by Mark Davidson

The engine overheats or boils over. The temperature gauge reads higher than normal or the instrument panel “hot” light is illuminated.

The most common cause of an engine overheating condition is a defective thermostat. However, there are numerous items that can cause an overheating condition. Coolant loss will also cause an overheating condition. If you are experiencing coolant loss in the form of a leak, refer to the “Coolant Loss” symptom on this site. If the vehicle overheats and has excessive white smoke from the tail pipe, refer to the “Excessive Smoke From Tail Pipe” symptom (also on this site).

You should start your diagnosis with a visual inspection of the cooling system. Ensure the coolant level is at the proper level. Do not remove the radiator cap when the engine is hot! Inspect for warped belts and leaking hoses. If the engine overheats a few minutes after start up, the most probable cause is a defective thermostat. If the engine overheats after several miles of driving or only overheats at low speeds, you should inspect the cooling fan operation and radiator. On vehicles that use an electric cooling system, ensure the cooling fan is coming on at when the engine reaches operating temperature, usually between 195F – 220F. Let the engine idle with all of the accessories in the off position (such as A/C). Monitor the engines temperature by observing the temperature gauge. The cooling fan should automatically come on prior to the gauge reading hot. If it does not, promptly shut the engine off. If the vehicle is equipped with air conditioning, turn it on and observe the cooling fan. If the fan comes on, the most likely cause is a cooling fan thermostatic switch. If the fan does not come on, further diagnosis of the fans electrical system will be necessary.

On vehicles with a conventional radiator fan, inspect the fan for damage. Ensure the fan clutch is operating properly. Have an assistant turn the engine off while observing the fan. The fan should only spin for about 2 seconds after the engine is shut off. If the fan continues to spin for several seconds after the engine is shut off and the engine only overheats at low vehicle and engine speeds, fan clutch replacement may be necessary. If the cooling fan operates properly, inspect the radiators external cooling fins for excessive debris, such as bugs and dirt. When the engine has cooled down, remove the radiator cap and inspect the inside of the radiator for excessive scaling. The tubes that run accross from one tank to the other should be clear of build up. If the radiator is not equipped with a conventional cap, it may be necessary to remove one of the radiator hoses to perform an internal inspection. You should do this when the engine is cold. If necessary, the radiator can be removed and inspected by a radiator shop that will provide advice. A radiator that is plugged may require a “Rod Out” to restore proper operation. If the radiator, cooling fan, coolant level and thermostat are in proper working order further diagnosis will be necessary. It is possible that the impeller on the water pump is damaged or the vehicle has a blown head gasket or cracked cylinder head. White smoke from the tail pipe is a key indicator of such a failure.

Use extreme caution when working on the cooling system. The cooling system is under pressure when hot and can cause extreme burns if opened when hot. Use caution when working around hot or rotating engine parts. On vehicles with electric cooling fans the cooling fan starts automatically. Keep your body away from the cooling fan at all times. Radiator flush agents may be an aid in removing scaling from the radiator. Use these products as per the manufacturers recommendations.