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, then, 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, temperatures begin to rise and the engine starts to overheat.

Engine overheating can be caused by a combination of two factors: too much heat or not enough cooling.

Not enough cooling may be due to:

• Low coolant level due to a coolant leak (internal or external)

• Poor heat conductivity inside the radiator because of accumulated deposits. The most common problems radiators fall prey to are clogging (both internal and external) and leaks. Dirt, bugs and debris can block airflow through the outside of the core and reduce the radiator’s ability to dissipate heat. If clogged internally, the radiator must be removed for cleaning. Backflushing the cooling system and/or using chemical cleaners can remove the old antifreeze and some rust and scale, but will do little to open up a clogged radiator.

• A defective thermostat that doesn’t open

• An inoperative electric cooling fan or fan clutch. With an electric cooling fan, check to see that the fan cycles on when the engine gets hot or when the air conditioner is on. If the fan fails to come on, the problem may be with the fan motor, fan motor relay, or temperature sensor

• A collapsed radiator hose

• An eroded or loose water pump impeller

• A defective radiator cap.

• Excessive heat buildup due to a defective engine.

• Excessive heat buildup due to conditions not connected to the cooling system, such as: front end out of alignment, restricted exhaust system, defective transmission.

Your engine may not be overheating at all. Your temperature gauge or warning lamp may be coming on because of a faulty coolant temperature sensor. Sometimes this can be caused by a low coolant level or air trapped under the sensor. If the cooling system has a bleeder valve, you have to temporarily loosen it to get all the air out of the system. Air pockets in the head(s), heater core and below the thermostat can also interfere with proper coolant circulation.