changed front o2sensor now car's pig rich. why?
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This is a discussion on changed front o2sensor now car's pig rich. why? within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; my car doesn't idle too well. been tryin to fix that. anyway i had a broken wire on the o2 ...

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    changed front o2sensor now car's pig rich. why?

    my car doesn't idle too well. been tryin to fix that. anyway i had a broken wire on the o2 sensor so i replaced it, then i test drove the car in a snowstorm all seemed fine but i didn't drive far on account of the snow. brought the car back home and then couldn't get it started for like 5 days and brought it to the subaru place. They replaced the spark plugs and told me it is running rich. I'm on a custom tune from a reputable tuner and i've been driving around on that tune for a long time now. What happened and why is my car running rich now?

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    just prior to this i had the same mechanic look at it to try and figure out what was wrong with the idling, the only thing he could find was that i had some higher than normal back pressure, measured behind the downpipe, so he figured i had a melted cat. I sawed the (cobb dp) in half to look at the cat and theres nothing wrong with it, i'm temporarilly running a 2.5" catless dp i had till i get this sorted out. Could that change in back pressure going to the smaller dp have caused the rich condition? I don't want to risk ruining my new 2 piece (2 cat) perrin dp that i haven't had a chance to install yet.

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    is there any way a clogged fuel filter could cause a rich condition, cause i've never changed it.

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    Quote Originally Posted by autowagon View Post
    is there any way a clogged fuel filter could cause a rich condition, cause i've never changed it.
    No. If anything it would cause a lean condition.

    I dont know much about tuning, but if u had a bad 02 sensor, wouldnt that mean that when the guy was tuning the car, he wasnt getting correct information back? I would assume this would make the tune different and knock things out of spec. Maybe because of that, the tune itself is rich?

    Someone feel free to correct me if im wrong.

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    i do not think that that is the case. and i replaced the sensor because it had a cut wire not because it was a bad sensor. The old sensor was white on one side, does that mean anything? Is there any chance that snow packed in the front of the car could have cracked something? and if so would that cause the car to not start in the first place or run rich now?
    Last edited by autowagon; 01-22-2007 at 06:03 PM.

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    Quote Originally Posted by autowagon View Post
    i do not think that that is the case. and i replaced the sensor because it had a cut wire not because it was a bad sensor. The old sensor was white on one side, does that mean anything? Is there any chance that snow packed in the front of the car could have cracked something? and if so would that cause the car to not start in the first place or run rich now?
    Well my concern was with if the wire was cut before the tune, which may have cause the tuner to get inaccurate o2 readings while he was doing the tune. Once you replaced the o2 sensor it started to run rich right?

    I dont think snow packed into the front would cause a no start condition. Maybe damage the radiator or lower front end, but not a no start unless u went plowing thru some snow and smacked the starter on the snow. (not sure about the positioning of the starter on these cars, so that may or may not be possible.) Could be a lose connection with the wire to the starter. I would check that. I've seen that take down multiple cars before.

    Hopefully someone else will chime in for you. Thats about all the help i can offer.

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    well 260 dollars later it now starts because they changed my plugs. but basically i get the "there must be something wrong with your car" for my money. It now runs rich and i don't know why. (the check engine light only recently lit up for the o2sensor having a cut wire, i would have gotten a cel earlier if that were the case) I'm gonna have to pull the manifold and uppipe to look and see if it cracked, i just read a thread on that diagnosis, and its the only thing i can do right now. if thats not it i'm screwed (well more screwed, 6hours to my tuner) i can't figure whats wrong with my idling either and no one can help me round here. If i can't find the problem i'm pretty much out of luck.

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    i have a turboxs uppipe with the flex section. the flex section has black soot running up it. i think it is leaking, has anyone else had this problem? and would this thearetically cause my car to run rich?? edit: yup uppipe leaking at flex section from deep powdery snow for sure. (now if i can get that idling problem under control)
    Last edited by autowagon; 01-23-2007 at 09:52 AM.

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    The front O2 sensor only operates in closed loop fuel control, so during idle and light load (cruise). A tune is typically only concerned with open loop fueling at moderate to high loads. I assume the tuner didn't use your O2 sensor during tuning, and used a real wideband O2 sensor for tuning. If they used the on-board O2 sensor, run away fast and get the car re-tuned. The on-board O2 sensor can be off by 5 in either direction depending on the manifold pressure. If the tuner was reputable, and used a proper wideband O2 sensor, I'd be willing to bet you're only running rich at low load and idle. That would explain why you're having a rough idle. If he relied on your on-board O2 sensor, I'm surprised it hasn't blown up yet.

    First, did you reset the ECU after switching O2 sensors? Each sensor is different due to manufacturing differences, and using the learning curve from the original sensor on the new one could be causing your problem. Resetting the ECU will let it learn the new sensor.

    Second, did you buy the right sensor? Some sites claim they have the right one, but will sell you one from a NA car, or something else. The Subaru front O2 sensor is only available through Subaru dealerships, from what I've found.

    Third, a leak in the uppipe, which is downstream from the O2 sensor would not likely cause it to read incorrectly. Leaking headers would be a different story.

    Last, reading spark plugs is questionable at best. Unless you're sure you're running rich by measuring the exhaust with a wideband O2 downstream of the turbo, it's tough to claim you're rich. A couple of stops along the way to the dealership could have put enough soot on there to get the techs to default to "its rich", since it sounds like they didn't even look hard enough to find a leak in the uppipe. Fix the uppipe and check it again.

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    first, i reset the ecu (about 20 times because after the test drive the car wouldn't start for a week). second i dished out the extra money and got the sensor from subaru. third, i'll retorque my exhaust manifold when i change the uppipe although on another thread more than one person was claiming running rich and idling problems with cracks or holes in their uppipes being the culprit. I don't know why either exactly, could it suck in cold air and tell the sensor to add fuel? I don't have a wideband myself but it was tuned by christian at cobb a long time ago. I sure as hell hope this is my one and only problem though, i've never gotten good #'s for my setup yet and i have some cams to put in but i am not going to do that until i figure this idling problem.
    Last edited by autowagon; 01-23-2007 at 02:23 PM.

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    car seems to be running fine now. idle is still a little choppy jumping between 12 and 10 inhg though, didn't die on my yet. the boost needle jumps up and down more consistantly and steadilly now (at idle), i think it helped a little. edit: the idle problem seemed to go away over the day, so i think it was a cure all i'm stoked. (f that txs uppipe pos)
    Last edited by autowagon; 01-27-2007 at 06:17 PM.

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