I have a 2001 with four o2 sensors, 1 for 1-3, 1 for 4-6 and 2 for the mini-cats. The mileage seems reasonably (normally) bad, 14.6 mpg. It is has 130k, the four speed auto and is throwing no codes. Synthetic oil in the engine, tranny and diffs. Cone filter with heat shield and spacer. 235/75r tires, no lift.

Can O2 sensors degrade without throwing codes? I keep thinking I should be getting better mileage than this. It's recommended that the sensors get replaced at 100k

Just curious.

 

I'd replace the upstream (before the cats) sensors, if any.. as far as I know, they are the only ones that affect mpg. The downstream ones only show if the cats aren't up to snuff

 

Yes the sensors can degrade. They get dirty and age and "slow". With a scan tool you can watch them work. Mine were updating slow. I replaced my 2 upstream sensors and got a nice jump in mpg. I also cleaned the tb and intake. You might also want to research running synthetic in your aw4. Its not needed and can actually be harmful. Its too thin and slippery.

 

I think I'll replace the O2 sensors and see what happens. No problems with the synthetic oils, besides the MPG deal, it runs great.

 

Absolutely oxygen sensors can get "lazy" long before they trip a code. And that can lead to a drop in gas mileage.

Oxygen sensors with 130k don't owe you anything. The average lifespan of oxygen sensors is between 100k and 150k, depending on the efficiency of the engine.

If you replace, try to find NTK. They are excellent oxygen sensors and NTK is the original equipment supplier to Jeep.

 

Yea the sensors do get bogged up with carbon. You can use the IAC as an example. Take if off and you'll see how that dirty just imagine after the engine. Also change the MAP sensor also.

 

Yes, O2 sensors slowly give up the ghost over time and often with out throwing codes.
As they get bad, the mixture gets richer and richer, often burning out the cat as it gets hotter and hotter.

I had a Ford T-bird see a fuel mileage jump from 20-22mpg(imp) to 28-32mpg(imp) with only O2 sensor replacement. Paid for themselves in a few months.

Steve

 

FYI Amazon has the o2 sensors for the best price I found plus free shipping.

 

I also like rockauto cause they have OEM sensors but beware not every place has the upstream front of cat sensors read the description always

 

Yea I got mine from rockauto for 30$ best place to buy parts from.

 

I got denso upstream sensors off of amazon for $30ish each with shipping. They seemed to have good reviews. I ordered a sensor plug wrench also. They could come today.

The only cat sensors I saw were bosch and I ordered them, but now I'm thinking about returning them because the check engine light isn't coming on.

Map sensor seems slightly pricey at $75. Maybe I'll see how the new upstream sensors do then work my way out to the MAP if I'm not happy. Anybody ever try to clean the Map sensor? http://www.cumminsforum.com/forum/6-...or-w-pics.html

 

I just changed my map with a denso and a CEL won't come on for a while with an o2 sensor unless something is really wrong with with but a dirty one won't pop a
Light fast

 

synthetic oil in the transmission could absolutely affect your fuel mileage as well. the thinner fluids really do increase friction on the clutch packs, also increasing heat.

synthetics in an aw4 have never been recommended. ATF+3 is what they like, and what they last with in my and many other's experiences.

 

But only on the type of fluid ATF-3, but the synthetic fluids take heat much better than any dino type of fluid whether they are thinner or not they will take heat much better.

 

aah, the religious dino vs synthetic wars. I tend to fall to the new tech side, kind of ironic considering that I bought a XJ.

I put in Mobil1 synthetic. The manual recommended Dex II or Mercon, the Mobile1 was Mercon compatible. No issues so far, but I'll keep an eye on it.