Coolant is what circulates in the engine to cool it. In most places, it is a 50/50 mix of anti freeze and water. If you are asking what do you put in the bottle marked Coolant under the hood, that is...
There could be several reasons. Under normal operation, the vehicle will experience this as the coolant heats up and expands into the over flow. A bad water pump, low level of coolant, as well as a...