Exhaust Muffler Back Pressure Issue

Discussion in 'Fox 5.0 Mustang Tech' started by MikeH686, Nov 1, 2013.


  1. MikeH686

    MikeH686 Mine is only two inches though.

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    Im having issues in my 91 mustang ever since i dropped my car off to have valves worked on it may be an awkward coincidence but running straight x pipe car runs flawless no surging or anything i put my mac pro dumps on and it immediately hunts at idles from 600-1200 rpms before slowly creeping down and bogging out wondering if a clogged up muffler would do this? Not letting enough air out and all
    #1
  2. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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    pull codes?
    #2
  3. RangerJoe

    RangerJoe Advanced Member

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    Did you weld on the car? I have heard mixed reviews regarding welding on a vehicle and its effects on the electronics if they are left attached. No idea if there is any truth to that.

    Joe
    #3
  4. MikeH686

    MikeH686 Mine is only two inches though.

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    Nope havent pulled codes yet and no i didnt weld on the car at all but ill pull codes this weekend and return to this
    #4
  5. stykthyn

    stykthyn Commander of the snuggie cultists

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    Can you unclamp the muffler and see of that's the case?
    #5
  6. MikeH686

    MikeH686 Mine is only two inches though.

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    What you mean? Im saying with just x pipe it idles fine and all put the mufflers on it bogs itself out
    #6
  7. stykthyn

    stykthyn Commander of the snuggie cultists

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    If everything is good without mufflers on I would think you may have a defective unit.
    #7
  8. MikeH686

    MikeH686 Mine is only two inches though.

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    I think a test i could do is leave it bolted and unbolt them one as a time to see if i cant get it to straighten out
    #8
  9. elarm1

    elarm1 Active Member

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    you need to pull the codes. After you pull the codes you should reset the base idle. That's what I'd do.
    #9
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  10. TOOLOW91

    TOOLOW91 Mustang Master

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    Sounds like maybe the inner baffles blew out of the muffler and are causing a major issue. If you pull the mufflers is there a way to pass light in to see if there is anything in there or shake them and see if you hear metal in there ?
    #10
  11. jozsefsz

    jozsefsz Active Member

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    You could also be running rich at idle (rich mixture or insufficient air from the IAC) and the extra back-pressure of the mufflers is just enough to put the IAC into hunting / surging mode. You might just want to try the whole IAC setting procedure to see if that helps. Clogged mufflers would probably keep you from idling at all.
    #11
  12. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    And the answer is...

    See and

    for a video on how to check exhaust system and catalytic converters that are restrictive or clogged. Watch the last half of the second video for tools and techniques that will work on a 5.0 Mustang.
    #12
  13. MikeH686

    MikeH686 Mine is only two inches though.

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    I pulled the mufflers yesterday and one muffler seems alright bit the other one i shook it and it sounded like i threw changed in there that was after i dropped it to the ground to get it to break loose
    #13
  14. TOOLOW91

    TOOLOW91 Mustang Master

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    Mike even if you dropped I don't think it would make it do that , they are flow masters right ?
    #14
  15. MikeH686

    MikeH686 Mine is only two inches though.

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    Nah macs but i dropped it to the ground to see if i could break something loose when the backfire happened
    #15
  16. MikeH686

    MikeH686 Mine is only two inches though.

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    Yeah forgot to say ive been contemplating what may have happened ... after the valves were worked on the motor builder cranked the motor with the key but left the coil unplugged so steady put fuel in cylinders enough to wjere i had to hold it full throttle to get it to kick over and when it kicked over it back fired real bad
    #16
  17. TOOLOW91

    TOOLOW91 Mustang Master

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    I would swap the mufflers out . I think that back fire is what did it
    #17
  18. MikeH686

    MikeH686 Mine is only two inches though.

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    Yeah me too i got a new set of mac pro dumps just gotta swap em out today
    #18
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  19. MikeH686

    MikeH686 Mine is only two inches though.

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    Well swap didnt happen yet gotta shorten up the lengths a few inches but that shouldnt be to hard to do
    #19
  20. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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    Always sumthin
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