CAUSES OF OVERHEATING
Overheating can be caused by anything that decreases the cooling system's ability to absorb, transport and dissipate heat: A low coolant level, a coolant leak (through internal or external leaks), poor heat conductivity inside the engine because of accumulated deposits in the water jackets, a defective thermostat that doesn't open, poor airflow through the radiator, a slipping fan clutch, an inoperative electric cooling fan, a collapsed lower radiator hose, an eroded or loose water pump impeller, or even a defective radiator cap.
One of nature's basic laws says that heat always flows from an area of higher temperature to an area of lesser temperature, never the other way around. The only way to cool hot metal, therefore, is to keep it in constant contact with a cooler liquid. And the only way to do that is to keep the coolant in constant circulation. As soon as the circulation stops, either because of a problem with the water pump, thermostat or loss of coolant, temperatures begin to rise and the engine starts to overheat.
The coolant also has to get rid of the heat it soaks up while passing through the block and head(s). So the radiator must be capable of doing its job, which requires the help of an efficient cooling fan at slow speeds.
The thermostat must be doing its job to keep the engine's average temperature within the normal range so the engine does not overheat. If the thermostat fails to open, it will effectively block the flow of coolant and the engine will overheat.
Your engine may not be overheating at all. Your temperature gauge or warning lamp may be coming on because of a faulty coolant sensors. Sometimes this can be caused by a low coolant level or air trapped under the sensor.
Diagnosis of the Engine Overheating
Bad Thermostat – Several engine overheating can often damage a good thermostat. If the engine has overheated because of another problem, therefore, the thermostat should be tested or replaced before the engine is returned to service.
One way to check the thermostat is to start the engine and feel the upper radiator hose (or use an infrared noncontact thermometer to read its temperature). The hose should not feel uncomfortably hot until the engine has warmed-up and the thermostat opens. If the hose does not get hot, it means the thermostat is not opening.
TIP: When refilling the cooling system, air can become trapped under the thermostat. This will form a steam pocket that prevents the thermostat from opening and may cause the engine to overheat. Some cooling systems have one or more bleeder valves that can be opened to vent air from the system while refilling the system. If your cooling system does not have a bleeder valve, you can drill a small hole in the thermostat as shown. This will allow air to escape past the thermostat so it is not trapped inside the engine block. Some thermostats come with a similar feature called a "jiggle valve." There is a small hole in the thermostat with a pin that allows air to escape.