Diagnosis And Testing: Aeration

If coolant level drops below a certain point, aeration will occur drawing air into the water pump resulting in the following:

High reading shown on the temperature gauge.
Loss of coolant flow through the heater core.
Corrosion in the cooling system.
Water pump seal may run dry, increasing the risk of premature seal failure.

NOTE:

Combustion gases leaking into the coolant can also cause the above problems.