Help With Codes Po171, Po300 And Freeze Frame Data

Discussion in 'GM Powertrain' started by jcwells, Feb 17, 2009.

  1. jcwells

    jcwells New Member

    2004 Suburban, 5.3 80000 K miles. Running fine until today. Rough idle and I pull codes PO171 System too lean, and PO300 Random/Multiple misfires detected.

    Cleaned the MAF sensor, and put new plugs in. When I pulled the plugs I could immediately see the problem. Passenger side 4 plugs looked worn but fine. Driver side, all 4 plugs fully carboned over. Put the new plugs in but as expected no change.

    When the car first starts up cold it seems fine, but within a minute or two the idle gets rough. If you rev it a bit it seems to smooth out, and it is drivable (it must be running on 8 not 4 or I would know it). After working on it, and revving it in the driveway, there is lots of black carbon spitting out the exhaust and leaving a mess on the driveway.

    The freeze frame data from my code reader is:

    Calc load 5%
    Coolant 82
    ST FTRM1 53.1%
    LT FTRM1 24.9%
    ST FTRM2 1.5%
    LT FTRM2 0.0
    MAP 9.1
    RPM 1091
    MAF 1.563
    ABSLT TPS 17.6%

    I am not sure what these codes represent, but I expect the 0.0 is not correct.

    Any comments or directions?

  2. kinglt1

    kinglt1 Member

    Leaking intake gasket! There is a gm service bulletin on this condition, it's a common problem! Just had mine fixed. starts up fine, runs ok for a few minutes, then starts surging at idle. if you rev the engine it smooth's out. Once the engine completely warms up the condition goes away.
  3. jcwells

    jcwells New Member

    Ok, I will look further at them and try the carb cleaner test, but the soot covered plugs and the soot coming out the exhaust makes me think the drivers side bank is too rich. Most report a rough idle when cold with the intake gasket problem, whereas mine is smooth when cold.

  4. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    I agree that the black smoke and fouled plugs indicate that the driver's side (bank 1) is running rich. However, the P0171 code means that the computer is seeing "lean" from the O2 sensor. It's been too long since I've had my scanner hooked up, so I don't remember for sure, but I would say that the LTFTRM2 (Long term fuel trim bank 2) of 0.0 is more correct than the LTFTRM1 number. The "contradiction" between running rich but seeing lean suggests to me a fault in the O2 sensor bank1 sensor 1 circuit .

    Will your code reader read PID's in real time, or only freeze frame data? It would be interesting to see what signal the computer is seeing from the O2 sensor bank 1 sensor 1 and compare it to O2 sensor bank 2 sensor 1. If O211 flat lines at 0, I would suspect a bad O2 sensor or a broken wire between O211 and the PCM.

    I would probably also put a fuel pressure gauge on it, just to make sure fuel pressure is correct, and that the system holds pressure.
  5. jcwells

    jcwells New Member

    Thanks, that makes sense to me. My local jobber wants $200.00 for the upstream O2 sensor (lousy Canadian pricing), so tonight I will swap the left and right sensors and see if the problem switches sides. If it does I have my answer.
  6. jcwells

    jcwells New Member

    Just completing the thread. I moved the O2 sensor and the problem shifted from Bank 1 to 2. So I replaced the O2 sensor that was originally in bank 1 and everything is fine.
    Thanks for the help.

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