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04 gmc envoy 5.3l

This topic contains 12 replies, has 2 voices, and was last updated by Nightflyr * Richard Kirshy 6 months ago.

  • Creator
    Topic
  • #963790
    Jeff Moore
    Jeff Moore
    Participant

    When engine is started it runs fine but after a couple minutes it starts to run rich and blowing black smoke out the tail pipe. You can turn the engine off and start it back up and it runs fine then after a couple minutes starts running rough again.

Viewing 12 replies - 1 through 12 (of 12 total)
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    Replies
  • #963791
    Nightflyr *
    Richard Kirshy
    Participant

    Might start at looking at the operation of the O2 sensors.
    If they all check out then move to the maf sensor

    #963795
    Jeff Moore
    Jeff Moore
    Participant

    No codes for 02 sensors the maf seems to have no affect on it plugged or unplugged. I sprayed the intake with break cleaner looking for leaks none to be found. The intake gaskets never been changed. It just seems weird that when it starts to run rough you just cut it off and start it right back up and it runs fine and smooth but not long.

    #963799
    Nightflyr *
    Richard Kirshy
    Participant

    That is why I say check the operation of the O2 via live data.
    You may have a lazy reaction time.
    Now the fact ” the maf seems to have no affect on it plugged or unplugged ” would have me concerned.
    Again this can be looked at thru live data to check its operation.

    #963815
    Jeff Moore
    Jeff Moore
    Participant

    I checked the o2 sensor readings after I replaced the intake gaskets found the gaskets leaking some. The bank 2 sensor 2 on both sides read .925 volts at 99% at start up and do not change even when it goes to open loop. Then bank 1 sensors seem to be reading ok. I took the exhaust loose from the engine to see if the cats where stopped up no change.

    #984880
    Jeff Moore
    Jeff Moore
    Participant

    I cleaned the cats out replaced the bad intake gaskets. The stft readings run at 53% the left stay at 25% positive. The stft goes to 0% when you go wot and let off the ltft stays 25%. The mass reads 5.6 GPS at idel goes high at 2500rpm. I have checked for vacuum leaks with water and break cleaner.

    #984881
    Nightflyr *
    Richard Kirshy
    Participant

    First I would suggest smoking the engine to see if there is a leak you might have missed.

    #986686
    Jeff Moore
    Jeff Moore
    Participant

    Did a smoke test today only leaks found where oil cap and Pcv grommet on the tube. Looked at the mass air flow readings at 700 rpm the readings where 9.95 grams per second. Below 700 rpm it jumps from 5.12 to 7.15 what should the correct readings be.

    #986689
    Nightflyr *
    Richard Kirshy
    Participant

    Generally, @ operating temp and idle g/s usually ~ equals the engine displacement.
    In your case ~ 5 – 6 g/s

    #986690
    Jeff Moore
    Jeff Moore
    Participant

    If it’s reading to high at idle could this be the cause of high stft readings making it go lean because the ltft never moves from 25%. The owner tried one before but that’s before I cleaned out the clogged up exhaust and blown intake gaskets.

    #986691
    Nightflyr *
    Richard Kirshy
    Participant

    I’m wondering if the MAF is actually functioning at all.
    Being you stated: ” the maf seems to have no affect on it plugged or unplugged ”
    and the engine is running on a default strategy.
    See if these help at all:

    #986717
    Jeff Moore
    Jeff Moore
    Participant

    Replaced maf and still running lean. The one thing I have not replaced is the spark plugs. The plugs have to be shot from all the lean running it has been doing. Correct me if wrong wouldn’t the plugs not firing good cause the stft and Ltft not to read correctly, due to the un burnt fuel going past the o2 sensors.

    #986718
    Nightflyr *
    Richard Kirshy
    Participant

    You state your running high positive (adding fuel) trims which actually means the engine is actually operating lean.
    If, by your statement ” wouldn’t the plugs not firing good cause the stft and Ltft not to read correctly, due to the un burnt fuel going past the o2 sensors ”
    That situation would tell the ECM that there is an excessive amount of fuel, in which to correct that would be taking fuel away (negative fuel trims)

    I would think if the plug(s) are your issue to that extent.
    You would be see / feeling it in the operation of the engine.

    You do understand:
    Higher( + 10% and above )fuel trims indicates a lean condition
    Lower ( – 10% and below )fuel trims indicates a rich condition
    What your stating in your last post is opposite to the actual operation of the fuel mapping system.

Viewing 12 replies - 1 through 12 (of 12 total)

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