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Slight idle bounce 2004 8.1L

Discussion in 'Maintenance' started by recycleoldgrowth, Jan 20, 2017.

  1. recycleoldgrowth

    recycleoldgrowth New Member

    Messages:
    13
    Just recently replaced my alternator thinking because it's diodes were going bad was why my idle was rough. Turns out that was not the case. The voltage swing is gone (except now when the idle drops, so does the voltage) but the idle bounce is still there. (We're talking from 600, to 500 then back every couple seconds)

    I've replaced plugs and wires, cleaned the MAF and TB, all grounds ohm out, there isn't an EGR valve on this truck to clean or replace.

    I've looked at the live data using my scan tool. The STFT and LTFT on both banks are within +-5% and all O2 sensors read .2 - .85 with a good throttle snap. All closed loop data, idle is more or less perfect on cold start up OL.

    Any other ideas? Appreciate your time and thoughtfulness on this matter.

    Regards,
    Kyle

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  2. Ginger

    Ginger Bubbles

    Messages:
    24,040
    fuel pressure steady?
     
  3. recycleoldgrowth

    recycleoldgrowth New Member

    Messages:
    13
    Does that data show on the scan tool or would I need to connect to the fuel rail? So I guess my answer would be I'm not sure. Could it only affect it at idle? Thanks for the quick response.

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  4. Ginger

    Ginger Bubbles

    Messages:
    24,040
    normally people use the connection on the fuel rail and a mechanical guage,
     
  5. recycleoldgrowth

    recycleoldgrowth New Member

    Messages:
    13
    That I can do.

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  6. recycleoldgrowth

    recycleoldgrowth New Member

    Messages:
    13
    What numbers should I be looking out for?

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  7. recycleoldgrowth

    recycleoldgrowth New Member

    Messages:
    13
    If I remember correctly it's like 55 psi with KOEO, a little less with the engine running?

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  8. TechieTruckGuy

    TechieTruckGuy 3.5 EcoHell

    Messages:
    4,790
    A turbo will fix that idle bounce.
     
  9. recycleoldgrowth

    recycleoldgrowth New Member

    Messages:
    13
    Tell me more?

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  10. TechieTruckGuy

    TechieTruckGuy 3.5 EcoHell

    Messages:
    4,790
  11. recycleoldgrowth

    recycleoldgrowth New Member

    Messages:
    13
    I just had time to review 3 sets of live data across multiple RPM ranges I'm seeing a common thread.

    The LTFT on both banks seems to me to be very lean at idle -9% to -14% (not getting enough air?) At higher RPM'S it reading +5% to +9% (getting to much fuel?) I was under the impression that LTFT should stay pretty much the same even with added throttle, am I wrong?

    The STFT stays pretty much +/-6% on both banks.

    Is this pointing to a bad throttle body assembly or a MAF on its way out (No codes).

    Both O2S2 are not holding a consistent voltage through a range of RPM'S, it's ranging from .295 to .820. Shouldn't they also be hold a consistent voltage as well? Or is that just at idle? Both O2S1 seem to be doing their job right. Bad CAT'S or defective O2 sensors?

    Thoughts?

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  12. Ginger

    Ginger Bubbles

    Messages:
    24,040
    is the truck pretty much stock or does it have a tune?

    any leak in the intake after the MAF will cause incorrect readings.
     
  13. recycleoldgrowth

    recycleoldgrowth New Member

    Messages:
    13
    Bought it used, but I'm sure it's all stock, definitely no tune.

    Correct me if I'm wrong but wouldn't a vacuum leak effect + FT, not - FT? A leak would increase the air mixture so the PCM would add(+FT) (enrich) because of a perceived lack of fuel for that condition. Am I wrong here?

    Thanks for your input.

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