Recently noticed a big drop in mileage, which surprised me because it coincided with finally regearing to 4.11’s after running through stock gears with 32” BFG M/Ts. I took those tires off, swapped them for Falken Wildpeak’s in 265’s and did the gears expecting to see something of a bump in better mileage.

I have 150k+ miles on this thing and am owner number who knows how many, but I can say I myself have never replaced the O2 sensors. After extensive reading on these forums, it seems like this is a likely culprit? I already plan on replacing the header, so if it’s cracked that will be dealt with.

I guess my question is this: the CA emissions setup has three O2 sensors, correct? Two upstream, one down stream? I’m trying to find which sensors I need to order. Seems like the NTK’s on RockAuto are what people here recommend?

 

2 upstream and 2 downstream.
The 2 upstream are in your 2 piece exhaust manifold set up.
Assuming you still have it in there.
The 2 downstream are below each cat in the dual front cat.
Guessing here but thinking downstream does not effect gas mileage.

 

Gotcha! Ok, yeah I figured the downstream wouldn’t impact it. Worth it to just replace all four or should I just do the two up streams? Even if they aren’t bad I suppose at this point it’s inevitable and just change them anyway?

 

I have read that life of them is about 100K to 150K.
I swapped out all 4 on my '00 XJ with CA Emissions just to do it.
Had about 150K on her at the time. Thinking they were original.
They get lazy as people will say but still not throw a code.
If you have a scanner that will show live data, which I do, maybe you can tell by how they behave.
Voltages and whatnot. Gonna be frank here. I can see the data but can't interpret it worth a damn. LOL.
With the regearing and the tires and the blah blah blah we are even more out of my wheel house.
So I really can't tell you if that is your problem or not.
I do know this. If I was pretty sure that they were original and I was replacing my exhaust manifolds I am putting in new O2 sensors.
Would probably to the downstream too.
But that is just part of my "Well while we are here" way of thinking.
Oh by the way. Nothing but NTK O2 sensors.

 

I would recommend to only replace the 2 upstream O2 sensors with NTK. I replaced all 4 with NTK & had a check engine light come on for inefficient cataltic converters. That was a several hundred dollar fix to replace the cataltic converters.

 

The downstream sensors DO participate in the fuel calculations:

 

There are no coincidences.

 

True, but what would regearing have to do with a mileage drop? I would expect, if anything, better MPG due to taller gears? I deliberately chose 4.11’s based on the chart floating around on these forums since I highway/daily drive a lot. Seemed to be the correct size for better mileage. I’m open to suggestions though, this thing is eating gas like it’s going out of style. It starts up strong, runs smooth and quiet, so I’m at a loss. That’s why I figured maybe the O2’s? Would a hairline header crack cause that much of a mileage drop?

 

Exactly. The early ones, no, but the later ones with waste spark do.


If it's the O2 sensors you'll have a sooty tailpipe. Check it.

 

What chart is that? If you lower the gear ratio, and if the new tires are the same or a little bit smaller in diameter (measure both) then your RPMs will increase for a given MPH and your MPG will decrease.

 

Oh this chart



So it looks like you moved back 3 blocks in MPG (whatever that comes out to) but overall have a balanced setup.

 

That one! I should have explained it better as well: the mileage drop wasn't immediately after the gear change, but it was within a week or so I'd say. Initially I felt like my mileage was near the same, then I noticed I was suddenly getting really poor mileage. I also ran a couple of cans of sea foam through since the Jeep had been sitting for nearly a year without being properly stored. Possible that fouled up the sensors? Regardless, I have a full set of O2 sensors on the way. Hopefully that's it.....