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[Insignia] [08---] P0171 but no engine issue - high STFT at idle

Discussion in 'Cavalier, Calibra, Vectra, Signum, Sintra & Insignia owners forum' started by Naz_S, May 19, 2017 at 8:30 PM.

  1. Naz_S

    Naz_S Member

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    Hi all. Newbie here. Been lurking and reading up for a bit as I've been trying to pinpoint an issue with my insignia. It's 2009 1.8 SRi Petrol 70k miles.

    I keep getting the P0171 code without any loss of power etc. I clear the code and after a while it comes back. As there is no power issue, I tried narrowing down when and why this might happen. So my findings after monitoring STFT and the O2 sensor voltage after the car is properly warmed up and in a closed loop is:

    Whilst revving / driving: STFT is fluctuating around 0 (zero) and the O2 sensor voltage is fluctuating up and down from (around 0.8 to 0.1). Now, I believe this to be normal - please correct me if wrong.

    When idling: STFT starts to creep up all the way past 20% and the O2 sensor voltage remains at 0.1 (or there abouts), it doesn't fluctuate. I was under the impression the O2 sensor should constantly fluctuate in a closed loop system?

    My initial thought was a faulty O2 sensor, but as it does produce voltage under revving then I guess it's working?

    So - can anyone shed any light on this? I'm pretty handy with a spanner but as I've only recently purchased this as my other car needed a rebuild then I'm not too familiar with the in's and out's of it. There's no power loss and I can't hear any kind of vacuum leak (although I haven't done a smoke test). I just don't want to have to reset the ECU light every two days.

    Appreciate any help you guys could provide.
     
  2. M32Guy

    M32Guy M32 Specialist

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    Not heard many P0171 faults on this engine. It's a fairly safe bet it's not the O2 sensor. It's reading fine when driving according to what's the you've. I believe it's picking up a genuine fault at idle. It's reading a rich mixture and asking the ECU to lean it out. I'd be suspecting the MAP sensor is giving an incorrect reading at this point
     
  3. Naz_S

    Naz_S Member

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    Thanks M32Guy - does the 1.8 petrol have a MAP, I thought it was just a MAF? (or did you mean MAF?). Thanks again.
     
  4. M32Guy

    M32Guy M32 Specialist

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    I did mean to put MAF yes. Technically it's known as an Air Mass Sensor as I believe it does a bit of both. Anyway it's the part that looks like this
    52566.jpg
     
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  5. grant19841

    grant19841 Well-Known Member

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    You have capabilities to do some live data yes?
    Well then do some data on the maf to see if this reading correctly, or use a multimeter to check ohms and voltage.

    By the readings of the stft and o2, ecu is adding fuel to compensate for the lean o2 reading.
    I would be checking the intakes etc too.... one way to see if it's a vacuum or intake leak is to;
    Live data @ idle as you have been doing, then open up the throttle.
    If you see the stft drop dramatically it's a good indication of a leak.

    The reason and theory behind this is because when the car is at idle the ecu is adding fuel based on the o2 lean voltage, but when you open that throttle the ecu will add more and more fuel this will eventually bring the fuel trim down.









    Sent from my iPhone using Tapatalk
     
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  6. Naz_S

    Naz_S Member

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    STFT does drop when opening up throttle because then the O2 sensor is making voltage. The O2 sensor doesn't seem to be creating any voltage when at idle which is why STFT would increase. Weird one this.

    Ill clean the MAF and see if it makes any difference to start with.

    Any common places i should check for leaks?
     
  7. grant19841

    grant19841 Well-Known Member

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    What are the stft when you rev the car @ 3000 rpm for abit (20-30 secs)??

    The o2 sensor is running lean (more air than fuel), so the ecu will keep adding fuel to correct the air & fuel ratio mix.

    If you have a leak you need to be checking all vacuum and inlet hoses for splits,
    Best thing to do is a smoke test as this will show a leak in seconds.

    If you pull a vacuum pipe off you can try blow smoke through that pipe, I have done this by constantly blowing cigarette smoke down it before and seen leaks.... lol bit red neck way of doing it but it works!!


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  8. Nick H

    Nick H "Red One.." VxON Club Member

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    Agreed with Grant that if unmetered air is entering the engine after the airflow meter, the o2 sensor readings will read lean to the ECM and this would cause the trims to run high.

    When the engine is idling the intake system is under the most vacuum so if the leak is minor this is where it'll rear its head.

    This would also explain the low voltage reading from the o2 sensor at idle as the sensor reads low in lean conditions (from memory they're still a narrowband on these).
     
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