Misfire Cylinder 4

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

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  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
  11. ASRDogman

    ASRDogman Senior Member

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    I bought ngk plugs from autozone, because they didn't have the bosch ones I usually get.
    2 failed in less than 1000 miles......
    It's easy to switch the coil packs, and just slightly more work to switch the plugs....

     
    #11 ASRDogman, Dec 17, 2025 at 11:28 AM
  12. ChapmanF

    ChapmanF Senior Member

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    I had heard some newer Prius models can calculate fuel trims for each cylinder (amazing tech, that).

    I don't think gen 3 could, though. Just a long-term and a short-term trim for the engine overall.
     
    #12 ChapmanF, Dec 17, 2025 at 11:28 AM
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  13. gjkristi

    gjkristi Junior Member

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    I didn't see independant fuel trims only all cylinders (gen 3) This weekend I will do a compression test on 3 and 4 to see if there's a difference in cylinders. Also put a camera down it to see if I see if there are any issues. 3 days of driving and see 3 miss fires on #4. Maybe just on start ups throws a missfire.
     
    #13 gjkristi, Dec 19, 2025 at 3:01 PM
  14. ASRDogman

    ASRDogman Senior Member

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    :cautious:(n) Wonder why it's called a "miss fire"? :whistle::eek::rolleyes:o_O
     
    #14 ASRDogman, Dec 19, 2025 at 3:11 PM
  15. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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    Try OEM plugs from the stealer.
    I almost never give that advice but it's a fairly good way to avoid knock-off plugs.

    If you know somebody with a borescope you may want to ask nicely if you can borrow it to look inside the hole.
     
    #15 ETC(SS), Dec 19, 2025 at 5:54 PM
  16. ASRDogman

    ASRDogman Senior Member

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    The dealership isn't holding a gun on you to buy from them.
    You don't have to purchase anything from them...
     
    #16 ASRDogman, Dec 19, 2025 at 6:22 PM
  17. gjkristi

    gjkristi Junior Member

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    Threw a check engine on #4 again. I think it comes from start ups. Make a slight rattle noise during the first few seconds. This weekend did a compression test on 4 and compared to 2 and 3 and all 3 looked the same about 85 to 90. also swapped injector 3 and 4 to see if problem moves.
     
    #17 gjkristi, Dec 21, 2025 at 2:15 PM
  18. ASRDogman

    ASRDogman Senior Member

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    If the spark plug doesn't first, the computer will see it as a misfire, whether it's the plug
    or coil. And the plug won't fire if there is coolant in the cylinder.
    I know, you did the head gasket.... just something to look into...
     
    #18 ASRDogman, Dec 21, 2025 at 5:39 PM
  19. gjkristi

    gjkristi Junior Member

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    Update. the PSI's are not correct, it was a new compression tester that I need to return. I tested it against my compressor which was 120 lbs and the gauge showed only 60lbs. So I believe the compression is OK.
     
    #19 gjkristi, Dec 21, 2025 at 8:41 PM
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  20. ASRDogman

    ASRDogman Senior Member

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    You could have 0 compression, and NOT get a misfire.....
     
    #20 ASRDogman, Dec 21, 2025 at 8:44 PM
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