer accurateish after the regear?

I did install the correct Speedo gear according to the chart I found. I need to check it with a gps. I actually hadn’t thought of the fact that it could be off.