thermostat, water pump, temperature gauge, or coolant? listen for the sound of boiling coolant (rule out temperature gauge, or coolant if present). Stop immediately, if presen
…t, and allow the engine to cool. Generally, the temperature goes higher when it is the thermostat that malfunctions as opposed to the water pump. Many water pumps are belt driven, when the belt goes, the water pump stops, and the engine will overheat, although not as high as when the thermostat won't open because you have a larger cooling resevoir. You can also check the hoses for the feel of flowing water into the radiator (functioning water pump if the thermostat is open), and a temperature differential on the input/output hoses to the engine (indicating water flow through the engine). (MORE) 
