P0301 misfire on cylinder one

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

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  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
     
    #1 danielz000, Nov 23, 2025
  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.
     
    #2 PriusCamper, Nov 23, 2025
  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
     
    #3 danielz000, Nov 23, 2025
  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
     
    #4 danielz000, Feb 8, 2026
  5. bisco

    bisco cookie crumbler

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    #5 bisco, Feb 8, 2026
  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
    Last edited: Feb 8, 2026
  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
    Last edited: Feb 8, 2026
  8. danielz000

    danielz000 Junior Member

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    Bisco, cheers! I think I've cleared the two holes. Hopefully the seal doesn't need replacing too... I hit it with a hose for a bit and it was dry but no idea what prolonged rain will do to it.

    PrimalPrius, thank you for that! I'll be certain to be careful when I change the oil. I can certainly see a little spillage has occurred there in the past. Also, yes it definitely rained, I live in the UK! I did consider some silicone but thought better of it in the end. If water is still getting in there maybe that's what I'll do :)
     
    #8 danielz000, Feb 9, 2026
  9. Mendel Leisk

    Mendel Leisk MMX GEN III

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    The air holds water, it can condensate.
     
    #9 Mendel Leisk, Feb 10, 2026 at 9:14 AM
  10. PrimalPrius

    PrimalPrius Junior Member

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    To me it seems very unlikely that this could result in meaningful amounts of water collecting in the plugwells.
    And of course even much more unlikely that it would happen to only some of the wells and not others.
     
    #10 PrimalPrius, Feb 10, 2026 at 11:01 AM
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  11. danielz000

    danielz000 Junior Member

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    Hi guys,

    So after cleaning the drains out some heavy rain has deposited water in the plug holes again.

    Lifting up the bonnet causes a few droplets which makes tracing the problem tricky but here's what I think is causing the problem...

    The car is parked on the road and the road has camber.

    When I opened up the bonnet the cowl was full of water (up to the brim) where the rubber tube is in the attached photo.

    I think it's filling up and over flowing into the engine bay. The drains are on the other side and would only work if the car is level.

    Has anyone ever seen this before? Is it a known problem?
     

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    #11 danielz000, Feb 15, 2026 at 9:21 AM
  12. BiomedO1

    BiomedO1 Senior Member

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    You need to remove the cowl and clean out all the debris clogging it up. There shouldn't be standing water anywhere. I don't remember reading that you need to park your car in a certain position to prevent water pooling???? I believe it should be draining behind and between the outer fender and door panels. There are also visible drain hole at the bottom of your door panels. You just need to open the door and stick your head under there.
     
    #12 BiomedO1, Feb 15, 2026 at 2:05 PM
  13. danielz000

    danielz000 Junior Member

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    Hi Biomed,

    I'll get another (better) picture of it but you can see in the picture there's literally no holes for the water to drain out of when it's sitting on the camber of the road.

    I'm debating on taking the cowl off and drilling extra holes in to it but need to be careful to make sure that doesn't cause it to drain off somewhere even worse!

    Was hoping there might be a known fix for it or that maybe Toyota updated the part...

    Cheers

    Dan
     
    #13 danielz000, Feb 15, 2026 at 3:05 PM
  14. PrimalPrius

    PrimalPrius Junior Member

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    I have parked my car at all known angles and never had water collect at the cowl.
    As said above, take the cowl apart and clean everything. If you have access to pressurized air and a blow gun that can help. Otherwise use metal wire or metal cable or a scrap of electrical wire to clear all passages. Use a hose or bucket water to verify that you got it flowing.
     
    #14 PrimalPrius, Feb 15, 2026 at 7:13 PM
  15. dolj

    dolj Senior Member

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    Yest you do, and make it landscape rather than portrait. Including the engine bay is of no use in this case.
    That is not necessary. There are two large drains at either end of the tray. The drains are not accessible unless you take off the top part of the cowl if there are large pieces of debris, like leaves and such. I managed to clear mine by using a plastic rod to clear the debris, as it was mostly gathered dust and grit, so I could stir it up and get it through the drains. The design is such that most water will drain off from one side or the other, regardless of which way the car is angled. Clear the drains (as already suggested), and you will be all sorted.
     
    #15 dolj, Feb 15, 2026 at 11:00 PM
  16. danielz000

    danielz000 Junior Member

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    As mentioned, the drains behind the cowls are clear and water that lands either side of the cowl drains out behind the wheel arches without a problem.

    For anyone curious... Here's the water puddling in the cowl due to the camber of the street.

    From there it drips down directly on to spark plug 4 but then trickles down into the cylinder 1 spark plug hole.

    As a temporary solution I've drilled 3 little holes into each cowl, but I'm not sure if that'll be enough during heavy rain.

     
    #16 danielz000, Feb 16, 2026 at 11:25 AM
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  17. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Maybe the difference in propensity for pooling water, North America versus Great Britain, is due to the left/right differences. There is some rearrangement of engine bay components, and wipers, IIRC.
     
    #17 Mendel Leisk, Feb 16, 2026 at 11:28 AM
  18. PrimalPrius

    PrimalPrius Junior Member

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    The problem I see with that is that in Japan they also drive on the "wrong" side of the road, so UK vehicles would/should not have been an "afterthought" to the original designers.

    Seems like a hole through the "divider" or a "notch" at the top could be a solution. A notch at the top would still leave plenty of water splashing there and spilling down.
    I don't like "hack" solutions that can leave water leaking into unintended places with unintended consequences. with that said, I would be tempted to drill a hole though the divider near the bottom that is a tight fit (interference fit) for a short piece of hose, preferably black to make it look somewhat factory.
     
    #18 PrimalPrius, Feb 16, 2026 at 12:11 PM
    Last edited: Feb 16, 2026 at 12:17 PM
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  19. PrimalPrius

    PrimalPrius Junior Member

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    Sorry that I didn't believe you. I expected that Toyota engineers would have had this.
     
    #19 PrimalPrius, Feb 16, 2026 at 12:12 PM
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  20. danielz000

    danielz000 Junior Member

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    Cheers guys! The (UK) passenger side of the cowl came off without any issues which allowed me to look at what was behind the cowl... Basically a large metal gutter... Nothing sensitive there and it's designed to drain water away... So based on that, I drilled 3 5mm holes into each cowl.

    I just can't believe there's no talk of this anywhere on the internet and no one on here seemed to know about it.

    Also, as PrimalPrius said... You'd think the Toyota engineers would have had this under control.

    It's only been fairly typical UK weather here and all streets have camber...

    What a weird situation.

    Anyway, I guess the car will be good again until them holes get blocked up!
     

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    #20 danielz000, Feb 16, 2026 at 12:41 PM
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