Coolant Overflow Tank

[coopns]

02 Dodge Grand Caravan 130k. My coolant overflow tank was low. Can someone give me the dummy version of how that tank interacts with the rad? I did fill it up to min (when cold). I was thinking it draws from the rad. (Didn't know, just trying to be logical). So if I fill up the overflow it doesn't go to rad?

 

[The_Jeff]

If it's not pressurized and the rad has a cap, there's a little flap on the cap that opens when the rad draws vacuum from coolant contracting from cooling down. This flap sucks coolant from the overflow. A pressurized overflow is generally called a "surge tank" and the pressure cap is on it not the rad. You see these more and more with the surge tank being the high spot in the system and the rad down low for aerodynamics.

Conversely when the rad gets hot the coolant expands and goes out the little rubber hose to the overflow.

If the rad has a small leak or the cap a small malfunction, it will lose coolant on expansion but not draw it back when it cools down, as the leak will be a path of less resistance than the cap.

 

[Burt]

engine coolant expands when hot, increases the pressure in the radiator to the point that the radiator cap relieves pressure by opening, allowing the excess coolant to flow into the Overflow reservior. Process reverses itself when the engine cools.

One purpose of the overflow is to provide a buffer, the other is to keep air/oxygen out of the cooling system.

 

[mkIIIman089]

Fill it to the full mark on the tank, they are designed to be to the full line when cold with addition expansion room (on my car its at least 2x the full line) for when the engine warms up and temporarily purges some of the coolant as Burt described.

 

[Hokiefyd]

On the '02 Caravan, the overflow tank is not pressurized.

 

[mechtech2]

In the old days, radiators were NOT filled to the top cold.
There was an inch or two left. When they got hot, the fluid and parts would expand and coolant would simply blow out the vent tube to the ground [after the cap's pressure rating was exceeded].

This was replaced with the common reservoir systems, which are far superior.
The radiator is totally full now, and any expansion is caught in the 'puke' tank. When it cools, it is sucked back into the radiator. Due to evaporation, you may need to add a bit of fluid to the reservoir every 6 months to a year.