Stock XJ on 235's 146,000kms
did an injector flush, tune up, high flow cat and 2 O2 sensors and now getting 25mpg and to think i was going to sell it and buy a civic.

first off, wow, holy ****. second, that is awesome. i have had this really annoying hesitation when driving. my gas mileage has been steadily dropping from like 18, to 15, to 11... i have spent and wasted alot of time and even more money trying to get rid of this gremlin. new TPS, IAC, plugs, wires... etc. but it never threw a CEL, untill just the other day when we were sitting there with a code reader plugged in, looking to see it it had stored any codes. it said that on my last trip over to the shop the downstream o2 sensor reading was not swinging properly, and was in fact quite steady (not good). not but a few seconds later, the CEL pops on. we run it and it says its the downstream o2 sensor again. so wed. both o2's are getting changed. i really hope this solves it. there was a thought that an exhaust gasket was sticking open, so we sea foamed the manifold w/ it running. after that, the jeep deff. ran better and even jumped up to 17.5 mpgs!

Is it credible that replacing the cat is the answer to lousy MPG problems?

Well, if the oxygen sensors are failed to the point that you're getting 8mpg, it probably plugged the cat and had to be replaced as collateral damage.

Sounds like something I need to do. For an injector flush are you talking about just that gas tank additive? and where did you get your high flow cat and o2 sensors?

FYI guys, the downstream O2 does NOT affect short or long term fuel trim. Which means that if it throws a code for downstream, if you don't replace it, it won't hurt fuel mileage. The wire on mine broke off God knows how long ago. Just found out, and haven't even noticed a change in fuel milage.

The not affecting fuel trims isn't speculation either. I go to Wyotech, and using the Snap-On Modis, this is the information I got.

I go to UTI (same Snap-on modis scan tools but tested on a mustang instead) and can say yes downstream o2 sensor will affect gas mileage, not as much as the upstream, but it affects it nonetheless. the downstream sensor measures the effectiveness of the cat and a bad sensor will tell the ECU to compensate for the lack of a cat, thus making fuel/air mix different. Will it be noticeable, probably not, might only be a 1/5 of an MPG, but it will affect it.

What about the fact that just because you get a CEL pointing the finger at the O2 sensor,that don't mean the sensor it self is bad.Isn't it true that basically any poor running condition could cause the O2 sensor to give improper readings?

False... that would be mean o2 sensor is there for no reason at all... that's just silly... I worked at Nissan as a tech but aircraft's are my thing so I left...

True... however I believe they both work equally... there not there to save or hurt gas mileage... they are there to read exhaust gasses going in and out of the cat in order to fine tune the rest of the system and will also let you know if the cats bad or clogged since it won't get a proper reading... however they burn up normally before the cat gets clogged... lol


Only if the running condition clogs up the cat for the o2 sensors to read... and the o2 themselves can allow the cat to clog... just put my buddies 04 wrangler in the shop cause hesbhad o2 sensor codes the last two years... well it lost power and almost just wouldn't drive.... o2 sensors were working at first... cat was clogged... but he didn't get it taken care of so then all the o2 sensors burnt up from the excessive heat... luckily he falls under the 8 year 80k fed emissions warranty... lol

So even if the cat is not clogged and say the vehicle was running rich/lean or even burning a lot of oil,you would not get a code for the O2 sensor?

I'm not a pro mechanic or anything but I have seen a lot of O2 sensor codes the lead to other issues rather than the sensor it self.
I had a 89 s-10 pick up a couple of years ago,gave me O2 code,replaced the O2 sensor and that did not resolve the code.Had a brand new exhaust and all that.The engine was a pos though that had other issues.Of course I didnt bother much with it since the truck it self was pretty much at the end of its life.
I'm not saying the O2 sensor is not the issue in this case though.Just m 2 cent

How many miles did you drive to confirm the much better gas mileage?
Lets us know how the mileage is after a couple hundred miles please.
kilometers/miles.... either ;-p

Hey could you share where you picked up your high flow cat from, and how much? I'm looking for one, I have a 99 and I live in Canada so you might have the same stores that I have. I have found a couple but if you had such success with yours on a Jeep similar to mine I'd rather get the exact same one if I know it works lol. Thanks

magnaflow direct-fit

Let us know mileage after a couple tank fulls

That's a Mustang. I tested my XJ, and my fuel trims were not any different from stock.

The downstream O2 tests NoX, and other emissions. Your upstream is the O2 that adjusts fuel trims.

UTI is a joke and a half. My buddy goes to UTI Chicago, and he says that it's 90% crowding around an instructor trying to learn, 10% tests.