Overflow hose from the coolant expansion tank goes where?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by pasadena_commut, Mar 27, 2025.

  1. pasadena_commut

    pasadena_commut Senior Member

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    At the radiator neck there is a small black hose which goes down and then into the bottom of the coolant expansion tank. That is, the big white one right behind the radiator. This I understand - as the coolant expands with heat some coolant will go into the tank, and when it cools down it will be sucked back into the radiator.

    On the coolant expansion tank itself, just below its cap, there is another small black hose which goes towards the driver's side and then turns down. It seems to be attached somehow to the radiator. What exactly is going on there? That tube cannot be on a nipple that connects to the inside of the radiator as that would defeat the purpose of the cap on the radiator. Is the end of the hose just clipped to something solid so that if the expansion tank also overflows (which could happen in an extreme over heating event) the coolant will dump out in a preferred location? Why would that spot be the preferred location, rather than extending the hose all the way to the bottom of the car so that it doesn't spray the inside of the engine compartment?
     
    #1 pasadena_commut, Mar 27, 2025
  2. MAX2

    MAX2 Senior Member

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    Are you describing this branch from the expansion tank?

    Yellow arrow indicator.


    This is a drain from overflowing the tank, when the coolant from heating fills the entire space, some of it will freely leave the tank. When it cools down, air will enter through this tube. If the tube is lowered to the ground, then apparently it can become dirty from road dust and dirt and not perform its functions.
     
    #2 MAX2, Mar 27, 2025
  3. Tombukt2

    Tombukt2 Senior Member

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    In the US we call it a blow off tube in the shop it's where the coolant will blow off when the idiot who owns the car overfills the reservoir tank not knowing that there's crap in the radiator that's going to blow off into the tank and then onto his driveway. Essentially that is where people see shiny liquid all over their plastics under the hood of the car because this is blown off and squirted right down on the plastic and now they're looking for a leak It's to make people ask questions It ends if you just wiggle that tube you'll see it's in a plastic clip and it dies it's over it's cut it's just sitting there squirting between the fans right down to the plastic or the core support or however you like to see things.
     
    #3 Tombukt2, Mar 27, 2025
  4. pasadena_commut

    pasadena_commut Senior Member

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    Yes that's the tube. It isn't exactly in between the fans though, it slightly overlaps the one on the picture on the left. So if fluid is coming through there when the fans are on, or the car is on the highway, it will be blown, under the inverter and on top of the transaxle? Yes?

    We just had thread about a mystery coolant leak in exactly that location

    Coolant leak.. can't find the culprit. Please help! | PriusChat

    Possibility?
     
    #4 pasadena_commut, Mar 27, 2025
  5. MAX2

    MAX2 Senior Member

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    In a normally filled radiator and a normally functioning cooling system, nothing will drip or flow from this tube.
     
    #5 MAX2, Mar 27, 2025
  6. pasadena_commut

    pasadena_commut Senior Member

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    Agreed.

    What if the radiator cap is opening at too low a pressure? Problem might only show up during a long highway drive.
     
    #6 pasadena_commut, Mar 27, 2025
  7. MAX2

    MAX2 Senior Member

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    #7 MAX2, Mar 27, 2025
  8. Tombukt2

    Tombukt2 Senior Member

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    Yeah it's usually overfilling but note this when you're filling your radiator and you're trying to do it without spilling and making a mess and you don't have the fancy air business air funnel whatever it is I disconnect that tube from the radiator neck and put a cap on it because when I'm feeling any water that drips into that little trial for around the neck runs into the overflow tank thereby time you get this kind of car filled up you'll have the radiator overflow tank full to the top of its neck I've done it two or three times so now I pull that tube put my little cap on that radiator tube get everything full everything working right driven around for a while all that Make sure I'm good then I connect that tube to the filler neck.
     
    #8 Tombukt2, Mar 27, 2025
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