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Thread: Back Fire at Cylinder 3 392 Hemi and Limp Mode

  1. #1
    Tuner in Training
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    Sep 2019
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    Luling Louisiana
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    Back Fire at Cylinder 3 392 Hemi and Limp Mode

    I have been working on this motor for some time and getting it sounding/running better, but I still can't get rid of this backfire. Here is the scenario:
    The truck starts, runs, and upon start it backfires and doesn't stop. The backfire goes from a load pop to a muffled pop. I went through and removed individually and upon getting to cylinder 3 and removing the power to the coil pack the engine stops backfiring. After a minute or less I can replace the power to the coil pack and the motor does not backfire anymore. However, if I shut the truck off and restart the motor the backfire returns. I have replaced all of the coil packs, all the plugs, gapped the plugs, and swapped injectors from 1 to 3 and 3 to 1 and the backfire continues.
    Does anyone have any thoughts? Can the timing be advanced on just one coil or cylinder?

    I appreciate all of the past help and anything going forward.

  2. #2
    Advanced Tuner
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    Mar 2018
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    Have you done a compression test on the motor to confirm it is not mechanical?

  3. #3
    Tuner in Training
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    I checked cylinder 3 today it was 180psi. I was worried about broken spring on a valve, but at 180 I think it is good but I guess I should check the other cylinders. I just don't get the unplug coil power and plug it back and backfire is gone.

  4. #4
    Advanced Tuner
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    Quote Originally Posted by TunaSRT View Post
    I checked cylinder 3 today it was 180psi. I was worried about broken spring on a valve, but at 180 I think it is good but I guess I should check the other cylinders. I just don't get the unplug coil power and plug it back and backfire is gone.
    Always a good idea to check engine compression with a persistent misfire on the same cylinder as it is easy to do. Have seen several guys on the CT forum where they tried to diagnose a misfire only to find out that the compression on that cylinder was either too high/low compared to the rest of the cylinders (usually a sign of bad exhaust/intake lifter or cam lobe). If you unplug and then plug the coil in and backfire disappears it makes me wonder if there is an issue with the PCM?

  5. #5
    Tuner in Training
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    Sep 2019
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    Luling Louisiana
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    Cylinder 3 is the one the backfire has been happening in. The truck is not throwing a code for misfire since changing plugs and coil packs. I just got the obd mx scanner in and I am looking through alphaobd to see what I find.

    If I could get it out of limp mode their may be a chance the computer can learn of the Ann, but in limp I can't get it over 5 miles per hour.