My 98 trooper (3.5L 78K )has been having pretty bad gas mileage for quite a while (approx 13mpg city.
The other day it was about 100 degrees outside when my CEL light came on. I pulled off the highway, checked oil, radiator and a general check of the engine. No gross problems noted engine still running fine.
I pulled the codes that evening and the only code that showed was,
PO157 O2 sensor circuit low voltage (bank 2 sensor 2).
The printout suggested that this code can be caused by many different things including air, fuel, bad sensor etc.
When I checked the air filter (K&N)I saw that there was excess oil on the filter and some slight pooling in the bottom of the air box (half a thimble full). The air filter had been cleaned lubed and dried 5 months ago. My thought is that the excessive heat may have made the oil more fluid and caused the drippage. I haven't had a chance to check the MAF sensor yet. Edit..I just pulled off the MAF sensor and while its not sparkly I wouldn't consider it dirty (no fluid, no build up no gunk)
My thoughts are Check the fuel filter (read somewhere that a clogged or sluggish filter may cause a decrease in gas mileage and may lead to O2 sensor fault and finally search for the post detailing how to check the o2 sensor (IIRC I read a post about a year ago here detailing the procedure (any help with this would be appreciated).
If the CEL is still on does this indicate that the problem is ongoing? I know the code won't clear the memory for about 40 warm up cycles after the problem is fixed.
Am I on the right track? Any suggestions?
Thanks

My take is that a plugged fuel filter would likely cause the engine to starve/stumble under acceleration rather than give you poor mileage.

If the MAF looks good that doesn't prove it is ok. However that and the fact you are only getting one bank with a CEL would make me look elsewhere.

Depending on how tough the sensors are to reach and how stuck in place they are I would strongly consider swapping B1S2 and B2S2. If the problem moves that says sender to me.

A leaking intake manifold gasket would also give you the CEL. You having a surging idle problem? If so that says IMG problem to me.

The CEL will turn off after three trip cylces where it isn't tripped. It will clear memory after 40 trip cycles.

"A leaking intake manifold gasket "


THis can cause lower MPG.

K+N typically will give you lower MPG during city driving.

Wheel alignment can cause lower MPG as can a plugged EGR and EGR port.

this may be a longshot...but my average for mainly highway driving is in the 14s, so 13 doesn't sound so bad in city driving. this is on a '97 3.2l with automatic and 265s. i don't know how it is in your area, but i've been convinced that bad gas was hurting my vehicles. it started with the trooper. it sounded like it was rattling more when i was using cheap/sams club gas. it improved after some techron and a switch to chevron and shell gas. then, my wife's minivan's check engine light also started coming on, but cleared when i "asked" her to start using chevron or shell gas. i recently came across an article about top tier gas, which talked about how certain companies have not cut back on their fuels' detergent levels while others have. i don't know if this is fluff, but it's just a thought. good luck.

Thanks for the replies,
The IMG was replaced a year and a half ago. There are none of the original signs of the leaaking gasket (surging, wierd idle, etc).
Last night I went out for a drive and <img src="/forums/images/graemlins/confused.gif" alt="" />the CEL was gone. I think I will try to switch the O2 sensors to see if that changes the codes.
BTW the auto parts store Checkers lists 2 options for the 2 front o2 sensors ($54.99 and $79.99) and only one option for the rear($79.99 IIRC) all made by bosch. Are all 4 o2 sensors the same or are the rear two different?
As far as the fuel filter causing the problem, I've read that a sluggish fuel filter may allow enough fuel to properly run the vehicle at normal operating conditions, However problems may show up in extreme conditions such as flooring down the Highway with the ac blaring on a 100+ deg day trying to make it to an event on time (I do not deny or confirm that I was in fact "speeding" officer....)

Did they do all 3 IMGs or just one? If not all of them you could be in for a repeat of this problem. Since you are not getting any symptoms like you did before I will assume this is not your problem though.

If the fuel filter was compromised enough to effect your milege, like I noted before you would very likely feel the vehicle stumble at speed and when accelerating heavily. So I don't think this is your problem. Still if you haven't changed it lately, it would be a good idea to change it now in any event.

My parts manual shows that ALL 4 oxygen sensors are the same. Only change is that different year 97+ Troopers take different sensors but each model takes 4 of the same sensor. You may want to call another parts store and get their take. Could be Bosch/Isuzu has susequently speced different plugs since my manual came out, but I doubt it.

To elaborate on my swap sensors suggestion. There are not a lot of posts about oxygen sensor problems on the newer Troopers. Thus, it seems that they are fairly robust. So I for one wouldn't assume one failed and automatically buy a new one and hope this fixed the problem. I would do the noted testing. Then again if you did buy a new sensor, and it didn't fix the problem, you would have a spare you could likely use in the future.

Lastly, any chance your area converted to low pollution gas about the time your mileage took a hit? Typically low pollution gas is less dense and while you get about the same miles per pound of fuel you get a lower miles per gallon. That could explain some of the mileage hit.