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Thread: I can't Kill this backfire and it's driving me BONKERS!!!

  1. #1
    Fuel Injected!
    Join Date
    Mar 2019
    Location
    Camarillo, CA
    Age
    42
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    61

    I can't Kill this backfire and it's driving me BONKERS!!!

    AMC 401, warmed over, 454 TBI, 16197427, $0E, Dizzy locked at 0, computer controlled timing, brand new MSD TBI coil, brand new Taylor 8mm spiral core wires, brand new Autolite Copper core plugs (gapped at .0454), New brass terminal Duraspark wide cap, new brass terminal rotor.

    My truck runs AMAZING when it is stone cold and going less than 50MPH. If I accelerate to get on the freeway, at about 60 or 65 it starts to backfire out the exhaust. It has been doing this since my rebuild finished (including the installation of my EFI). I have replaced rotor, cap, wires, plugs, and coil. When it is warm, under steady throttle, at 55 or so, it will backfire and run pretty poorly. I started with BMHK bin, made some minor changes and run the stock VE and timing tables. Then I tried to lean out the VE tables a little, back the timing off a little, and no change.

    1) Could I have a Mechanical engine problem causing this? Cam timing? pushrods too long? obstruction in my intake? leaks out my injectors?
    2) How do I look for issues in my data logs? My oxygen sesnor readings swing like they should be (I think??) my timing looks smooth and where it should be...
    3) I plugged my PCV port for now. I plugged all the other vacuum ports for now, other than trans modulator and brake booster.
    4) I don't hear any exhaust leaks... My 4 wire O2 Sensor is in my header collector on the passenger side (long tube headers), going to a 3"Y-pipe and out through a single 3" exhaust, no cat, but I do have a Borla Turbo muffler.

    I am running out of ideas. I am starting to think I need to pull my EFI and go back to carb... Not sure where to go! This is killing me!!!!

  2. #2
    Fuel Injected! vilefly's Avatar
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    Sep 2017
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    One "fun" thing I experienced working on this tbi setup is that when the distributor pickup coil goes bad, it does it when hot, and causes lots of spark timing scatter. To test for this, get it hot enough for the symptom to show up, then disconnect the "set timing" connnector with the tan/black colored wire, and see if it goes away. You might even go a step further and set base timing to 8-12 degrees with the tan/blk wire disconnected and drive it to see if it likes it.
    The only other thing I can think of right now is exhaust leak that is sucking in air.

  3. #3
    Fuel Injected!
    Join Date
    Sep 2018
    Posts
    63
    Hi rang a stang

    spark advance tables of 304 and 401 are nearly the same (pic spark1 and spark2), if you want you may try my sparktable.
    In tunerpro i see nearly no knocks , while in trimalyzer there are small knocks.
    But this could be caused by the placement of the knock sensors!
    BTW. Have you already checked your fuel suply (fuel pump,filter)?
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    Last edited by mecanicus; 09-06-2019 at 11:35 PM.

  4. #4
    Fuel Injected!
    Join Date
    Mar 2019
    Location
    Camarillo, CA
    Age
    42
    Posts
    61
    Great info, guys! I am heading out on a business trip and will not be able to tinker until I get home. I will try your spark table soon, mecanicus.

    Yeah, fuel pressure is solid 12PSI.

  5. #5
    Fuel Injected!
    Join Date
    Mar 2019
    Location
    Camarillo, CA
    Age
    42
    Posts
    61
    It was distributor phasing! I had locked down my pickup in the wrong spot. I moved it about 10 degrees and no more backfiring! Woo Hoo!!!

  6. #6
    Fuel Injected! vilefly's Avatar
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    Quick! To the paint marker! Attack...attack....attack!

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