Water out of over flow

[bitwrx]

Important to have a rad cap that maintains pressure because it raises the boiling temperature of the coolant. Not a worry at the rad, which will not get above 90 deg or so, but when you switch the engine off, stagnant coolant at the hottest bits of the engine will boil, expand and push coolant out.

When running, any steam produced at the engine will reduce the heat transfer and you will get hot spots. Hot bits of engine are bad.