Missfire at Idle

Discussion in 'V8 - Maintenance & Troubleshooting' started by gamefanatic, Sep 16, 2019.

  1. gamefanatic

    gamefanatic SHO Member

    Joined:
    Feb 20, 2005
    Messages:
    428
    Likes Received:
    133
    Location:
    Barstow, CA
    So I am troubleshooting a missfire at idle. It appears for the moment to be coming out of cylinder 5. I have to validate if it moves to any other cylinder.

    The problem appears to occur after closed loop (need to validate) or after the car has warmed up, and never under a load. So to me that rules out mechanical issues. Though I'll likely do a leak down this weekend.

    Looking at fuel trims I notice the long term of both banks goes high at idle after closed loop / idle, but are normal once driving.
    Short Term appears to be normal.

    I did fix a small vacuum leak at the PCV to upper intake hose this weekend while looking for unmetered air leaks. Haven't found any others yet. Vacuum seems to stay around 21 at idle even with the fixed hose.

    Thought's?

    As a seperate note this car also appears to be about 3-4% richer on LTF bank 2, but LTF trims move up and down equally with bank 1. STF's are in sync with each other at most all times. I am thinking this is a separate issue that may be developing. I have been watch it for about a year. The subject problem just started ocurring.
     
  2. E1

    E1 Crash Tested

    Joined:
    Jan 24, 2003
    Messages:
    501
    Likes Received:
    204
    Occupation:
    Printing mechanic/engineer
    Location:
    Chicago area
    Home Page:
    Coil, sparkplug.
     
  3. GEN 3 SHO FAN

    GEN 3 SHO FAN SHO Member

    Joined:
    Jan 31, 2015
    Messages:
    659
    Likes Received:
    187
    Occupation:
    Records manager
    Location:
    Canada
    Gamefanatic you probably already saw the post on EGR issue. The EGR sensors can affect the idle. They will only cause misfires at idle rpm from what I observed (on park or when car is at a stop).
     
  4. 99sho-time

    99sho-time SHO Member

    Joined:
    Dec 15, 2015
    Messages:
    284
    Likes Received:
    45
    Gender:
    Male
    if its truly after its warmed up then are the o2 sensor switching lean/rich? is the maf clean whats the voltage on the maf?? ive never really seen a vacuum leak cause a misfire at idle maybe rough running unless maybe the vac leak is very big but then it will stall. Im guessing when you say high fuel trims you mean high negatives? High negatives - are because a vacuum leak. fuel trims can do some weird things when driving seems normal to me only time where fuel trims matter to me is idle where vacuum is strongest. short terms being good makes me feel like its learning something fromabad o2 sensor or something.

    I do remember trhese coils being random misfiring idle and not always noticing it under load.
     
    gamefanatic likes this.
  5. gamefanatic

    gamefanatic SHO Member

    Joined:
    Feb 20, 2005
    Messages:
    428
    Likes Received:
    133
    Location:
    Barstow, CA
    Haven't had much time to look at this further. Hopefully this weekend.

    Just a note high negative trims would mean its cutting back on fuel (rich) vs high positive numbers is adding fuel (lean).

    Only the long term is going up, but doesn't appear to get above 11% but affects both banks at the same time.

    I did confirm that the miss is staying in Cylinder 5, so swapped with #6. So far no more miss, but have had the trims raise again. It acted like it was going to do something but never did.
     

Share This Page

If you wish to help keep SHOforum running, please click the donation button below