Overheating can be caused by a water pump with a loose or corroded impeller. Check the pump flow by holding the top radiator hose with the engine running and the thermostat open at normal operating temperature. The surge of the coolant can be felt when the engine is accelerated. Besides obvious cooling system problems, overheating can be caused by retarded ignition timing, a partially blocked catalytic converter or exhaust system, or even by detonation due to an inoperative EGR valve.