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Sorry another P0171 but stumped

Discussion in '1st Gen. Tacomas (1995-2004)' started by RMMT, May 12, 2022.

  1. May 12, 2022 at 2:07 PM
    #1
    RMMT

    RMMT [OP] New Member

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    Hi, I’ve got the P0171 CEL. First CEL I’ve ever had on this truck.

    2003 Tacoma 3RZ Supercharged (LC Engineering 4psi)
    AEM AFR gauge
    180k miles, we’ll maintained

    Symptoms:

    Having the afr gauge has been huge in seeing real time values while driving. AFR is always normal when driving 14.7ish unless I’m into boost with pedal to the floor and it takes a few seconds to drop into the 13’s. So I’m not lean when driving. However, when at idle (at a stop light, etc.) it will climb to mid 15’s and beyond to 16’s and 17’s and idle pretty rough like it has a lope to it. Anything above idle and it runs like normal. The colder it is outside (Montana winters) the more pronounced the lean condition at idle. Also it seems like when pedal to the floor the AFR only goes down to high 13’s where as before the P0171 CEL it would drop into the 12’s.

    What I’ve done:

    - inspected all vacuum hoses, all appear fine (not brittle or cracked)
    - cleaned MAF with special MAF cleaner
    - injector service by LC Engineering (pink tops cleaned and flow tested with before/after data)
    - have new spark plugs but haven’t put them in yet (due for being changed anyways)
    - no exhaust leaks and my AFR gauge seems to be confirming that my O2 sensors are working properly

    Seems odd to me that it is only overly lean at idle but that I don’t appear to have vacuum leaks. Any ideas? Could it be fuel pump or IAC? Any help is greatly appreciated!
     
  2. May 12, 2022 at 5:23 PM
    #2
    Bivouac

    Bivouac Well-Known Member

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    Remains to be seen I bought the tires and wheels the rest came along
    Is the PO171 the only code nothing pending any test cycles not completed?

    Is your idle rpm where it should be?

    Auto or manual??

    What do your spark plugs look like? Can`t hurt to change them?

    When was your last fuel filter change?
     
  3. May 12, 2022 at 7:11 PM
    #3
    RMMT

    RMMT [OP] New Member

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    P0171 has been the only code active and no stored codes (I have a basic ScanGauge II code reader).

    Manual transmission (SR5 extended cab, 4x4)

    pulled spark plugs (Denso K16R-U11) a couple weeks ago when this started and they looked good color wise and no fouling. Electrode prong was a little rounded/eroded from heat. never any misfire codes or anything. I just bought NGK BKR5EGP platinums to throw in since the last spark plug change was at 135k when I put the supercharger on. Same with last fuel filter change… 135k. I did it b/c SC necessitated removal of NA intake so easy access to fuel filter… not looking forward to doing it again PITA spot but I’ve heard you can go through the fender well.

    idle is around 700 or 800 rpm. So example: I’m approaching a light that has just changed red, go into neutral & stop, rpm will be at 900 or 1000 briefly at idle and normal 14.8 AFR then after 3 to 5 seconds it’ll drop to around 700 at which point idle gets tougher and AFR goes to 15’s if it’s mildly lean but then idle gets even rougher and has more of a lope if it gets into the 16’s/17’s but still at that same 600 or 700 rpm. Never stalls or feels like it’s going to die… just rough idle and super lean. All other driving above idle is great.
     
  4. May 12, 2022 at 7:46 PM
    #4
    Deathbysnusnu

    Deathbysnusnu Work is just a daily detour to happy hour.

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    Dog, camper.
    I'd start by changing out the vacuum lines, even if they look good.
    They can get porous over time.
    No need to buy factory or any kit, just buy a roll of the stuff in the diameters you need. Very inexpensive and prolly needs done if they are original at that mileage.
     
  5. May 12, 2022 at 10:01 PM
    #5
    Dm93

    Dm93 Test Don't Guess

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    This is just a hunch but since you can duplicate it pinch off or disconnect and plug your brake booster hose when your AFR goes lean and see if it corrects it, if it does you have a leaking brake booster.
     
    Bivouac likes this.
  6. May 13, 2022 at 6:00 AM
    #6
    RMMT

    RMMT [OP] New Member

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    Is there a reputable brand for vacuum lines? I don’t want to get crap rubber that rots in a year or two.

    I’ll try pinching off the brake booster vac line this weekend and see if that does anything.

    Any chance the IAC being dirty would be inline with these symptoms? Sounds like IAC issues usually manifest themselves in a racing idle and/or the wanting to stall?

    thank you for the replies thus far… I’ve lurked here for a longtime for how-tos and technical data, this forum is a great resource.
     
    Dm93 likes this.
  7. May 13, 2022 at 6:34 AM
    #7
    Dm93

    Dm93 Test Don't Guess

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    An IAC issue shouldn't cause a lean condition just a low or high idle since all air that goes through the IAC valve is still metered.
     
  8. May 14, 2022 at 9:20 AM
    #8
    Bivouac

    Bivouac Well-Known Member

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    Remains to be seen I bought the tires and wheels the rest came along
    I buy my rubber hoses from McMaster-Carr i have never received poor quality products from them.
     
  9. May 24, 2022 at 10:03 AM
    #9
    RMMT

    RMMT [OP] New Member

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    So a couple weekends ago I replaced all the small 3mm vacuum lines and also dropped in new spark plugs. CEL is now off and has stayed off so I think the issue is resolved.

    All the stock vacuum lines I removed seemed to be in good shape (still had supple rubber, no cracks or chafing). The spark plugs I replaced with iridium ones were basic denso coppers but were at the 50k mileage and were overdue for replacement.

    I can’t imagine spark plugs would cause a lean condition at idle but correct me if I’m wrong. With how good of shape the factory vacuum lines were in, again I’d be surprised if they were the culprit. Anyways just glad the rig is running right again.

    thanks for the help everyone.
     

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