expansion tank function

[wengr]

So if I understand correctly, when the cooling system pressure rises beyond the rating of the radiator cap, the cap opens and coolant is forced into the expansion tank. Maybe a dumb question but how then does it return to the radiator when needed? As the system cools, would the cap not close before suction would siphon enough coolant back into the system? Is this a typical function that's happening all the time, or is the expansion tank just a catch in the event of overheating and not intended to self drain?

[baltobernie]

There's a separate spring-loaded valve that allows the coolant to flow back into the radiator. It is sucked back in by the vacuum created in the radiator when the coolant cools. There is fluid movement back and forth between the expansion tank and radiator throughout the typical duty cycle.

[wengr]

thank you Bernie, I did not know about the second valve. that explains everything.