Burgy

You probably have a exhaust system like I have in my 2001. There are 4 o2 sensors. Two upstream and two downstream. The cat under the vehicle before the muffler isn't the only cat, there are two small ones immediate after the Exhaust Manifold. One is for cylinders 1-3 and the other for 4 -6. Thus there is an O2 sensor before each small cat, and one after the cat. Also, all the O2 sensors are NOT the same. Each has a separate part number. I have them written down somewhere but can't seem to find them. If you call O'Riely Auto Parts or your local Jeep dealer parts department they should be able to give you the separate part numbers. As far as which ones to replace first, I would say your 2 upstream sensors before the cats, these would be the dirtiest as they are receiving exhaust fresh out of the cylinder. Hope this helps.

Please update us if your mileage improves after changing these sensors out.

askingxforxit

Awesome thanks for the info. I'll look in to getting those first two replaced and let ya know

I see the two that are before those two small cats. One (closest to the front) has longer wires than the other one. Can I use the same sensor in both ports? Or are they different? The sensor I bought from napa has a longer wire set.

Also can I replace one at a time or is it better to replace two at a time?

Burgy

They are not interchangeable, there should be a specific o2 sensor for the front and one for the rear. I would change both at once, and I believe it will take a bit of time before the ecu adjusts to the new sensors. Can't wait to hear if you see an improvement in your MPGs.

askingxforxit

Oh ok cool, I'll have to go back to napa and get the second one. Right now I'm getting around 16-17.5 mpg mostly highway driving.

askingxforxit

Got both parts from napa and installing this morning. I'll monitoring the next 5 fill ups to see if there is any changer in fuel mileage as I'm sure the o2 sensors in now are original with 145,000 miles on them.


EDIT: Wow that was probably the easiest thing I've ever done lol took all of 10 minutes with nothing more than a Crescent wrench. As stated above I'll be monitoring my mileage for the next 5 or more fill ups to see any gains in fuel mileage. I assume the original O2 sensors were affecting a little but that won't be conclusive for another few weeks.

askingxforxit

Almost through my first fill up after replacing the upstream o2 sensors.

Haven't done any calculations but doesn't seem to be any increase in mileage

Burgy

Thanks for the update. Hopefully once the ECU adjusts you will start getting better mileage.

askingxforxit

Yep. I'm hoping to gain 1-3 mpg. This is what it looks like at this moment

askingxforxit

I'm now sitting in traffic..ugh...

askingxforxit

Ok so the gas light came on at exactly 330 miles. Typically it used to come on at around 315. I just filled up again right after the gas light came on and it put 15 gallons in.. Had I gone 330 miles on 15 gallons??? We'll see when this tank reaches gas light cuz I don't feel good about running it completely empty.

413maxwedge

To help some of you guys out, they sell books on your year xj's that will tell you alot about your jeep's maybe you could pick one up. The rest of us have them and also read them. GL, some of you seem to need it more than others.

Burgy

Wow, that's 22mpg! Big improvement over over your 17.5. Typically my light comes on around the 15 gallon mark as well. I think it's pretty standard for Cherokee's to have a 5 gallon reserve. What size tires are you running, what's your Gear Ratio, and what kind of fuel do you run?

Burgy

Are you referring to a Haynes or Chilton Manual? If so I've had one of those for years, and it's absolutely no help on this issue. The exhaust systems changed for California models in 2000 and in all models for 2001. If we could get the information we needed form those books this forum wouldn't exist.

askingxforxit

I think on that last full up I put more than 20 gallons.

I'm running 235/75R15 with stock gearing (3.55 I assume) and I usually go to shell for my gas 87 octane

Burgy

Any updates on the mileage?