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  1. #1
    Senior Member Three Rings
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    May 09 2009
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    Code P0171, bad MAF or vac leak?

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    Hi all. I'm new to Audis and I just got a 2001 A4 1.8TQM. I love it, it's a great car but it has a check engine light on. I went to Autozone and scanned it and got the following code:

    Code P0171: System Too Lean (Bank 1)

    I looked that up on here and vortex and it led me to believe it was the MAF. I tried cleaning the MAF with isopropyl alcohol. However, this only made the car idle funny (RPMs bouncing around between 750 and 250. It even conked out once.)

    So I went back to Autozone and bought some MAF cleaner and tried that. Now it's running a little bit better, it no longer jumps at idle (It still drops low when I start it though.). When I was taking out the MAF to clean it, I noticed what looked to be a leak in something (check valve? Is that what it is?) Could that be the source of my code and idling problems?




    Thanks.
    Last edited by turbo_bootin; 04-21-2010 at 02:27 PM. Reason: Add pics

  2. #2
    Veteran Member Four Rings
    Join Date
    Feb 27 2005
    AZ Member #
    5482
    Location
    North Wales, PA
    Items for Sale

    Re: Code P0171, bad MAF or vac leak?

    Yes get a new check valve
    2001 A4 Avant 2.0t comp CT4 5858 Maestro Tune
    2012 Volkswagen CC TSI 6 speed.

  3. #3
    Established Member Two Rings slingshot's Avatar
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    Jun 02 2007
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    18488
    My Garage
    frankenstein turbo a4
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    virginia beach

    Re: Code P0171, bad MAF or vac leak?

    There's your leak right there.

  4. #4
    Senior Member Three Rings
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    May 09 2009
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    Syracuse, NY

    Re: Code P0171, bad MAF or vac leak?

    Alright, thanks. But why would it start idling funny after I cleaned the MAF?

  5. #5
    Established Member Two Rings slingshot's Avatar
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    Jun 02 2007
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    frankenstein turbo a4
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    Re: Code P0171, bad MAF or vac leak?

    Quote Originally Posted by turbo_bootin View Post
    Alright, thanks. But why would it start idling funny after I cleaned the MAF?
    It should have been idling somewhat funny to begin with since you have at least one vacuum leak. But you may have damaged the MAF or got some dirt or something in the connector.
    Also possible you aggravated your one or multiple vacuum leak(s) when you were in there poking around.

    Go through the engine bay and check all the vacuum lines. It's easy since there are only 55,000 of them on this engine. Inspect all those little check valves and doohickies and make sure you've taken care of every little hose and connection that might be leaking air. Then re-check the MAF and connetors. Alcohol is generally pretty safe, but you have to be careful how you apply it. MAF's are very sensitive. Double check the clamp on the hose attached to the MAF as well. Could be something silly like that.
    Also, I'm not familiar with the MAF on this car, but in many cases the actual MAF sensor is tucked up into the tube, and the part you can see is actually the intake air temp sensor or something similar.

  6. #6
    Forum Moderator Four Rings biketsai's Avatar
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    Jun 04 2007
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    18558
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    01' A4 1.8TQM;05 USP; Cannondale CAAD9
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    Austin/DFW, TX

    Re: Code P0171, bad MAF or vac leak?

    I hate the P0171!!!!
    BetaAlphaTau Member #12
    01' A4 1.8TQMS 5spd swapped
    GT2871R|CTAPP MAFless|034 EM|Godspeed FMIC|APR 2.5"|EVO MS DV|CM FX400-LWFW
    18" RS4 Reps.|S4 Front/Skirts/Seats/Brakes|RS4 Grill|CF Trim
    Stasis SS|LLTEK Tower|KW F/R Sways|Profec Spec 2
    My 5-Speed Swap DIY
    - B6 USP
    Chris:Mod|PM me with any questions

  7. #7
    Veteran Member Four Rings McGovs2.7TB5's Avatar
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    May 28 2008
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    8 driven wheels and 10 doors
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    Denver, CO

    Re: Code P0171, bad MAF or vac leak?

    OK, so this code comes and goes on my '01, CEL will be on for a couple days then off for 5 then back again. Most of the time when its on I'm in limp mode. My gauge doesnt show any abnormal vac pressure amounts so I'm leaning towards the MAF. Has cleaner ever really fixed the problem or is it a stop gap to buying a new one? Thanks
    01.5 Brilliant Black S4 avant - JFonz 2+ / Piggies+SSAC / X34 intake / ST coils / Podi Stepper / Samcos / 007s / A8 flat fives / Thor skid plate
    tips need love too

  8. #8
    Veteran Member Four Rings Seerlah's Avatar
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    Re: Code P0171, bad MAF or vac leak?

    It is always worth a try to use CRC MAF cleaner before purchasing a new MAF. Figure $5 (MAF cleaner) vs. $125 (New MAF). Never hurts to try first. And yes, CRC MAf cleaner has fixed a MAF in many instances. That is the case when the MAF is not faulty, but simply dirty (which can lead to it being faulty). And you won't know, unless you try and clean it first.
    I hate it when my car acts like a little bitch, treating me like the bitch

  9. #9
    Forum Moderator Four Rings biketsai's Avatar
    Join Date
    Jun 04 2007
    AZ Member #
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    01' A4 1.8TQM;05 USP; Cannondale CAAD9
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    Austin/DFW, TX

    Re: Code P0171, bad MAF or vac leak?

    Quote Originally Posted by McGovs1.8TB5 View Post
    OK, so this code comes and goes on my '01, CEL will be on for a couple days then off for 5 then back again. Most of the time when its on I'm in limp mode. My gauge doesnt show any abnormal vac pressure amounts so I'm leaning towards the MAF. Has cleaner ever really fixed the problem or is it a stop gap to buying a new one? Thanks
    Def the MAF. I used to have that issue, replaced it and I was completely fine. You can try the cleaner, but if your sensor is just bad, then it may not work out.
    BetaAlphaTau Member #12
    01' A4 1.8TQMS 5spd swapped
    GT2871R|CTAPP MAFless|034 EM|Godspeed FMIC|APR 2.5"|EVO MS DV|CM FX400-LWFW
    18" RS4 Reps.|S4 Front/Skirts/Seats/Brakes|RS4 Grill|CF Trim
    Stasis SS|LLTEK Tower|KW F/R Sways|Profec Spec 2
    My 5-Speed Swap DIY
    - B6 USP
    Chris:Mod|PM me with any questions

  10. #10
    Registered Member One Ring
    Join Date
    Jun 08 2009
    AZ Member #
    43492
    Location
    TX

    Re: Code P0171, bad MAF or vac leak?

    I have the 2004 Audi 1.8T.

    Check engine light is on. I have my car diagonesed at Autozone, code P0171 & P0507 were read. See the picture, is that the problem that caused check engine light?



    This is what I am planning to do. I will go to the dealership and get the part, then drive up to a mechanic shop and ask them to change it for me, or any forum members can provide some good tips to change the part.

    Just wonder is the car still safe to drive in the next couple days until I have time to bring the car to the shop.

    How much is it costs to fix the problem in dealership? Thanks.

  11. #11
    Veteran Member Four Rings dcampana's Avatar
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    Nov 01 2006
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    04 Allroad, 01a41.8Q, 90 CQ
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    Nor Cal

    Re: Code P0171, bad MAF or vac leak?

    ^^ dude, just buy the check valves and then go buy some new hose clams. take old one off and out new one in (just put it on the right direction.... dont pay someone for something this easy... save your cash for real problems:):) if you can turn a screwdriver, you can handle this!!! there are better things to buy than labor.
    my ride: 2001 A4qtm... 2871r, 440cc, 3 bar, turbo xs dv, custom 3" exhaust, test pipe, Cm fx400 billet LWFW...
    90 CQ.. 2bennett... work in progress
    2004 AR... bone stock
    Timing belts, clutches, and all done in the North Bay. Pm me.

  12. #12
    Registered Member One Ring
    Join Date
    Jun 08 2009
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    Location
    TX

    Re: Code P0171, bad MAF or vac leak?


  13. #13
    Active Member Two Rings
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    Mar 19 2009
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    40097
    My Garage
    1999 A4 Quattro
    Location
    Kansas City

    Re: Code P0171, bad MAF or vac leak?

    is there a way to check if the maf sensor is bad and needs to be replaced?

  14. #14
    Registered Member One Ring
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    Oct 01 2009
    AZ Member #
    48631
    Location
    brooklyn

    Re: Code P0171, bad MAF or vac leak?

    can the in-line check valve really cause so many problems? burning rich, heavy idling, loss of power among other symptoms...

  15. #15
    Senior Member Two Rings RockyMountainB5's Avatar
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    Nov 01 2009
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    Location
    Boulder, CO

    Re: Code P0171, bad MAF or vac leak?

    Quote Originally Posted by audijet View Post
    Not unless you want to pay $30 for that stupid little valve.
    heres the one you need: $10....
    http://www.ecstuning.com/Audi-TT_MKI...alve/ES259630/

    or, you can just purchase a generic one from Napa, Autozone, etc - its just a diaphragm valve that prevents gasses from going in a certain direction. So, any check valve will work - one that can handle up to -25 in/HG vacuum, and up to +25 PSI boost.
    2001.5 A4 1.8TQMS

  16. #16
    Registered Member One Ring
    Join Date
    Oct 01 2009
    AZ Member #
    48631
    Location
    brooklyn

    Re: Code P0171, bad MAF or vac leak?

    i just replaced the inline valve check that was corroding. after driving a few minutes on the new one i notice tthat im still burning rich as well as my turbocharger is glowing red.
    any ideas as to why this may be?



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