Only getting about 6 MPG

[Logans99TJ]

No, I didn't get a TJ because of the fantastic mileage...I was around 12mpg after installing 33" tires. Haven't done anything different, just dropped off to about 6mpg. Exhaust manifold had a big crack, replaced it but didn't help the mileage. Thoughts?

 

[Chris]

When was the last time you replaced all your O2 sensors?

 

[astjp2]

reset pcm, pull battery cable and then try to start to drain the electrical system of any power. Reinstall cable and then drive it.

 

[jjvw]

O2 sensors, cats.

 

[Jerry Bransford]

No, I didn't get a TJ because of the fantastic mileage...I was around 12mpg after installing 33" tires. Haven't done anything different, just dropped off to about 6mpg. Exhaust manifold had a big crack, replaced it but didn't help the mileage. Thoughts?

First and only thought at this point is that your upstream O2 sensor has gone t.u. Your '99 only has two O2 sensors and only the upstream O2 sensor can affect mpg. The downstream O2 sensor is mounted on the catalytic converter and its sole job is to monitor the catalytic converter, is has nothing to do with the engine.

If you decide to replace the upstream O2 sensor, make sure to ONLY go with an NTK or NGK (both are the same company). Avoid Bosch O2 sensors in our Jeeps, they don't work properly for long.

This is the one I'd get... https://www.amazon.com/dp/B001PNCKV8/?tag=wranglerorg-20

There would be no mpg or performance benefit gained by replacing the downstream O2 sensor too.

 

[Logans99TJ]

When was the last time you replaced all your O2 sensors?

Haven't ever replaced them...looks like I will be!

 

[Logans99TJ]

First and only thought at this point is that your upstream O2 sensor has gone t.u. Your '99 only has two O2 sensors and only the upstream O2 sensor can affect mpg. The downstream O2 sensor is mounted on the catalytic converter and its sole job is to monitor the catalytic converter, is has nothing to do with the engine.

If you decide to replace the upstream O2 sensor, make sure to ONLY go with an NTK or NGK (both are the same company). Avoid Bosch O2 sensors in our Jeeps, they don't work properly for long.

This is the one I'd get... https://www.amazon.com/dp/B001PNCKV8/?tag=wranglerorg-20

There would be no mpg or performance benefit gained by replacing the downstream O2 sensor too.

On order...thanks!

 

[Jerry Bransford]

When removing the old O2 sensor, it may be seized from age and extremely hard to remove. Get the engine good & hot before trying to remove it might help. I use a standard 7/8" box-end wrench which grips the O2 sensor well enough to be able to put considerable pressure against it. To gain more leverage as is often needed for an O2 sensor, you can slip a second box-end wrench over the open-end of the wrench.

The special O2 sensor sockets with the slot running down the side commonly don't have enough strength to remove a seized O2 sensor, they're more for installing new O2 sensors. The slot is there to pass the wiring through. When removing the old O2 sensor just cut its wiring off first.

 

[Logans99TJ]

So I ordered the O2 sensors and special socket...while waiting for them to arrive I sprayed PB Blaster on the sensor 3 nights. Today I removed the old sensor with ease. I actually thought it would take 1/2 and hour...most people say 15 min...but it took about 10 min. I will have to replace the cat sensor next week (going out of town).
Thanks for the help, I'll keep you posted on the mileage.
Oh here are a couple pics of the new and old sensors. The old one was NTK. (japan)

 

[Stockli]

When removing the old O2 sensor, it may be seized from age and extremely hard to remove. Get the engine good & hot before trying to remove it might help. I use a standard 7/8" box-end wrench which grips the O2 sensor well enough to be able to put considerable pressure against it. To gain more leverage as is often needed for an O2 sensor, you can slip a second box-end wrench over the open-end of the wrench.

View attachment 80168

The special O2 sensor sockets with the slot running down the side commonly don't have enough strength to remove a seized O2 sensor, they're more for installing new O2 sensors. The slot is there to pass the wiring through. When removing the old O2 sensor just cut its wiring off first.

I found kroil and a pipe wrench work really well for this too.

 

[Logans99TJ]

Changed the CAT sensor this afternoon.
I was really dreading this thinking about taking the skip plate off.
I had applied some PB blaster a couple times last week since it worked so well on the upstream o2 sensor. Before tackling the project I thought I'd just crawl under to see what was what...due to the sun beating down on the passenger side of the TJ I went in from the drivers side. To my surprise I was able to easily reach over the drive shaft and get the o2 socket wrench on the sensor...a little pushing on the wrench and BAM, it came out with ease.
I figured this would take a couple hours but ended up taking 10min. Thanks for all the advice.
Here is a pic of the old sensor...don't know what is normal for discoloration.