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  1. #1
    Active Member Two Rings
    Join Date
    Dec 17 2009
    AZ Member #
    52220
    Location
    Indiana

    P0171 Code, fuel trims seem off also.

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    Hey guys,

    Few questions:

    I am getting a P0171 code and i have been trying to pin point the issue. I did some fuel trim logging and saw that my LTFT will go to around -15% and my STFT will go from 15% to -27%. This was a short run after resetting the code using torque, but that seems off.

    Am I looking at a vacuum leak? I need to clean my MAF and see if it makes a difference.

    Thanks,

  2. #2
    Senior Member Three Rings
    Join Date
    Jun 26 2015
    AZ Member #
    339463
    Location
    Tampa, FL

    Your fuel trims are way off. Could be a bad O2 sensor, but I doubt it. Could be a bad MAF. Probably a vac leak though. The O2 sensor is reading lean, likely because of unmetered air entering the combustion chamber.

  3. #3
    Active Member Two Rings
    Join Date
    Dec 17 2009
    AZ Member #
    52220
    Location
    Indiana

    ok, would i be able to tell if its the MAF by unplugging it and seeing if there is a difference? Would that rule out MAF? Guess I will have to start really looking for some cracked or broken vac lines.

  4. #4
    Veteran Member Four Rings BARRY's Avatar
    Join Date
    Nov 02 2007
    AZ Member #
    22288
    My Garage
    AE86 20v, UZZ30 LS2, 2002 A4 B6 1.8t, E63 AMG
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    Items for Sale

    i have the same issue. i'm pretty sure my DV/BOV connection hose is torn.
    2007 Mercedes E63 AMG - Nardo Grey, Pano, LTH X-Pipe, Res Del, ROW Box Mod, Programa ELM, UPD Arms
    2003 Audi A4 1.8t Q Avant - Maestro, 225TT Inj, TP, FMIC, Thermal R&D Exh, KW V3, BBK / Ferrodo DS2500
    1992 SC400 - LS2 6.0, LTH, Tomei 2-Way, KW V3, BattleVersion Arms
    1986 Toyota Corolla GT-S SOLD - 20 Valve, Haltech E8, Toda 272 8.5mm, Toda Cam Gears, Silkroad High Rise Headers, TRD 2-Way, TRD SS Race

  5. #5
    Active Member Two Rings
    Join Date
    Dec 17 2009
    AZ Member #
    52220
    Location
    Indiana

    Quote Originally Posted by BARRY View Post
    i have the same issue. i'm pretty sure my DV/BOV connection hose is torn.
    Ill have to check that area out

  6. #6
    Active Member Two Rings
    Join Date
    Dec 17 2009
    AZ Member #
    52220
    Location
    Indiana

    Checked for any vacuum leaks, strayed some areas with carb cleaner to see if i could get a reaction from the engine also, and took my MAF off and cleaned it. My STFT idle was pegged at -25% and +25% but not it seems to be in the normal range, but while driving its all over the place. I am starting to think O2 sensor. Anything else I can look at to further determine the issue?

  7. #7
    Active Member Two Rings
    Join Date
    Dec 17 2009
    AZ Member #
    52220
    Location
    Indiana

    The trims seem to sometimes be ok, maybe when the car is colder, and it doesnt always peg to -25 at idle, sometimes its at a normal range. Which makes me believe its not a vac leak but more of a bad sensor.

  8. #8
    Veteran Member Four Rings BARRY's Avatar
    Join Date
    Nov 02 2007
    AZ Member #
    22288
    My Garage
    AE86 20v, UZZ30 LS2, 2002 A4 B6 1.8t, E63 AMG
    Location
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    Items for Sale

    mine moves around as well...without knowing as i've yet to take apart my car...i assume the hole in my DV/BOV hose is on a part that is part of a lot of back and forth flex. sometimes the hole is more open than other times. won't be able to work the issue with my car for a while, but my STFT are all over the place. sometimes +, sometimes - and by a large margin. 7-35% in either direction. but the read does correctly correlate with engine stutter or not. when my LTFT is at +25% and for some reason my STFT is -15% or more throttle inputs will cause my engine to stutter due to running real real lean. STFT in + have yet to cause issue as they have not went past +15%
    2007 Mercedes E63 AMG - Nardo Grey, Pano, LTH X-Pipe, Res Del, ROW Box Mod, Programa ELM, UPD Arms
    2003 Audi A4 1.8t Q Avant - Maestro, 225TT Inj, TP, FMIC, Thermal R&D Exh, KW V3, BBK / Ferrodo DS2500
    1992 SC400 - LS2 6.0, LTH, Tomei 2-Way, KW V3, BattleVersion Arms
    1986 Toyota Corolla GT-S SOLD - 20 Valve, Haltech E8, Toda 272 8.5mm, Toda Cam Gears, Silkroad High Rise Headers, TRD 2-Way, TRD SS Race

  9. #9
    Established Member Two Rings
    Join Date
    Apr 01 2015
    AZ Member #
    324392
    My Garage
    2004 Audi A4 B6 3.0 Quattro, 2015 GMC Terrain, Infiniti G35 Coupe
    Location
    United States

    I've done a lot of trouble shooting with fuel trims on my Audi and G35. When you're idling the STFT should be fairly close to 0 and the LTFT should be close to 0, but +-10% is also acceptable depending on the age of the car. When you rev up the engine does the STFT get richer? You'd be looking for a vacuum leak if it does. You're checking these values after it's in closed loop mode as well right? Also, pulling the MAF isn't a make/break test, however, it should start running in a +25 rich mode. That doesn't mean your MAF is bad though, so don't just buy a new one and if you have a Hitachi MAF they are usually pretty reliable. If your STFT is going all over the place do the opposite and introduce both a vacuum leak and a rich condition to see how the computer responds. Pull a small vacuum line and it should start adding more fuel and then start adding like unlit propane or carb cleaner into that same line to see if it starts taking fuel away. That will let you know if the sensors are working correctly.

    Best method to find a vacuum leak if nothing stands out is to smoke your engine. I built a DIY smoke machine to track down a 171 code in my G35 using some mineral oil, paint can, wick, 12v battery, compressed air, and metal wire to heat up the oil. It ended up being an o-ring behind my brake booster causing a huge amount of air to enter the system.

  10. #10
    Veteran Member Four Rings BARRY's Avatar
    Join Date
    Nov 02 2007
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    My Garage
    AE86 20v, UZZ30 LS2, 2002 A4 B6 1.8t, E63 AMG
    Location
    San Diego, Ca
    Items for Sale

    smoke test is good...also check valves fail within the PCV system which so no external signs of failure.
    2007 Mercedes E63 AMG - Nardo Grey, Pano, LTH X-Pipe, Res Del, ROW Box Mod, Programa ELM, UPD Arms
    2003 Audi A4 1.8t Q Avant - Maestro, 225TT Inj, TP, FMIC, Thermal R&D Exh, KW V3, BBK / Ferrodo DS2500
    1992 SC400 - LS2 6.0, LTH, Tomei 2-Way, KW V3, BattleVersion Arms
    1986 Toyota Corolla GT-S SOLD - 20 Valve, Haltech E8, Toda 272 8.5mm, Toda Cam Gears, Silkroad High Rise Headers, TRD 2-Way, TRD SS Race

  11. #11
    Senior Member Two Rings Gin+'s Avatar
    Join Date
    Apr 16 2015
    AZ Member #
    327575
    Location
    CNY Syracuse

    In my experience, vacuum leaks caused misfires at idle, but it's certainly not a rule. Keep in mind when you reset your DTC's the trims zero out. You also (I believe) do not have the opportunity to check LTFT at idle with the generic OBD2 readers. I like to look at LTFT on the highway at a steady speed with DashCmd (Torque for iPhone) to get decent idea of where I stand vs idling in the driveway but I'd do both in your case.

    This vid is gold... https://www.youtube.com/watch?v=yM3iarhSP68
    They're not scared of you. They're scared of what you represent to 'em.

  12. #12
    Senior Member Three Rings
    Join Date
    Jun 26 2015
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    339463
    Location
    Tampa, FL

    Quote Originally Posted by goalie990 View Post
    ok, would i be able to tell if its the MAF by unplugging it and seeing if there is a difference? Would that rule out MAF? Guess I will have to start really looking for some cracked or broken vac lines.
    That's a good initial test, but it is not definitive. If you unplug the MAF and things improve, then your MAF is suspect.

  13. #13
    Veteran Member Four Rings BARRY's Avatar
    Join Date
    Nov 02 2007
    AZ Member #
    22288
    My Garage
    AE86 20v, UZZ30 LS2, 2002 A4 B6 1.8t, E63 AMG
    Location
    San Diego, Ca
    Items for Sale

    the only time a vacuum leak caused misfires for me was when the SJP was split in half causing a huge vacuum leak. smaller vacuum leaks will only cause a lean reading.
    2007 Mercedes E63 AMG - Nardo Grey, Pano, LTH X-Pipe, Res Del, ROW Box Mod, Programa ELM, UPD Arms
    2003 Audi A4 1.8t Q Avant - Maestro, 225TT Inj, TP, FMIC, Thermal R&D Exh, KW V3, BBK / Ferrodo DS2500
    1992 SC400 - LS2 6.0, LTH, Tomei 2-Way, KW V3, BattleVersion Arms
    1986 Toyota Corolla GT-S SOLD - 20 Valve, Haltech E8, Toda 272 8.5mm, Toda Cam Gears, Silkroad High Rise Headers, TRD 2-Way, TRD SS Race

  14. #14
    Active Member Two Rings
    Join Date
    Dec 17 2009
    AZ Member #
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    Location
    Indiana

    Quote Originally Posted by BARRY View Post
    the only time a vacuum leak caused misfires for me was when the SJP was split in half causing a huge vacuum leak. smaller vacuum leaks will only cause a lean reading.
    Ok so I swapped out my O2 sensor and the STFT are defiantly less erratic but it still randomly pegs to positive 25% and doesn’t seem to go past 10-12% negative stft. Another weird this is sometimes at idle it is around zero and sometimes it sticks at 25%. If I rev it it will come back down to normal... is something sticking?

  15. #15
    Veteran Member Four Rings BARRY's Avatar
    Join Date
    Nov 02 2007
    AZ Member #
    22288
    My Garage
    AE86 20v, UZZ30 LS2, 2002 A4 B6 1.8t, E63 AMG
    Location
    San Diego, Ca
    Items for Sale

    i'm not too sure...what do your air/fuel readings look like? my afr's are pretty dead on, so the ECU is appropriately correcting. i won't be able to disassemble my front end and inspect for another 2 weeks. my issue is around the DV/BOV area because my turbo is extra loud since this started happening.

    have you looked at the PCV area under the intake manifold? my 1st B6 had lean issues and I eventually found that the plastic breather fitting was snapped. it looked fine, but when you actually moved it you'd see it was split. things like this are why you could have erratic running conditions as on vacuum it can be a large or small leak depending on alignment, where as on boost your turbo will simply be pushed harder to maintain the same levels of boost.
    2007 Mercedes E63 AMG - Nardo Grey, Pano, LTH X-Pipe, Res Del, ROW Box Mod, Programa ELM, UPD Arms
    2003 Audi A4 1.8t Q Avant - Maestro, 225TT Inj, TP, FMIC, Thermal R&D Exh, KW V3, BBK / Ferrodo DS2500
    1992 SC400 - LS2 6.0, LTH, Tomei 2-Way, KW V3, BattleVersion Arms
    1986 Toyota Corolla GT-S SOLD - 20 Valve, Haltech E8, Toda 272 8.5mm, Toda Cam Gears, Silkroad High Rise Headers, TRD 2-Way, TRD SS Race

  16. #16
    Veteran Member Four Rings BARRY's Avatar
    Join Date
    Nov 02 2007
    AZ Member #
    22288
    My Garage
    AE86 20v, UZZ30 LS2, 2002 A4 B6 1.8t, E63 AMG
    Location
    San Diego, Ca
    Items for Sale



    http://www.pelicanparts.com/techarti...eplacement.htm



    https://www.fcpeuro.com/products/aud...YaApF8EALw_wcB

    you don't have to remove the intake manifold to replace that fitting.
    2007 Mercedes E63 AMG - Nardo Grey, Pano, LTH X-Pipe, Res Del, ROW Box Mod, Programa ELM, UPD Arms
    2003 Audi A4 1.8t Q Avant - Maestro, 225TT Inj, TP, FMIC, Thermal R&D Exh, KW V3, BBK / Ferrodo DS2500
    1992 SC400 - LS2 6.0, LTH, Tomei 2-Way, KW V3, BattleVersion Arms
    1986 Toyota Corolla GT-S SOLD - 20 Valve, Haltech E8, Toda 272 8.5mm, Toda Cam Gears, Silkroad High Rise Headers, TRD 2-Way, TRD SS Race

  17. #17
    Active Member Two Rings
    Join Date
    Dec 17 2009
    AZ Member #
    52220
    Location
    Indiana

    I built a little diy vac leak tester, hopefully this will guide me in the right direction. I just keep thinking if it was a leak, it would happen all the time. I will log my AFR on the way home and see how it looks. What are normal readings for AFR?
    Last edited by goalie990; 11-21-2017 at 09:07 AM.

  18. #18
    Veteran Member Four Rings BARRY's Avatar
    Join Date
    Nov 02 2007
    AZ Member #
    22288
    My Garage
    AE86 20v, UZZ30 LS2, 2002 A4 B6 1.8t, E63 AMG
    Location
    San Diego, Ca
    Items for Sale

    any luck? i was able to resolve mine. my Turbo Inlet Pipe (TIP) was off the turbo.
    2007 Mercedes E63 AMG - Nardo Grey, Pano, LTH X-Pipe, Res Del, ROW Box Mod, Programa ELM, UPD Arms
    2003 Audi A4 1.8t Q Avant - Maestro, 225TT Inj, TP, FMIC, Thermal R&D Exh, KW V3, BBK / Ferrodo DS2500
    1992 SC400 - LS2 6.0, LTH, Tomei 2-Way, KW V3, BattleVersion Arms
    1986 Toyota Corolla GT-S SOLD - 20 Valve, Haltech E8, Toda 272 8.5mm, Toda Cam Gears, Silkroad High Rise Headers, TRD 2-Way, TRD SS Race

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