Cooling System Low Coolant Level
AERATIONIf the coolant level in the radiator drops below the top of the radiator core tubes, air will enter the system.
Low coolant level can cause the thermostat pellet to be suspended in air instead of coolant. This will cause the thermostat to open later, which in turn causes higher coolant temperature. Air trapped in cooling system also reduces the amount of coolant circulating in the heater core. This may result in low heat output.
DEAERATION
As the engine operates, air trapped in the cooling system gathers under the radiator cap. The next time engine is operated, thermal expansion of coolant will push trapped air past radiator cap into coolant reserve/overflow tank. Here it escapes to atmosphere in the tank. When engine cools down the coolant, it will be drawn from reserve/overflow tank into radiator to replace removed air.