Back in my cylinder head

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by LazuliNyx, Nov 3, 2025 at 11:06 AM.

  1. LazuliNyx

    LazuliNyx Member

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    changed my headgasket earlier this year 15k miles ago. I’ve been having oil consumption and began getting a P0304 at high acceleration. I pulled the plugs and found oil fouling on cyl 4 and slight fouling on cyl 3. I opened the valve cover to check the valve stem seals. This is the one on the left intake valve for cylinder 4, viewed through a cheap borescope for angle. I can’t tell if it’s torn/deformed and I’m trying to tell if there’s a better way to check it without pulling the head off again. Thank you!
     

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  2. ASRDogman

    ASRDogman Senior Member

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    Hard to tell from that photo.
    How many total miles on the engine? Oil burning is an issue with these engines.

    The only way to fix it correctly is to replace the pistons and rings with the updated ones.
    Or a short block, or engine...
     
  3. LazuliNyx

    LazuliNyx Member

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    Mhmm. 192k miles. It could be pistons and I’m concerned, however I strongly suspect the HG job was the trigger for the oil consumption. I was able to check the seating of the valve stem seals and seemed ok.
     
  4. BiomedO1

    BiomedO1 Senior Member

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    On a gen3; oil consumption is usually a problem with the low tension piston rings - though the valve seals are suspect. Whomever did the head should've replaced them as a preventive measure while it was out.
    Bottom-line, the oil can be coming from the bottom-up or the valve seals, top down. Since we don't know the history; you'll need to figure that out before proceeding. You can't always determine an issue by visual inspection ONLY.

    Good Luck
     
    #4 BiomedO1, Nov 3, 2025 at 12:29 PM
    Last edited: Nov 3, 2025 at 12:39 PM
  5. ASRDogman

    ASRDogman Senior Member

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    Were you NOT burning oil BEFORE the head gasket replacement?
    If it's leaking afterward, it's likely a seal or "O" ring was missed.

    192,000 is a lot, and depending on how often you changed the oil, and how fast/hard you
    drive, consumption varies.

     
  6. LazuliNyx

    LazuliNyx Member

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    I pu
    hi I did replace them when I did the head. They seemed to be in place but I have concerns now. For example one tore when I put the valves in so I had to get more.
     
  7. BiomedO1

    BiomedO1 Senior Member

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    Valve stem seals are usually pretty robust and takes a LOT of effort to cut or tear. None of the valve stem ends should be sharp enough to do that; otherwise they should've been wire wheeled to smooth out those burrs and sharp edges before getting pushed through the head. A very light touch of assembly grease would also help.

    Just my opinion and the way I do things......
     
  8. ASRDogman

    ASRDogman Senior Member

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    Using a small socket and small hammer to "tap" them on helps. But usually pressing them
    is enough.
    Even if they were leaking, it wouldn't be as much as the weak oil rings...

    How is your driving and how much are you using between the 5000 mile oil changes?

     
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  9. LazuliNyx

    LazuliNyx Member

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    So, I found a problem too where the oil drain plug was leaking with the last oil change. I thought i torqued it right, but it backed out a little bit, oops. I'm estimating 3/4 quart every 1000 miles from the previous change. If it was the piston rings, would it be relatively even or in one or two cylinders? Also, I have an oil catch can, does not pickup anywhere near that much oil so it's not all going out the PCV.

    Regarding driving, I drive hard, often going 80 on the freeway. So I know I'm a likely candidate for oil consumption. Might have to do things a little lighter on the pedal.
     
  10. BiomedO1

    BiomedO1 Senior Member

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    and use an old seal as a strike plate - NEVER hammer directly onto a seal. Professionals can do that because they know, by experience and feel, how much force they can place on it, without damaging or deforming it. For someone that doesn't do this kind of work all the time, it's easy to 'mess it up'. The dark side of YouTube videos, where seeing it done verses doing it yourself can yield vastly different results...
     
  11. ASRDogman

    ASRDogman Senior Member

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    If you drove 65mph, you'll really reduce the oil consumption. Even 70 would help.

    You should check for other leaks too.
    Are you 100% the gasket was in place correctly? If it's pinched, you could be leaking oil
    between those two cylinders.

    But I would guess it's the 80mph that is really sucking oil.

    Does the engine run well?

     
  12. PriusCamper

    PriusCamper Senior Member

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    Early year Gen3 they choose ultra high MPG piston rings that turned out to have a much shorter lifespan than expected and they corrected it in the last years of Gen3 2014-2015. I'm bummed you didn't have this info before the headgasket replacement decision.