O2 sensor replacment

I have a 98 4.0 automatic 31" tires and was considering replacing my O2 sensors. I did a full tune up about 6 months ago and it seam like my mpg has been going downhill lately. It's got about 160k miles. Runs great no cel and no other issues. I know our jeeps get horrible gas milage as it is but if gaining just 1 mpg is all I get then i think it's worth it. Right now I'm getting about 12 in town and 14-15 on the freeway with a tail wind. Any thoughts on this? Maybe just the up stream or both?

My 00 usually got around 16 to 17. I had two upstreams so i put them in and now get 18 to 19.

Auto tranny and 31" tires 40psi and baby the hell out of the throttle as well as throw it in neutral down hills.

The OD kicks in at 38 mph in 3 so if im gonna drive 38 to 43 ill leave it there, over 43 ill put it in D

I would just do the upstream, the downstream just monitors cat efficency. Get one from rock auto, they have the OE ones and the prices are cheap.

I baby the hell out of the throttle too. Think I might try upstream first and see how it goes. Ah dreams of 16+ mpg sounds good

It won't hurt to do the upstream, but I would get used to the fact that XJ's are:

1. Heavy
2. Aerodynamically Challenged
3. Almost ancient.

Add to that bigger than stock tires, what I assume is factory 3.55 gearing, and 160K. I'd call what youre getting pretty good. I'm in exactly the same position as you. Right at 160K, 31's, 3.55's, 4.0 and Automatic. I also have a JCR front bumper adding weight, tons of tools in the back adding weight, and a surco roof rack with my spare on it making it even more aerodynamically challenged. I used to pull 18-19 highway even with the AC on until I started racking up more miles, then I fell to 14-15 highway on a good day and a solid 13-13.5 in the city. If I babied it I saw 13.5, if I opened it up I saw 13....so I started to be a bit more spirited in my city driving.

My solution was I bought an Acura a couple of weeks ago. Solid 30+mpg no matter how hard I drive it, and now the Jeep is no longer a DD which means I can wheel it (read: fix it) more.

Oxygen sensors have a finite lifespan of between 100k and 150k in general, depending on engine efficiency.

If these are the original sensors, by all means, replace them. And replace both. They certainly don't owe you anything at 160k.

I look for NTK, as they are the original equipment supplier to Jeep and you can usually source them cheaper aftermarket than from the dealer, for exactly the same sensor.

Try NAPA or Rock Auto.

Normally I would say rockauto but I just recently got my o2 sensor off amazon for a bit cheaper. I think I saved 10 bucks including shipping for the same part number.

The Bosch ones from any parts house are pretty reliable. Just make sure whatever you do, get an Exact Fit sensor and NOT a universal sensor, or you'll be doing it again.

Also, make sure you run the Jeep to warm up the exhaust before trying to remove the sensor. Other wise you will most likely strip out the bung in the exhaust.

If it's any consolation, I get about 15-16mpg with about 60/40 highway city driving. Same setup as you with the same mileage. That's after replacing my O2 sensor and exhaust manifold.

Right now I'm trying the "cruise at 70mph with the tranny in 3 instead of D" advice. We'll see if it helps at all.

Well after replacing just the upstream I'm getting about 30 more miles per tank so that is a huge improvement. I'm pretty sure the one I pulled out was the original. I replaced it with the Bosch oem. Not only got a little better mpg but also seems to run smoother and throttle response seems to be way better. All in all worth the investment

Rather than an O2 sensor, I would run a can of BG44K through the fuel. It is much more likely to help your mileage by cleaning the injectors and removing carbon.

X2 on the BG44k. I like to do it every few oil changes. I went through another O2 sensor in my '96 (replaced original at about 100k, and again at 160k), but didn't see any gain. Someone suggested testing the Coolant Temp Sensor. The figures are here on somewhere. Mine was dead! I replaced it with one from Advance, and it seemed to be a little better. But after a week or so, back down. Got another, and here a few months later, I'm well into the lower twenties for MPG. It seems the upstream (first) O2 sensor, and the CTS are the two major sensors which allow the engine to attain closed loop operation. I can see and feel the difference, and also hear it, as the engine will lean-ping slightly under heavy load, absent for a few years, as an indication that the ratio is right at the edge. And, I replaced the injectors last November, hoping for better MPGs. Now it seems to be paying off!

old_man said:

Rather than an O2 sensor, I would run a can of BG44K through the fuel. It is much more likely to help your mileage by cleaning the injectors and removing carbon.

Click to expand...

Try this first and see what happens. It's good stuff.

After I did my lift and put my 15x8" Cragar soft 8's on with 33x12.50 KM2s I got around 8mpg. Replaced my upstream O2 sensor and bumped it back up to 16 city 18 highway. I have stock 3.55 gears and always keep it in 3, never put it in overdrive. Somebody mentioned running 40psi...wtf? Don't do that, you'll wear down the middle of your tires like that. I run mine at 30psi just because the Cherokee is NOT a heavy vehicle.