Settle a coolant debate for me

[SDHNTR]

So when a radiator overheats I know it sends excess coolant into the overflow bottle, but if the radiator was low, would it also suck the necessary amount from the bottle? I understood it to be an overflow bottle only.

 

[00328]

It will pull coolant from the overflow bottle if it is needed.

 

[cztheman64]

it somewhat depends on what rad cap you have on it. most only allow it to be an over flow only, but it is supposed to pull back what it needs.

 

[oldskoolvws]

it somewhat depends on what rad cap you have on it. most only allow it to be an over flow only, but it is supposed to pull back what it needs.

This is correct. You need to ensure that your radiator cap does both overflow and suction. That is the key.

 

[SDHNTR]

And how would one know?

 

[00328]

Easy, look at your overflow bottle when your engine is stone cold, it will be lower than when your engine is hot.
When the coolant is hot, it expands and the extra fills the bottle. Likewise, when the engine is cold it pulls coolant from the bottle back into the engine.