New O2 sensors = really bad gas mileage

I just replaced both of the O2 sensors on my '96 due to a CEL code for Bank 1 O2 Sensor Lean Condition and Bank 1 O2 Sensor Heating Circuit Open. I was getting about 15 MPG with the old sensors. I've run 2 tanks through the XJ after changing the sensors, and I'm now getting 11 MPG. After the first 11 MPG tank I took the XJ in and had it aligned and had the tires rebalanced, but it didn't make any noticeable difference. I used the Bosch O2 sensors listed on Bosch's website for a '96 XJ, the OEM style, not the "Universal" type. Any ideas? With $3.65 gas this new low mileage is killing me.

Disconnect your battery and let it sit for a half hour or so then reconnect, start and DRIVE IT, don't just let it idle. It will drive like crap for 20 or 30 minutes as the system relearns it's settings, right now it's probably running off the old parameters. The other thing is the new O2 could have been bad out of the box.

other than resetting the computer, is the engine getting up to operating temperture?

what octane gas are you running for 3.65 a gallon

89xj said:

other than resetting the computer, is the engine getting up to operating temperture?

what octane gas are you running for 3.65 a gallon

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Yes, since I rebuilt the cooling system a few months ago, the engine warms up quickly to 195* and pretty much stays there unless I'm sitting idling for a long time.

Here in Simi, it's $3.65 / gal for the "cheapo" low-octane stuff.

Rich is right...... As usual!!!! :cheers:

crystalship1 said:

Rich is right...... As usual!!!! :cheers:

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I will disconnect my battery when I get home from work tonight and see how she does after sitting an hour or so. Rich, I will owe you.... I dunno, something if this fixes it.

If you haven't changed the spark plugs yet, consider doing so...When the old sensors went bad with Lean error codes, the PCM would most likely over enrich your fuel mixture....your plugs may be fouled a bit...I have found that changing my spark plugs on my 01 every 25000 miles approx ( or sooner) gives me my best mileage performance...

It ended up being a bad O2 sensor. I got a replacement and my XJ is back to it's 15-ish MPG. I was hoping for an increase, but I guess that's as good as it's going to get.

Daedalus454 said:

It ended up being a bad O2 sensor. I got a replacement and my XJ is back to it's 15-ish MPG. I was hoping for an increase, but I guess that's as good as it's going to get.

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How did you verify the bad out of the box O2 sensor ? did it throw a code ?

No, I did some research on how O2 sensors work, and figured that since that was the only thing I changed, it made sense that I had a bad one. I replaced it and my mileage immediately went up to ~15MPG.

Correct me if I'm wrong here, this is my understanding of what was wrong. Apparently an O2 sensor puts out a voltage between 0 and 1.1 volts in the form of a wave. .45 volts average is considered stoichiometric. A good O2 sensor should pass through .45 volts several times per second, if it doesn't, the PCM thinks that the O2 sensor isn't hot enough yet and stays in closed loop mode, not throwing a code. I think this is what is wrong with my original replacement O2 sensor. I will dust off the oscilloscope this weekend and test it out with a propane torch to verify.

Daedalus454 said:

I will disconnect my battery when I get home from work tonight and see how she does after sitting an hour or so. Rich, I will owe you.... I dunno, something if this fixes it.

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Never tell Rich that! He will hold you to it lol

Closed loop is the optimal operating mode. The ECM is looking for rapid changes above and below ~ .45vdc, which signify proper operation. With the engine temp above a predetermined degree, and a rapidly changing O2 reading, the ECM goes into closed loop, which means the ECM is now using the O2 signal to adjust fuel mixture.
A bad O2 heater will keep the O2 sensor from reaching operating temp, and keep the ECM in open loop. Othr failures of the O2 will result in the same thing: open loop.
A "lazy" O2 sensor will appear to the ECM as operational, but swings back and forth accross the .45vdc point so slow as to result in an overly rich mixture, adversely affecting fuel mileage, but not setting any codes.