As I quickly read this thread, I didn't notice if anyone has already responded on how to easily tell if it is the water pump or not.
One of the ways to tell if it is the water pump impeller failure is to see if you have heat from the heater. If the heater is hot, the water pump is working. If the heater gives off no heat when the engine is warm and there is enough coolant to reach the heater core, it is not the water pump. In this case, it is usually a thermostat that is stuck shut.
