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Prius C engine cooling related question

Discussion in 'Prius c Care, Maintenance and Troubleshooting' started by z80, Sep 11, 2022.

  1. z80

    z80 Junior Member

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    Hello!

    I've realized that I don't understand how Prius C's engine cooling works exactly. My concern is I don't understand the piece which includes upper radiator hose, radiator cap, and the expansion tank.

    I would appreciate any explanation! Please see the drawing below.

    The expansion tank cap doesn't seem to be designed to hold any pressure at all. How does it happen that the expansion tank is not under pressure if it is connected to the radiator cap area (which is under pressure) via the thin hose?

    Thank you!
    engine_cooling_scheme.jpg
     
  2. ChapmanF

    ChapmanF Senior Member

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    Prius c differs from the other Gen 3 Prii in that it still has the older-style "overflow" or "expansion" bottle, while the Gen 3 liftback and Prius v have the newer-style "degas" bottle that operates under pressure.

    In an overflow or expansion system, the radiator cap is where the system pressure ends. Its rubber gasket presses down on a neck inside of that fitting, so only points below that (the hoses to the radiator and to the engine) are under pressure.

    The non-clamped hose to the bottle attaches just beneath the cap, above that neck, where the pressure is atmospheric. The pressure in the bottle is atmospheric. When coolant in the pressurized system expands and pushes out from under the rubber gasket in the radiator cap, it flows (just at atmospheric pressure now) into the bottle.

    When the engine cools and the coolant contracts, a lightly-sprung vacuum-relief disc in the radiator cap opens, and allows the coolant from the bottle to be drawn back in. The bottle doesn't need a fancy cap, because the coolant in it is never under pressure.

    In the degas-style systems of the liftback and the Prius v, the degas bottle is a full-fledged part of the cooling system; the coolant circulates right through it, and it holds the same pressure as the rest of the system. It has enough headspace for any bubbles in the rest of the system to end up there, after being delivered through the skinny return hoses from some high points in the system. So in the degas systems, the cap on the bottle is a pressure cap.
     
  3. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    The missing link is that the cap itself contains a one-way pressure valve, and the filler neck bore to the expansion tank fitting is carefully placed past the pressure seal of the cap.

    So as the engine warms up and the coolant expands, the cap allows excess volume to expand into the overflow bottle.

    After the car cools down, the coolant is drawn back through that hose. Trick one-way valve means no pressure needed for return trip.
     
  4. ivanhoe

    ivanhoe Member

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    I'm planning of changing my daughter's 2014 Prius C coolant in her car and hoping the process goes without any hiccups, will I have any problems burping the system?