Cylinder Misfire or problems with head gasket

Discussion in 'Gen 3 Prius Technical Discussion' started by Mellyman666, Aug 28, 2025 at 10:13 AM.

  1. Mellyman666

    Mellyman666 Member

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    Hi!

    I may be jumping the gun here but just wanted so expert opinions.

    I have a 2010 Gen 3 Prius. It had a blown head gasket 2 years ago, which was completely changed as a DIY (with supervision and help from a toyota mechanic, especially for the timing). The job was completed 2 years ago and I haven't had any trouble since with regards to engine issues and no longer had signs (car losing acceleration, kicking noises in engine etc). 2010 Prius with coolant and spark plug issue | PriusChat

    Yesterday, I got a P0300, P0301, and P0302 code. I have no signs of a cylinder misfire or worse (i.e signs I had at first cylinder misfire and eventual head gasket problem). It also says I have the P0300 code in permanence.
    Today, I'm gonna start looking at spark plugs, fuel injectors etc all the normal basic stuff potentially leading to a cylinder misfire. But I'm starting to get a tad paranoid and scared that it's going to be another HG replacement. So I wanted to know is it possible or common for a HG to fail twice? Are cylinder misfires commonly related to HG replacements 2 years after, or should I treat it as if it's just a cylinder misfire and not even think of the HG seeing as it was completely redone?

    When I changed the head gasket I also changed, amongst other things, :
    -all other gaskets in the felpro big HG kit
    -all spark plugs
    -1 fuel injector (because I lost the little black rubber piece)- I do not remember if it was a Denso or a fake
    -cleaned EGR
    -cleaned intake

    HG failure was at 133k KMs; I'm currently at 158k KMs.

    Oil from oil dipstick looks perfectly fine and antifreeze level is normal; no loss.


    Also want to add, a few months ago, I had an O2 sensor code that appeared on a really hot and humid day, and went away on it's own the next day and never came back. Was reading that a cylinder misfire can sometimes be related to O2 sensors so thought I'd mention.

    Thank you so much, I always appreciate the great help I receive here
     
    #1 Mellyman666, Aug 28, 2025 at 10:13 AM
    Last edited: Aug 28, 2025 at 10:44 AM
  2. Mendel Leisk

    Mendel Leisk MMX GEN III

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    I'd swap the coils/plugs around, see if the code follows. If not, borescope the combustion chambers, morning after a previous day driving. Preferably with cooling system pressurized.

    Make-shift coolant pressurizing gizmo:

    IMG_1378.jpeg
    IMG_1379.jpeg

    Also check condition of plugs.
     
    #2 Mendel Leisk, Aug 28, 2025 at 1:47 PM
  3. bisco

    bisco cookie crumbler

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    did you make sure nothing was warped?
     
    #3 bisco, Aug 28, 2025 at 1:49 PM
  4. ASRDogman

    ASRDogman Senior Member

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    "I do not remember if it was a Denso or a fake"

    You should find out. It could be the issue.
    You could always take the "O" rings and seals from the new one and put it on
    the original one.

    What about the spark plugs? Where did you get them?
    If they are the cheap chinese copies, they are likely bad.

    If you aren't loosing coolant, the head gasket is likely okay.

    Did you make sure the head was not warped?

     
    #4 ASRDogman, Aug 28, 2025 at 4:39 PM
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