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Thread: Mixture too lean, P0171, P0174, P0313

  1. #1
    nathancarter is offline Stretch Haters Club #1 BMW CCA Member
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    Mixture too lean, P0171, P0174, P0313

    I've got the Service Engine Soon light on in my 2001 325i, 58k miles. I had it for a checkup at the dealership recently, and they performed some other work under my 2000mile used-car warranty (ECU said the thermostat was sticking open; said warranty has since expired). They gave me a printout of the DTCs, and the other prevalent codes besides the thermostat were P0171 and P0174 with an occasional P0313, plus one that indicated low voltage across the MAF (can't remember the code number). The service advisor suggested that they reset the codes and see if they come back after the thermostat replacement, since the warranty didn't cover the MAF. The light has since come back on and my scan tool says it's the same codes: P0171, P0174, P0313. Fuel mixture too lean, causing an occasional misfire.

    Yesterday, I gently cleaned the MAF using electrical contact cleaner (it didn't look dirty, but I cleaned it anyway with three cycles of spraying it then letting it dry). I then cleared the codes and took it for a test drive and the codes didn't come back. I checked it again this morning after driving to work and P0171 and P0174 are pending; I expect the light will come on when I drive it home.

    Possibly relevant: there's a high-pitched whistling noise at a certain throttle position. It sounds somewhat like a turbo whistle, and is independent of engine or car speed, dependent only on throttle pedal position. The dealer service advisor said he didn't hear anything unusual.

    Any suggestions before I drop a couple hundred on a new MAF?

    Thanks.

  2. #2
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    Lean mixture codes or O2 sensor codes can be caused by intake leaks. Good luck!
    128i

  3. #3
    hooRAH is offline Certified Ziptie Mechanic
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    Yep, intake leaks looks like. Also, good chance thats where your whistling is coming from.

    Good news is that whistling makes it real easy to find vacuum leaks. Move vacuum lines around until you hear the whistle change pitch - itll be in that line.

    Good luck!
    01 330Ci 5spd / 07 Honda Pilot EX-L
    Will read/reset SRS codes in South Florida for beer money


  4. #4
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    So your saying the a vaccum leak is what makes that whistling noise? Mine does that too. How much should it cost to fix a leak at a BMW dealership, or is it easy enough to DIY. Im only 16, so I dont have much experience with DIY work.

  5. #5
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    With your engine running, try to slowly and slighty remove your oil cap. There should be a slight vac, but if it's a very strong vac. and the engine stumbles hard when you remove it then it probally needs a "Crank case vent valve." Other places to look for air leaks is the air boot that "Ys" off at the throttle body and Idle control valve (follow your MAF back to your Intake and you will see it). The small hose on the boot that goes to the Idle control valve cracks on some older cars and allows "false air" in, causing your lean mixture faults.

    If none of these give you anything, look at your MAF signal as the engine is running (operating temperature WITH THE A/C OFF). IF the MAF reads something other than 11-13 kg (I think it's KG) then you probally have a faulty MAF.
    BMW LEVEL 1 AUTOMOTIVE TECH AT WEDGE KYES
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  6. #6
    nathancarter is offline Stretch Haters Club #1 BMW CCA Member
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    Got a lot to learn

    Thanks for the help and advice!
    As I expected, the SES light came on on the way home with P0174. The pending P0171 from the morning drive had gone away, though. I'll check out these suggestions as soon as I can, but I'll be out of town over the weekend. If my father-in-law has a good tool kit I'll look at it at his place.

    I have practically no knowledge of the fuel and intake system on a gasoline engine (and the Haynes is lacking). Can anyone give me a quick primer? (edit: pardon the pun, I just got it)

    With the ol' diesel that I've driven all my life, each cylinder draws in a constant volume of air on each cycle, regardless of engine speed or pedal position. The pedal mechanically controls the fuel injection pump, and more fuel makes more power as long as the intake stroke gets a full cylinder's worth of air. Vacuum is created entirely separately by a belt-driven vacuum pump. The intake is extremely simple: there's the outside, then a filter, then the valve, then the chamber. Well, also there's some ductwork and the manifold, and some models also get some EGR stuff, but the only complex piece in the whole system is the injection pump, which is entirely self-contained.

  7. #7
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    an easy way to find an air leak is with a can of carb cleaner, or brake cleaner. run the engine, then go around spraying stuff, and listen for the idle to rise up. spray on the intake boot, and around the hoses and such going to the intake. I've seen plenty of torn intake boots, some dry rotted torn hoses from the PCV to the dipstick, etc.
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  8. #8
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    Well??
    BMW LEVEL 1 AUTOMOTIVE TECH AT WEDGE KYES
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  9. #9
    nathancarter is offline Stretch Haters Club #1 BMW CCA Member
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    It's not the Crankcase Vent (Oil separator). With much trepidation, I opened the oil filler cap while it was warm and idling. There was no noise and only a slight suction.

    I was out of town over the weekend so didn't dig in any deeper to look at the vacuum hoses. I'll try to get to it tonight, spray a little bit of brake cleaner and see if I can find any leaks. I don't plan to spray it all over the place, but is there anything (other than paint) that I should definitely avoid?

    It seems that the whistling noise is getting louder or perhaps is occurring over a wider throttle range, but it may just be my imagination. Overall the car performed quite well over the ~800-mile round trip to the in-laws' house, and averaged about 28mpg.

  10. #10
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    nah, brake clean is fairly mild compared to like carb cleaner or starting fluid. i've never seen it hurt anything.
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  11. #11
    nathancarter is offline Stretch Haters Club #1 BMW CCA Member
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    Didn't get to it last night, the wife made me do some other chores instead ...

    But the SES light turned off on the way to work this morning?!? Huh.
    Oh well, I still need to check it out but I don't feel quite as pressured to do it "ASAP" since it's driving just fine. Will try to get to it tonight if I can shirk my other obligations...

  12. #12
    nathancarter is offline Stretch Haters Club #1 BMW CCA Member
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    Found it!
    As suggested, the intake boot was decayed and torn open on the elbow leading into the idle control valve. The other boot (immediately after the MAF) is also in mediocre condition, so I think I'll replace them both. In the meantime, I've applied my ghetto fix of closing up the hole with packing tape (couldn't find the duct tape). The whistling noise is practically gone and I've driven about 100 miles without any new fault codes.

    The hardest part of the job, by far, is that the last two hose clamps were installed (in my opinion) upside down, so they were VERY difficult to access and loosen. When I replaced them I put them right-side-up so I can get to them again when the replacement parts arrive.

    Thanks for everyone's advice! I'll post some photos as soon as I have enough posts, and maybe write a quick DIY.

  13. #13
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    yup. chalk up another one for the torn intake boots!
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  14. #14
    nathancarter is offline Stretch Haters Club #1 BMW CCA Member
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    First photo is the location of the failure, with some other parts (Intake, throttle body) moved out of the way. The red arrow is the boot where the hole is. The yellow arrow is one of the two hose clamps that were the hardest part of the job. If you look closely, you may be able to notice that the head of the screw on the hose clamp is not visible - because it's pointing downwards! When I put it back together I reversed the hose clamps so the screw is pointing up.

    Second photo is the big-ass hole in the intake boot.

    Third photo is my ghetto repair job (yes, I've already ordered replacement parts)
    Attached Images Attached Images

  15. #15
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    My car was throwing P0171 as well...but only that code and no CEL...I changed plugs and fuel filter...so we'll see if it returns or not...

    Meanwhile I will be double checking for vacuum leaks...


  16. #16
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    I've got a lean code, so I search for fuel filter, this is the first thread I open, and sure enough, I've got a whistle. So I go look and it's a torn boot. Thanks Bimmer Forums!

  17. #17
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    i have had the exact problems.. replaced the same things.

    at the end of my journey of 1300.00 it was 2 $60.oo coils causing the problem.

    would make a whistling sound coming from the air box location and miss. not all the time though. it was hard to fix because when i took it to the shop it wouldnt do it. and when i would leaqve it would come back..

  18. #18
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    I replaced the precat o2 sensors and the problem is now 99% gone.

    There is a ***tiny*** little stumble aroun 2400rpm...but I think that may be a remnant of the messed up fuel trims from the previous o2 sensors.


  19. #19
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    I have same problem, and have located the tare at the y-pipe does anyone happen to know what part number it is?

  20. #20
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    i had the same problem but i'll replace those boots three month ago.. but recently i have p0171 p0174 as pending mil ligh haven't came on yet . on the air filter box is a crack just on the uper side were the air pass true the air filter but way before the air flow sensor!!... is this could be the problem?.
    I know I has to replace the airbox , but I looking into a cold air intake and look for a good one!

  21. #21
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    there are several other hoses and things that can tear and develop leaks and cause the same codes and problems.
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  22. #22
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    E39 525i '02 with similar codes/issues

    I know this isn't the correct forum, but exact same issues, hoping for some insight...

    Picked up my first BMW used a couple months back. Within days, the SES light was on. First had Autozone run the codes (and reset, they were back on same day). They were Bank 1 and Bank 2 too lean. Also got P1083 and P1085. Today I rec'd my Peake decoder and my Bentley manuals. The last two codes also point to Fuel Control Limit Too Lean in both banks.

    Now, the Peake codes came up with:
    --Oxygen sensor control limit, Cyl #1-3
    --Oxygen sensor control limit, Cyl #4-6
    --Oxygen sensor adaptation limit, Cyl #1-3
    --Oxygen sensor adaptation limit, Cyl #4-6
    --Misfire detected, cylinder #4
    --Misfire detected, cylinder #5

    With my windows down, as I am driving thru a parking lot, I do hear a whistle noise. Either it goes away or I can't hear it above 15-20 MPH. I took off my whole intake back to the throttle body and didn't find any leaks. MAF looked OK.

    My best guesses, and open to input:
    1) did my O2 sensors go bad due to an air leak throwing the fuel mixture off?
    2) or, could I have bad coil(s), causing misfire(s) and making the O2 sensors bad?

    I am curious about the Crankcase vent valve, haven't checked it yet...does the M54 engine also have this?

    It ran much better before the service engine light...it would take off better and had good topend acceleration...still OK, but not like it was...

    Figure my next target would be the coils and plugs...

    ANY help from the experts is appreciated...thanks in advance!
    Last edited by KenBBMW; 05-01-2007 at 12:28 AM.

  23. #23
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    I love this forum... I just had a code P0171 so I replaced the bank 1 O2 sensor but the service engine light came back. When I read the codes this time it had both P0171 and P0174. Quick search and this thread is the first result. I just checked my intake boot based on nathancarter's picture and found a crack in exactly the same spot.

  24. #24
    nathancarter is offline Stretch Haters Club #1 BMW CCA Member
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    I'm glad it was useful. And the replacement parts are cheap!

  25. #25
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    nothing so far..
    <runs to driveway to check the damn elbow... oh the suspense>

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