2018 Toyota Prius 4 coolant leaking, water sloshing and strange noise in coolant reservoir

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by dv.1000, Dec 8, 2025 at 12:13 AM.

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  1. dv.1000

    dv.1000 New Member

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    Vehicle:
    2018 Prius
    Model:
    Three
    Hello,

    I need help troubleshooting a coolant leak on my 2018 Toyota Prius. Since I bought it used, the coolant has been leaking. It originally leaked into the engine oil due to high pressure, but after removing the reservoir cap O-ring, that issue stopped.


    Now the coolant keeps leaking—possibly from the reservoir or somewhere else. Today, after a long freeway drive, the engine overheated light came up. I added about a gallon of distilled water, but it kept pushing out of the reservoir, and I heard a strange noise that might be coming from the water pump.
    I recorded a video but unable to upload here, and it won’t let me post a YouTube link either.


    Any advice would be appreciated. Thank you.
     
    #1 dv.1000, Dec 8, 2025 at 12:13 AM
  2. syncro87

    syncro87 Member

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    Vehicle:
    2018 Prius
    Model:
    Two
    Your description sounds very similar to what I experienced with a Volvo I used to own. In that case, it was a blown head gasket. During the compression phase of the cycle, combustion gases are pushed past the failed portion of the head gasket, allowing said gases to push into the cooling system, causing overflow. It would make sense to me that removing the coolant cap o-ring alleviates the symptoms, as it provides a pressure release pathway.

    With a head gasket leak, you’ll often notice coolant in the oil as well. Once the engine is turned off, the residual pressure in the cooling system will often push coolant the opposite direction and into the combustion chamber chamber (cylinder). You’ll then see a milky apprearance to your engine oil which is at that point mixed with coolant.

    This all being said, hard to diagnose a car over the internet, so this is just what occurs to me to be likely based on your description. I don’t recall the 4th gen Prius being particularly known for head gasket failure like some of the prior gens were. Usually on a 4th gen, if you start losing coolant, it is the heat exchanger under the car where the exhaust gases heat the engine coolant up to operating temp more quickly. If I recall, the 4th gen’s are somewhat known for this failure point. I would venture a guess that this is not what you have going on due to the fact that the heat exchanger failure would likely not result in your overflow situation, since there wouldn’t be pressure introduced into the cooing system at that point. You’d just be experiencing coolant loss.

    If I was a gambler, which I’m not, I’d bet on a head gasket issue purely based on your description of events, which is pretty classic head gasket leak. The water pump might be making noise due to gas being in the system, so you have some cavitation going on.

    Just my 2c. Maybe someone more in the know than myself will chime in with better info.
     
    #2 syncro87, Dec 8, 2025 at 4:08 AM
  3. Brian1954

    Brian1954 Senior Member

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    It is time for a replacement engine!

    How many miles on the car?
     
    #3 Brian1954, Dec 8, 2025 at 8:57 AM
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