Misfire Cylinder 4

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by gjkristi, Dec 16, 2025 at 9:34 AM.

  1. gjkristi

    gjkristi Junior Member

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    I have a 2010 Prius and I'm getting a check engine with Cylinder 4 missfire. Head gasket has been replaced and the engine never goes down so don't think it's that. I replaced the EGR (with toyota new) cleaned the intake and all the pipes. Replaced the spark plugs with new one step hotter plug. Swapped coils 3 and 4. Never see a missfire driving but after a couple weeks check engine light comes on. Anything to try next?
     
    #1 gjkristi, Dec 16, 2025 at 9:34 AM
  2. BiomedO1

    BiomedO1 Senior Member

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    does the problem ever move off of #4?
     
    #2 BiomedO1, Dec 16, 2025 at 10:30 AM
  3. gjkristi

    gjkristi Junior Member

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    No it does not move from #4
     
    #3 gjkristi, Dec 16, 2025 at 12:54 PM
  4. ASRDogman

    ASRDogman Senior Member

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    Switch the #3 an 4 coils. If the problem stays at 4, switch the coils back,
    and switch the spark plugs.
    If it changes, it's the plug.
    Are the plugs from Toyota? Or ebay/amazon..... cheap chinese copies.

    Also, check the wiring harness. It's easy to mess something up without realizing it....

     
    #4 ASRDogman, Dec 16, 2025 at 1:51 PM
  5. BiomedO1

    BiomedO1 Senior Member

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    Well, that kinda makes it easier.
    You need to verify:
    Spark
    compression
    fuel
    in cylinder #4. New spark plugs and moving the coil probably rules out those as a cause, but is that #4 coil getting power or signal? Check the wiring harness for rodent chew marks. Whomever did the head gasket could've loosen up the connection or partially broke the wire.
    Then you need to see if you've got enough compression and fuel in that cylinder. I'd dump some Techron fuel injection cleaner in with your next fill-up and see if there's a compression tester available at your local auto parts store. They'll let you borrow it, if you put down a deposit.
     
    #5 BiomedO1, Dec 16, 2025 at 5:00 PM
  6. gjkristi

    gjkristi Junior Member

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    Already swapped coils no change. The wiring looked OK but will take a closer look. The clips are still there. Thanks Gregg
     
    #6 gjkristi, Dec 17, 2025 at 8:55 AM
  7. gjkristi

    gjkristi Junior Member

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    I did the head gasket and was careful taking it apart. Ran good for a year after head gasket. I can check the wiring again. The clips are good on the connectors and the car stays mainly in the garage. I have a compression gauge to test. I could send some cleaner down the pipe and see if that helps. Maybe I have a dirty injector. They are all original and would only want to replace one with OEM and kind of expensive. Do you think I could see something with the techsteam software? The code only comes on after a week or two. Driving I don't see any miss fires. Thanks for your help Gregg
     
    #7 gjkristi, Dec 17, 2025 at 9:04 AM
  8. ASRDogman

    ASRDogman Senior Member

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    Are the plugs from Toyota? Or ebay/amazon..... cheap chinese copies.

     
    #8 ASRDogman, Dec 17, 2025 at 9:11 AM
  9. BiomedO1

    BiomedO1 Senior Member

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    Yes, Techstream will tell you how often it's missing and look at the fuel trims for each cylinder. That'll tell you a lot on how the engine is performing and don't limit your search to cylinder #4; sometimes the computer gets it wrong. It can't think, just follows instructions you give it.

    Good Luck.....
     
    #9 BiomedO1, Dec 17, 2025 at 9:45 AM
  10. gjkristi

    gjkristi Junior Member

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    They are from a local parts store not ebay or amazon. Never buy elctronic parts from ebay or amazon
     
    #10 gjkristi, Dec 17, 2025 at 10:14 AM
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