Is your car starting to run a little hot? Is it overheating while sitting at idle? It's possible you may need to replace your water pump. An easy way to check your water pump is to look underneath the car and check for water seepage out of the pump. This means that water is leaking past the bearing and seals, indicating the pump is on its way out. Another way to check is to squeeze and hold the upper radiator hose for a few seconds (CAUTION: the hose may be hot) while the car is running. If you feel no pressure building up, it is a sign that water is not circulating and the pump may be bad. These units are brand new, not rebuilt.
Is your car's temperature too hot or cold? A coolant thermostat allows the coolant to circulate when the engine has reached a predetermined temperature. It should close when the engine is cold to allow it to warm up faster. Thermostats can fail and should be inspected for corrosion when servicing the cooling system. Thermostats come in different temperature settings for different climates.