Cooling System Aeration

If the coolant level in radiator drops below top of radiator core tubes, air will enter cooling system. On 2.5L engines, air can gather in the water box containing the thermostat.
Low coolant level can cause thermostat pellet to be suspended in air instead of coolant. This will cause thermostat to open later, which in turn causes higher coolant temperature. Air trapped in cooling system also reduces amount of coolant circulating in heater core resulting in low heat output.