Cooling fan operation is often taken for granted by many technicians because testing a cooling fan requires at least 15 minutes of fast idling or running until the coolant temperature reaches 220-230° F. If the cooling fan doesn't activate, a coolant temperature sensor, fan relay, fan electrical circuit or the fan itself may be at fault. In other cases, the engine may overheat when driving in slow-moving traffic or when climbing steep mountain roads. In these cases, more dynamic issues such as inadequate airflow through the radiator or engine compartment might be the cause. In either case, it's important to understand the basics of cooling system operation in order to determine the underlying cause of cooling fan failure.

Most modern liquid-cooled engines use a belt-driven water pump to circulate coolant through the lower cylinder area into the cylinder head, and from the cylinder head into the radiator. A few "reverse flow" engines are on the market in which coolant flows directly from the radiator to the cylinder head, which reduces cylinder head temperatures and better controls the rate of combustion in the cylinder head. In either configuration, a wax-pellet thermostat is used to prevent coolant circulation through the radiator until the engine assembly reaches a specified operating temperature of 170-195° F. Coolant temperatures in this range are the most conducive to maximum engine life, fuel economy and reduced exh...