P0301 misfire on cylinder one

Discussion in 'Gen 2 Prius Main Forum' started by danielz000, Nov 23, 2025.

  1. danielz000

    danielz000 Junior Member

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    Hey guys

    A few months ago I had some rough running and p0301 on my engine.

    I replaced all four spark plugs and the car has been doing ok since.

    When I replaced the spark plugs I *think* I saw some liquid in the spark plug hole for cylinder 1...

    I also *think* I swapped the cylinder 1 coil on to cylinder 3...

    The other day the car started to run rough and p0301 came back. The car's not losing any noticeable amount of oil or coolant and still returns +50mpg.

    Today I've checked cylinder 1 and it seems there was water in it again, about to the level of the top of the spark plug nut (the coil plug came out dry)...

    I'm thinking the water is a red herring (some how passing the coil gasket/boot) and unlikely to cause a problem.

    My thoughts are to replace the coil packs and see if this helps but I thought I'd check here to see if anyone has any thoughts or similar experience?

    It's a well looked after 2005 with 135k on the clock.

    Cheers

    Dan
     
  2. PriusCamper

    PriusCamper Senior Member

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    If the liquid was oil and not water I'd say you need to replace your valve cover gasket. But that symptom doesn't cause a misfire code. Does the vehicle have a history of running low on oil? Sometimes crankshaft bearing failure can cause a misfire code.

    At this point seems like a compression tests and borescope inspection is the next step.
     
  3. danielz000

    danielz000 Junior Member

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    Hi Camper,

    I just checked the tissues I dried it out with. It's definitely water. I don't believe coolant runs through the valve cover so I can only assume it's rain water some how... It's also got no real smell or colour to it.

    There's no history of coolant, oil or performance loss.

    I'll keep a compression test in mind. I'd need to find a friendly/decent garage for that tho.

    Cheers

    Dan
     
  4. danielz000

    danielz000 Junior Member

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    Hi all. I'm under the bonnet again and cylinder 1 is full of water again. I didn't clean it out long ago. Cylinder 2 also had some. I'm cleaning the mating surfaces with the hope the coil gaskets will seal but can anyone tell me how rain water is getting in there? It's definitely not oil (it evaporated real quick, and I'm not losing coolant). Cheers Dan
     
  5. bisco

    bisco cookie crumbler

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  6. PrimalPrius

    PrimalPrius Junior Member

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    In that situation first step is to just dry all the oil and see if it reappears. Plugwells 1 and 2 can end up with oil in them when oil is carelessly added to the engine.
    (If one does ALL their own maintenance, particularly ALL oil changes and top-offs, then it's of course possible to rule that out).
     
    #6 PrimalPrius, Feb 8, 2026 at 4:00 PM
    Last edited: Feb 8, 2026 at 4:06 PM
  7. PrimalPrius

    PrimalPrius Junior Member

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    Did it rain since you last dried them? If it's just clear water, then fixing it should require just finding out where all cowl area drain paths are and cleaning them out.

    Also, cleaning the mating surfaces is not going to stop water when it can't stop oil.
    If you wanted a temporary "fix", you could put RTV on the surface on the coil that meets the top of the valve cover.*
    *I never recommend temporary fixes, but I do them all the time for things that are not of primary focus to me.

    Just in case, to change coils and plugs on a Gen 2, none of the cowl related pieces need to be removed, only the fuse box (2x bolts that take a 10mm socket) needs to be pulled aside. A correct length plug socket is required, of course.
     
    #7 PrimalPrius, Feb 8, 2026 at 4:04 PM
    Last edited: Feb 8, 2026 at 5:33 PM