Backfire at idle

Posted by streetlegalcars, Apr 1, 2009
Newbie Forum - Beginner/newbie/general DSM modification questions. First mods, how to run 10's when you haven't run 12's yet, any tech question that doesn't fit in another tech forum.

  1. streetlegalcars

    streetlegalcars Probationary Member

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    Harrison, Michigan
    this has been bugging me for a while. I have 1 maybe 2 problems I have a backfire at idle only when the car reaches operating temperatures. I have a code 24 and a code 25 iac has been replaced the car idles 2 or 300 rpms higher than it should. also sometimes when i rev up the idle doesn't snap back down it idles high for a while then slowly goes back down. plugs and wires are fine. but the car burns rich. and the vacuum pressure is low. my vacuum tester says it has late ign timing. I cant find any leaks. any help is appreciated:cry:






    24 Throttle Position Sensor Voltage Disagrees With Manifold Absolute Pressure.
    24 Throttle Position Sensor Voltage High.
    24 Throttle Position Sensor Voltage Low.
    25 Idle Air Control Motor Circuits.
    25 Target Idle Not Reached (plus or minus 200 RPM).
    #1
  2. DSM_PWR

    DSM_PWR Proven Member

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    Austin, Texas
    Have you tried swapping out your TPS?

    Try unplugging the TPS during idle and see if it makes any difference, if nothing its possibly bad...

    Or the seals in the throttle body are bad as well, most commonly seen on N/A from my experience.. dunno why... but it will casue the low idle and bad readings like the ones your getting..
    #2
  3. lilchrishall

    lilchrishall Probationary Member

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    It sound like your manafold has a crack in it.
    #3
  4. streetlegalcars

    streetlegalcars Probationary Member

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    Im going to swap it out today i have a feeling it may be more than that
    #4
  5. streetlegalcars

    streetlegalcars Probationary Member

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    Harrison, Michigan
    nope not the tps
    #5
  6. brassc2007

    brassc2007 Proven Member

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    mason city, Iowa
    a backfire is usually to much unburnt fuel have you check the idle air control valve (iac), made sure you have good spark on a four cylinder and the plugs aint fouled out, what about the injectors you dont have an injector sticking open dumping fuel do you, what about the o2 sensors, it could be a number of things, any codes, does this happen only after the car is warmed up or only if the car is cold sensors read differently at different temps the sensor may work fine when cold but not at all when warmed up and vise versa kind of need more info here to really help ya LOL
    #6
  7. brassc2007

    brassc2007 Proven Member

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    mason city, Iowa
    another question does it surge at all at idle?
    #7
  8. streetlegalcars

    streetlegalcars Probationary Member

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    Harrison, Michigan
    Iac was replaced. new plugs, wires are fine. one o2 was replaced the codes are listed above.
    Im not able to check injectors.

    im going to remove the vacuum lines and block the egr and see what that does
    #8
  9. brassc2007

    brassc2007 Proven Member

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    mason city, Iowa
    I did a little seaching on here and and i found a few other posts that recommend checking the egr valve and vacuum lines but usually a vacuum line causes a rough idle or a miss. A easy way to check the injectors is to unplug them i mostly use it to see if they are firing but maybe you can do it do see if when you unplug the injector if the backfire goes away it sounds like it is maybe running a little rich to me at idle which may explain why it only does it at idle cause it would be using the fuel under a load. Let us know if you find out anything new.
    #9
  10. streetlegalcars

    streetlegalcars Probationary Member

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    Harrison, Michigan
    The backfire started before the high idle that's why I think there two different problems.
    #10
  11. streetlegalcars

    streetlegalcars Probationary Member

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    Harrison, Michigan
    egr is gone vacuum lines are gone in going to try to check the injectors if that dont work im pulling the intake off
    #11
  12. brassc2007

    brassc2007 Proven Member

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    mason city, Iowa
    any luck with trying the fuel injectors if it is a vacuum leak you can spray some wd-40 around the intake gasket and around the intake and if the idle speed goes up then comes back down when you spray a certian area that is your vacuum leak but dont spray when the motor is hot for safety purposes. to me it seems kinda quick to pull the intake did you check for exhaust leaks
    #12
  13. brassc2007

    brassc2007 Proven Member

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    mason city, Iowa
    a vacuum leak can cause both high idle and backfire depending on the air to fuel ratio
    #13
  14. streetlegalcars

    streetlegalcars Probationary Member

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    Harrison, Michigan
    i havent tried yet my radiator popped a hole in itself
    #14

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