Auto Repair Help

by Lance Wright

Radiators require a large amount of airflow in order to properly dissipate the heat from the engine. When a vehicle is moving at highway speeds, air flow across the radiator is sufficient to meet engine cooling demands. At idle and at low speeds however, there is not enough air flow across the radiator to cool the engine. To keep the engine from overheating at slow speeds, a cooling fan is used to provide the necessary air flow.

Almost all rear wheel drive, front engine design vehicles, use an engine driven cooling fan. The cooling fan is normally bolted to the water pump pulley, which is driven by a belt attached to the crankshaft pulley. Some cooling fans are designed to change pitch at higher speeds to lessen the fans drag on the engine.

Front wheel drive, transverse engine vehicles cannot use an engine driven fan, since the engine is mounted sideways in the engine compartment. These vehicles use a cooling fan that is driven by an electric motor. The electric motor is operated by a relay that is either controlled by a thermostatic switch or the engine control module. The cooling fan is only used when the engine reaches a predetermined temperature and/or a vehicle speed that is sufficient to provide proper airflow.

A fan clutch is used on some engine driven cooling fans, to disable the fan at higher speeds. It consists of a fluid coupling containing a silicone based oil. Most are controlled by a thermostatic spring that controls the amount of fluid that flows through the coupling. At higher engine speeds, the fan will spin slower in order to diminish the amount of load on the engine. The fan clutch is designed to provide greater airflow at low speeds and to limit fan rotation at higher speeds.

A fan clutch can lock up or seize after a period of time, causing a loud roar at higher speeds due to the fan not disengaging. They can also leak fluid and become ineffective at lower speeds causing engine overheating making repair mandatory. In some cases, a fan clutch can become excessively loose at the shaft outlet where it bolts to the water pump. Simple visual and physical inspection when the engine is off, can help to diagnose fan clutch related problems and should be the first step in an auto repair project involving such a component.

Engine cooling systems utilize a special cooling fluid to meet system requirements. The coolant used must resist freezing in order to prevent engine damage due to expansion. It also must resist rust and prevent corrosion of internal engine components. In order to accomplish this, a special mixture of ethylene glycol, silicates, and water is used.

CoolantEthylene glycol is the principle ingredient that prevents corrosion and lowers the freezing point of engine coolant. An engine needs to operate with a cooling system that has a higher boiling point than water. Water at sea level, boils at approximately 212 F and most engines operate near that temperature. If an engine cooling system were to boil, damage would result from the inability of vaporized water to conduct heat away from engine components and, as a result, you’d be facing a significant auto repair bill due to this damage. To raise the boiling point of the coolant, engine cooling systems operate under pressure. High pressure raises the boiling point of the coolant. The radiator pressure cap is designed to maintain the cooling system at a pre-determined pressure. By raising the operating pressure to 14 psi, the boiling point of engine coolant is raised about 249 F. The radiator pressure cap also will release pressure if cooling system pressure exceeds maximum levels.

In order to prevent costly engine and radiator damage, engine coolant should be changed at recommended intervals. If coolant is not changed regularly, rust and corrosion can build up in the radiator and the engine. Some internal components such as head gaskets and aluminum parts can be damaged due to the corrosive nature of water and other contaminants. Heavy metals in corroded coolant can cause damage by electrolysis to engine components as well. Radiator pressure caps should also be tested for proper release pressure and sealing ability.

(Lance owned his own auto repair shop for 30 years before retiring in 2006.)

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