The thermostat, which is usually located in a housing where the upper radiator hose connects to the engine, controls the operating temperature of the engine. It does this by blocking the flow of coolant from the engine to the radiator until the engine reaches a certain temperature (usually 190 to 195 degrees F.). When this temperature is reached, the thermostat opens and allows coolant to circulate from the engine to the radiator.
If the thermostat fails to open, which can happen due to mechanical failure or if a steam pocket forms under the thermostat due to incomplete filling of the cooling system or coolant loss, no coolant will circulate between the engine and radiator, and the engine will quickly overheat.
You can check for this condition by carefully touching the upper radiator hose when the engine is first started and is warming up. If the upper radiator hose does not become hot to the touch within several minutes after starting the engine, it means the thermostat is probably defective and needs to be replaced.
CAUTION: The replacement thermostat should always have the same temperature rating as the original. Do not substitute a colder or hotter thermostat on any vehicle that has computerized engine controls as engine operating temperature affects the operation of the fuel, ignition and emissions control systems.
On rear wheel drive vehicles with belt-driven cooling fan, a "fan clutch" is often used to improve fuel economy. The clutch is a viscous-coupling filled with silicone oil. The clutch allows the fan to slip at high speed, which reduces the parasitic horsepower drag on the engine. If the clutch slips too much, however, the fan may not turn fast enough to keep the engine cool.
The silicone fluid inside the clutch breaks down over time and can leak out due to wear, too. If you see oily streaks radiating outward on the clutch (and/or the fan can be spun by hand with little or no resistance when the engine is off), it means the clutch is bad and needs to be replaced. Any play or wobble in the fan due to wear in the clutch also signals the need for a new clutch.
On most front-wheel drive cars, the fan that cools the radiator is driven by an electric motor. A temperature switch or coolant sensor on the engine cycles the fan on and off as additional cooling is needed. If the temperature switch or coolant sensor (or the relay that routes power to the fan motor is bad), the fan won't come on when it is needed and the engine will overheat. Likewise, if the fan motor itself is bad, the fan won't work.
The system needs to be diagnosed to determine where the problem is so the correct component can be replaced.
Leaks in radiator or heater hoses, the water pump, radiator, heater core or engine freeze plugs can allow coolant to escape. No engine can tolerate the loss of coolant for very long, so it usually overheats as soon as a leak develops.
A visual inspection of the cooling system and engine will usually reveal where the coolant is going.
Leaks in hoses can only be fixed by replacing the hose. Leaks in the water pump also require replacing the pump. But leaks in a radiator, heater hose or freeze plug may sometimes respond to a sealer added to the cooling system.
If no leaks are apparent, the radiator cap should be pressure tested to make sure it is holding the specified pressure. If the spring inside the cap is weak (or the cap is the wrong one for the application), the engine will lose coolant out the overflow tube every time it gets hot.
If there are no visible coolant leaks, but the engine is using coolant, there may be a crack in the cylinder head or block, or a leaky head gasket that is allowing coolant to escape into the combustion chamber or crankcase.
See related question #16 for more information.
In some instances a severe exhaust restriction can produce enough backpressure to cause an engine to overheat. The most likely cause of the blockage would be a plugged catalytic converter or a crushed or damaged pipe. Checking intake vacuum and/or exhaust backpressure can diagnose this kind of problem.
In a high mileage engine, the impeller that pumps the coolant through the engine inside the water pump may be so badly corroded that the blades are loose or eaten away. If such is the case, the pump must be replaced.
Most pump failures, however, occur at the pump shaft bearing and seal. After tens of thousands of miles of operation, the bearing and seal wear out. Coolant starts to leak out past the shaft seal, which may cause the engine to overheat due to the loss of coolant. A sealer additive will not stop this kind of leak. Replacing the water pump is the only cure.
CAUTION: A leaky water pump should be replaced without delay, not only to reduce the risk of engine overheating but to prevent catastrophic pump failure. If the shaft breaks on a rear-wheel drive vehicle, the fan may go forward and chew into the radiator ruining the radiator.