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Discussion Starter #1
Has anyone had any experience with their O2 sensor going bad on their cars? I took my back to stock from stage 2 due to some financial issues and immediately noticed a MASSIVE drop in fuel econ, thinking that my O2 sensor might've taken some abuse when it was removed or something. When I was stock I was getting around 25 mpg combined with normal driving. I don't beat on my car (at all) and may take it full throttle once every couple weeks, if that. I normally drive like an old man, especially with gas prices. Now I'm getting around 19.5 mpg combined, bone stock, with driving like an old man. Granted I do mostly city driving, but I did mostly city driving before when I was pulling 25 mpg regularly. I have about 2900 miles on my oil, so I'm going to change it and my air filter out, but I don't anticipate either of them having gone that bad that quickly to cause that kind of drop. I was getting around 23 mpg with a Perrin stage 2 map before going back to stock. I've checked and re-checked all the seals and everything after the install, nothing looks awry. I haven't thrown any codes and the car appears to drive normally, though it's hard to tell since I went from stage 2 back to stock and was at stage 2 for about 12,000 miles. 16k miles on the car (09).

Any other ideas before I go and drop $100 on a new O2 sensor?
 

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Just throwing this out there but is there a possibility that with the now decreased power, you are into the gas more to make up for it? An O2 could affect fuel mileage but it will usually throw a code.
 

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Discussion Starter #3
Just throwing this out there but is there a possibility that with the now decreased power, you are into the gas more to make up for it? An O2 could affect fuel mileage but it will usually throw a code.
Definitely not; I hardly give it any gas, usually accelerate slowly and shift at around 2800 rpms and don't ever jam on it. For comparison, I used to also get around 27-28 when I did straight highway driving at around 65 mph, now it's around 24. That's with resetting after accelerating to speed, then cruising at that speed for about 30 miles on the way to work, which is a flat drive with no stops. I thought the same as far as throwing a code but I don't know what the threshold for throwing the code would be. Thought that there might be some sort of gray area that would cause fuel econ to drop but not yet be fowled enough to cause a code to be thrown, especially since I was running a higher flow cat from the Perrin pipe that would presumably let more goop to get through and gum up the sensor.
 

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I don't know exactly what the "tolerance" is but with a 25% drop in fuel mileage, I would only guess that it would throw a light. The rear O2 only slightly affects AFR, a huge majority of AFR adjustments are made using the front, which I'm assuming you didn't touch. Did you have maybe a CAI installed also? What other "stage 2" mods did you have (if any)?
 

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My guess and it's only a guess is that it has to do with the tuning or the map not being back to stock 100%. The O2 theory is also viable, we've seen this before w/o a CEL but usually on much older cars.

-mike
 

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Discussion Starter #6
That's a pretty valid guess.. Any ideas on how to fix it? I used the AP and did the return to stock option, unmarrying it and selling it to someone who is presumably using it now(sold it almost 2 months ago and haven't heard anything so I'm assuming it's unmarried correctly).
My guess and it's only a guess is that it has to do with the tuning or the map not being back to stock 100%. The O2 theory is also viable, we've seen this before w/o a CEL but usually on much older cars.

-mike
 

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If you unmarried it, it should be back to stock 100% then. So might as well throw that theory out.

-mike

That's a pretty valid guess.. Any ideas on how to fix it? I used the AP and did the return to stock option, unmarrying it and selling it to someone who is presumably using it now(sold it almost 2 months ago and haven't heard anything so I'm assuming it's unmarried correctly).
My guess and it's only a guess is that it has to do with the tuning or the map not being back to stock 100%. The O2 theory is also viable, we've seen this before w/o a CEL but usually on much older cars.

-mike
 

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Ever figure out what was going on with your MPG? I've been experiencing the same symptoms. I've been Stage 2 for over a year and this past month, have been experiencing some crappy ass MPG - from 19mpg-21'ish City to 16mpg within a month. I figured it was my tire pressure being off. Once that was adjust, it didn't help out at all. I just got back from a 3 hour road trip and only got 22.5 mpg. I use to get 25-27mpg every time for the past 1 year. I've come to the conclusion that my front O2 sensor is finally bite'ing it because I'm starting to experience rough initial starting in the morning. The car cranks normally but misses it's start every other day or so. It'll be $150 bucks out of my pocket for a new sensor but this can be the only thing causing symptoms like this. 2008 WRX Stage 2 with 50k miles on the ODO. :p
 

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Ever figure out what was going on with your MPG? I've been experiencing the same symptoms. I've been Stage 2 for over a year and this past month, have been experiencing some crappy ass MPG - from 19mpg-21'ish City to 16mpg within a month. I figured it was my tire pressure being off. Once that was adjust, it didn't help out at all. I just got back from a 3 hour road trip and only got 22.5 mpg. I use to get 25-27mpg every time for the past 1 year. I've come to the conclusion that my front O2 sensor is finally bite'ing it because I'm starting to experience rough initial starting in the morning. The car cranks normally but misses it's start every other day or so. It'll be 0 bucks out of my pocket for a new sensor but this can be the only thing causing symptoms like this. 2008 WRX Stage 2 with 50k miles on the ODO. :p
wouldnt a bad 02 sensor throw a check engine light?? i would say remove and clean your maf sensor. use electrical contact cleaner.
 

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wouldnt a bad 02 sensor throw a check engine light?? i would say remove and clean your maf sensor. use electrical contact cleaner.
I was thinking the same thing until I learned this over on NASIOC -
Found some info via SubieGal -
thought::

could be the initial signs of a failing front 02 sensor.

no CEL to be found.... and often when the sensor begins to fail
there is and will be no CEL

symptoms in my experience, include:

noticeably poor MPG
eventually a bit of a rough idle
also stuttering a bit on cold start up

just a thought
but when the sensor starts to fail
it works overtime to keep the car within tolerances (air fuel)

and this will KILL your mpg..... initially

other symptoms to follow as the sensor becomes progressively worse.


anyway, that's been my experience.
I've been experiencing this... Which makes me think I need to change it out.
 

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gotcha. just a thought on the maf sensor. i get a rough idle on cold mornings at stoplights,etc. maybe i have the same issue?
 

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It is January (where are your locations) My mileage has taken a huge hit lately, but mainly due to warming up the car in the morning and the outrageously high cold start idle. I can almost watch my gas gauge drop in the morning.
 

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gotcha. just a thought on the maf sensor. i get a rough idle on cold mornings at stoplights,etc. maybe i have the same issue?
I'll give it a shot. Wouldn't hurt to clean that thing before dropping $120 on a new sensor.
I'm in Los Angeles. Doesn't get snowy cold over here but the temps have definitely dropped.
 

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To the OP, did you change the o2 sensor? My car started getting rough idle this winter at stops but goes away when car warms up and my mpg sucks big time as opposed to other winters. I cleaned maf, intake and changed sparkplugs. Still on same boat.
 

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So I ended up cleaning the MAF Sensor, changing my oil with Rotella T6, and got a Stage 2 Protune. Gas mileage is back up there just as long as I lay off the throttle which is super hard to do considering the power/torque I now have. Super satisfied.
 
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