Brand new oxygen sensors in '01 - now getting worse gas mileage

Hi all,

First post. I just purchased a 2001 XJ (4 door, 4.0l, 4wd, no mods) with ~145k miles from a friend and have been fixing it up a bit (I am new newbie Jeep owner / total amateur mechanic). Unclogged the heater core, new heater hoses and thermostat, replaced shocks (and the infamous rear upper shock bolts), etc. The first tank of gas I only got 13.2 mpg, so I figured it wouldn't hurt to put some new oxygen sensors in. There is a lot of deferred maintenance on the rig, so I assumed that they were the original sensors and were well past their due date.

Last week I replaced all four oxygen sensors with new NGK sensors. I used the NGK website to identify the parts needed and also cross-referenced the OEM part numbers as I pulled them out one-by-one. I refilled the gas tank just after swapping all sensors and had 15.8 mpg on that tank - if I had known that it would go up so much I probably never would have bothered with the O2 sensors, but alas...

Now I went through nearly a half a tank of gas with the new sensors and got 8.9 mpg - Yikes! I know that the computer needs to reset (battery was disconnected for the sensor install), but would it really be this bad over 50 miles?

Unless this is something I need to wait out, I am left to guess that maybe one of the new sensors I put in was bad (or the wrong sensor, as the NGK sensors from Amazon had the part number on the box, but the part itself didn't have markings in the same format). No obvious fuel leaks. Oil, coolant, transmission fluid levels seem okay. Car seems to be running really well, and no excessive exhaust fumes from what I can tell. Exhaust fumes seems noticeable reduced vs. before, actually. There is no CEL and I have visually verified that all of the sensor connections are in place.

I would really appreciate thoughts on the best approach to troubleshoot. I have read some forum posts on testing sensors, testing connectors, cleaning connectors, etc but I would like to prioritize thoughtfully since I have 4 sensors to work through and am new to all this stuff. I don't have any fancy equipment (e.g., oscilloscope), but I would be willing to pick up a multimeter if that will help me. I figure that the last resort, tail-between-the-legs options would be to put the old sensors back in, but I'd rather try to identify the real issue.

This is an awesome forum - and has already helped me with some of the projects to date. Thanks!

KC

First of all, howdy! Second, unless you're getting a CEL of P0420, or something similar, there's no need to change the 'downstream' (of the cat) O2 sensors, as their only job is to monitor the effectiveness of the cat, and nothing else. The front ones, closest to the engine, are the ones that control the fuel mixture, and therefore the MPGs. It's good to disconnect the battery when changing sensors, so as to eliminate the possibility of damaging the computer, and to assist the computer in re-learning new sensor parameters. That being said, the computer will slowly recognize the new sensors, over the course of a few tanks of gas. There are also instructions of how to 're-boot' the computer, somewhere here deep within the bowels of . The O2 sensors do have a limited lifespan, somewhere around 80-100k miles, so you did do some good replacing at least the front ones.

Give it some time, a few tanks, maybe a month, see what you come up with.

Thanks, heyhar. I'll give her some more time and see how things shake out! Those bottom O2 sensors were a lot harder to get at than the top ones, too bad I didn't know better before replacing all 4. At least I am now wiser for it, with bruised knuckles to prove it.

1. Be sure you are doing the math on your gas mileage numbers. Don't use a trip computer or extrapolate by how many miles you get to a tank of gas. Miles driven divided by gallons consumed. Get at LEAST three samples before coming to any conclusions.

2. Do NOT put the old oxygen sensors back in! The new NTK sensors are exactly the right oxygen sensors to install. If you do have a mileage issue, it is not being caused by your new oxygen sensors.

3. Get some new spark plugs in there for a "new-to-you" vehicle. For an 01, I recommend NGK copper spark plugs and be sure they are gapped to .035 before installing.

Thanks, birchlake.

For #1, yes: I am calculating as miles driven by gallons consumed. I made the 8.9 gallon estimate on only 50 miles of driving, so very good point that I need some more data points before considering the next step.

I also like the idea of going with new plugs. Seems like an easy, cheap job. Thanks for the recommendation on the NGK copper.

I would reset your PCM. If you haven't, it's trying to translate your old air settings from your new parts. Free, easy and a good place to start. http://naxja.org/forum/showpost.php?p=244834826&postcount=8

A little correction for heyhar. Chrysler does use the post-cat O2 sensors for fuel management. You can call it the 'fine tune' adjustments.

Plata, I would be looking for problems with the MAP sensor. It is mounted on the throttle body and uses a little rubber elbow to connect to the vacuum source, and this elbow can harden and crack.
Also check for other sources of vacuum leaks, like the intake gasket and fuel injector o-rings. Exhaust leaks from a bad gasket or cracked manifold will cause poor fuel mileage.

just tossing ideas at ya.

Digger-I was under the impression that the downstream sensor merely reported the effectiveness of the cat. Thanks for the update. Does it contribute much, or is most of the work done by the upstream sensor?

I'm chasing a P0420 code in my daughter's '05 Subie Legacy. Just reset it today for the second time this week. We've had this same code pop up on our two '96 XJs, both with rattling/broken cats. One reset months ago, on each one, seems to have done the trick, both running fine and getting expected MPGs. Any thoughts would be welcome.

The PCM does use the upstream for the majority of fuel metering, and have the most effect when changing out poorly functioning ones. The primary job of the post cat O2 is catalyst monitoring. I just wanted to point out that they do have a secondary function in a Chrysler system as well.

Sounds to me that the cat needs replaced on the subie. the newer model year, the more aggressive the cat monitor algorithm.

Digger87xj said:

Sounds to me that the cat needs replaced on the subie. the newer model year, the more aggressive the cat monitor algorithm.

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Yeah, my online reading all points that way. The problem is that contributors on the various threads I've looked at have reported cat replacements of around a thousand bucks! If it were my car, I'd be under it right now. But as it's my daughter's, I'll try to shop around and see if there are any reasonably priced alternatives (that warrant their work) to me and a SawzAll. Also, dealers and more sophisticated independent repair shops have access to better diagnostic tools than my Actron auto scanner...

Quick update: I went for a longer drive a few days ago and noticed the gas gauge dropping at a much slower rate now, so I expect to see some improvement in mpg. Will give it a full tank before I refill. However, I had CEL come on for the first time. Nothing seemed different, but I just ordered a cheapo scanner so I can read the code (silly California, auto parts stores won't lend scanners anymore because of lawsuits).

I just got around to resetting the PCM today, and I also ordered some new spark plugs. I will report back in a few days once I can read the code, install the plugs, and see how the PCM reset affects things.

heyhar said:

Yeah, my online reading all points that way. The problem is that contributors on the various threads I've looked at have reported cat replacements of around a thousand bucks! If it were my car, I'd be under it right now. But as it's my daughter's, I'll try to shop around and see if there are any reasonably priced alternatives (that warrant their work) to me and a SawzAll. Also, dealers and more sophisticated independent repair shops have access to better diagnostic tools than my Actron auto scanner...

Click to expand...

If you have a laptop or smart phone you might look at some of the $20 OBD-II links with free software. They are as good as the $100 scanners, and if you buy the good software (several downloads to choose from) for about $50-$100 you can get and log live data to the laptop or smart phone.

When you get your scanner post up your fuel trims, STFT and LTFT percentages. I wouldn't keep resetting the computer, probably the fuel trim is at its limits and throwing a code.