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Thread: Check my logic. Common test method has faulty logic and is therefore inconclusive.

  1. #1
    Join Date
    Dec 2006
    Location
    Atlanta, GA
    Posts
    93
    My Cars
    2008 X3 3.0si

    Check my logic. Common test method has faulty logic and is therefore inconclusive.

    One of my fault codes is P1343- Misfire cylinder 1 with fuel cutoff. OK so "the internet" told me to unplug coil pack #1 and see if the idle changes. I did so and low and behold... no change in idle. And according to some on this and other BMW forums I should deduce that coil pack #1 is bad.

    Really? I think not. Here's my logic. Misfire with fuel cutoff means that the DME has cut the pulse to injector number 1 to save the catalytic converter. Therefore, in this case unplugging a perfectly good coil pack would make no change in idle. Am I right? I don't know. I just made it up. It seems right though. What do you think?

    Now I have been searching for over an hour for specs I can use in checking a coil pack with a multimeter. Plenty of results but no answers. Wow.

  2. #2
    Join Date
    Feb 2017
    Location
    Metairie, LA
    Posts
    596
    My Cars
    2001 325i; 2006 325i
    maybe try swapping the coil on #1 with another coil and see if the miss moves with the coil pack.

  3. #3
    Join Date
    Dec 2013
    Location
    Chicagoland area
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    4,372
    My Cars
    EXOTICS
    I'm not familiar with an exact interval, but if you turn off the ignition,
    upon restart the injector pulses normally, until the number of misfires reaches threshold,
    then fuel cutoff is once again triggered.
    I agree with Tony, do the swap as he suggests, I will only add that you inspect the condition of the sparkplugs also,
    and don't forget to check for oil filled sparkplug wells.

  4. #4
    Join Date
    May 2017
    Location
    Kansas
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    78
    My Cars
    525
    The computer saves the cat by reducing injector timings and hence fuel delivery across all cylinders. It wouldn't do that for just one cylinder. And a misfire code could mean a few things, coil wiring/connector, coil, spark plug, or localized vacuum leak. In the extreme it could even be the ecu. You need to troubleshoot.

    The first thing to do would be to get a coil from the adjacent cylinder, swap it with #1, clear codes, run the engine, and see if you get the code for #2 now. If you do then its the coil for sure. If you still get #1 its probably the plug. Change the plug and if you still get #1 its probably wiring.

    Actually the first thing to do is to visually inspect the wire and plug. Maybe think about any recent repair that could have affected things.

  5. #5
    Join Date
    Jun 2009
    Location
    South Carolina
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    31,073
    My Cars
    2004 330 vert 2004 X5
    The dme isn't cutting fuel to save the cat. It's cutting fuel to prevent damage to the engine.

    The only way to properly test a ignition coil is to connect a scope to it. Using an ohm meter only tests the primary coil. It does nothing to test the secondary coil. The secondary coil is what generates the high voltage for the spark. Since most people don't have a scope your only other choice is to do what the others said. Move the coil and see if the code follows.

  6. #6
    Join Date
    Jul 2007
    Location
    Philadelphia, PA
    Posts
    5,660
    My Cars
    2005 M3 & 2004 330i
    Technically, you're correct, the misfire could be due to something other than the ignition coil. Ignition coil is simply the most common cause of misfire on these cars. The test you did (removing coils until you got no change) is to see which cylinder is misfiring if you don't have a code reader available. Since you were able to read codes, you didn't even need to test that.

    If you want something more conclusive, try swapping the cylinder 1 coil with a different cylinder's. If the misfire moves, then it's the coil. If the misfire stays at cylinder 1, then it's something else.
    Last edited by TerraPhantm; 06-10-2017 at 08:50 AM.

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