Can somebody please tell me exactly what the coolant overflow bottle does, and what actually happens. Is coolant supposed to come out of the cooling system into the overflow bottle, or the other way around? What happens after that? Is it supposed to get sucked back in eventually, or spit back out? If so, when does this happen, and how much of the coolant actually goes back in or out?

 

Pretty much as the name says, usually when you start overheating to a certain point the fluid will overflow into the coolant overflow.

 

So if the car doesn't overheat, coolant should never enter or leave the overflow bottle?

 

As the coolant heats up, it expands. Once the pressure exceeds you rad. caps spring, excess air/coolant goes into the overflow bottle. As engine cools back down the coolant inside the engine contracts and sucks back in coolant from overflow bottle.

If you car overheats badly or blows a headgasket, excess pressure will go straight to the overflow bottle and cause ALOT of bubbles/burps.