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Intermittent Cylinder Misfire

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  • SQLGeek

    Muh state lines
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    The past month or two we've been getting an intermittent Cylinder Misfire in my 2011 Expedition with the 5.4 L engine. No CEL comes on and mostly does so at idle. The truck has about 120,000 on the clock.

    Last year we had the fuel pump and replaced and spark plugs replaced

    Took it in to the shop and they couldn't get anything to come up and there aren't any codes being thrown so we took it back.

    My wife said it happened again yesterday when she was low on fuel. Filled up, drove it for a bit and she said it happened a couple more times. I drove it last night and today and haven't seen it come up again. I have been in the truck when it misfires so I know she's not imagining things. Our fuel economy is also down about 1 MPG on average.

    I'm thinking the coil packs could be a culprit. I'd like to just forget about it but I have a feeling it'll come back, especially when the tank is low.

    Anybody have any thoughts on what to do or what to check?
    Guns International
     

    Bozz10mm

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    Interesting that you have a misfire, but no CEL. A bad coil pack should trigger the CEL, at least it did on mine. Look for a vacuum leak, that would cause it to run rough, especially at idle.
     

    SQLGeek

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    Hah!

    Bozz, agree it is weird. When the misfire was occurring last year due to the bad fuel pump, the CEL would flash indicating as such.

    This time the feeling is the same, a lurch in the engine every once in a while but no CEL. Its not really a continuous rough idle like a bad vacuum line.
     

    toddnjoyce

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    PCM module. Increased resistance of the new spark plugs is frying that bastard.

    ETA: it will die suddenly and will need to be replaced.
     
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    Brains

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    It's statistically never the PCM, don't bother looking there. It COULD be a coil pack, but also unlikely. EGR would likely show up as a random multiple misfire with a flashing check engine light. If there's no codes at all, the misfire might not last long enough to trip the test. You'll probably want a scan tool so you can watch the individual cylinder misfire counts in real time.

    Once you've isolated the cylinder, check the small parts first. Plug, wire, injector plug, etc.
     

    SQLGeek

    Muh state lines
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    I figured I'd have to get a scan tool on it to diagnose this further. So I'm going into some uncharted territory here having never used one but I want to try and figure this out myself.

    What kind of reader would I need to pick this up and how do I go about using it? Leave it hooked up while I drive in hopes that the misfire occurs? Will the code(s) be stored so that I can read them after the fact?

    I have tried searching on some of this but the searches I get back seem to be all over the place. I've never had great luck doing internet research to diagnose car issues unfortunately.
     

    Brains

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    Also:

    This came up as a lightning deal on Amazon so I might go ahead and claim it if you all think it's worthwhile?

    The reviews seem pretty good:

    https://smile.amazon.com/FOXWELL-NT301-Professional-Enhanced-Diagnostic/dp/B00UJV3E12
    From the bullet points it looks to support what you need (real time data). You'll want to drive and chart the misfire count for each cylinder. It's normal to see some here and there, but if there's a large number on any one cylinder that's where you start looking. Keep in mind the count may not get high enough to ever set a code. There's a timer running in the PCM, and if the timer resets before the count gets high enough it simply clears the misfire counter and keeps going.

    I guess I’m the statistical anomaly, then.
    Or you replaced a perfectly good PCM when something else was the actual problem. I've seen that more times I can count.
     

    SQLGeek

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    From the bullet points it looks to support what you need (real time data). You'll want to drive and chart the misfire count for each cylinder. It's normal to see some here and there, but if there's a large number on any one cylinder that's where you start looking. Keep in mind the count may not get high enough to ever set a code. There's a timer running in the PCM, and if the timer resets before the count gets high enough it simply clears the misfire counter and keeps going.

    Awesome. Thanks for the info!
     

    toddnjoyce

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    Or you replaced a perfectly good PCM when something else was the actual problem. I've seen that more times I can count.

    Unfortunately, not in my case. 05 Escape 3.0L. New plugs, new COPs installed, PCM board burned within a month. This was after the 2009 TSB that said replace spark plugs and before the 2010 TSB that said new plugs and different COPs.

    Ours was one of the reasons for the 2010 TSB; the OEM coil packs were not shielded appropriately.
     

    Brains

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    That's a very isolated case on a different vehicle than the OP. It's safe to assume the same problem hasn't become prevalent with the number of mod motors on the road not exhibiting a similar failure.

    Splitting hairs, the PCM is still repairable. It only takes out the coil drivers. A little past shade tree mechanic perhaps, but not too tough to fix.
     

    cbigclarke

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    It's statistically never the PCM, don't bother looking there. It COULD be a coil pack, but also unlikely. EGR would likely show up as a random multiple misfire with a flashing check engine light. If there's no codes at all, the misfire might not last long enough to trip the test. You'll probably want a scan tool so you can watch the individual cylinder misfire counts in real time.

    Once you've isolated the cylinder, check the small parts first. Plug, wire, injector plug, etc.

    That sounds like work


    Sent from my iPad using Tapatalk
     
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