1. Excessive antifreeze
If the antifreeze level is higher than the mark on the expansion tank, overflow will occur during driving.

2. Thermostat failure
A thermostat failure may result in too hot water, which in turn prevents antifreeze from entering the general engine water circulation system. As the antifreeze only flows in the small radiator circulation system, overflow can occur.

3. Damaged tank cover
If the tank cover is damaged due to excessive temperature in the expansion tank, the antifreeze may overflow.

Besides, water overflow from the expansion tank after flameout is also common. The auxiliary radiator is also an important part of the cooling system, serving the main purpose of balancing the pressure of the radiator.