Water pumps are simple devices. They force coolant through the engine block, hoses and radiator to remove the heat the engine produces. It is most commonly driven off the crankshaft pulley or in some cases the pump is gear-driven off the crankshaft. The coolant trapped between the impeller blades is thrown outward from centrifugal force. A suction (vacuum) is created in the center area of the pump housing. Coolant is drawn from the radiator via the lower radiator hose into the water pump. The coolant is then circulated through the engine where it picks up heat from the combustion process and then is sent back into the radiator where the heat is transferred to outside air. Warranty analysis reveals the most common causes of water pump failures are due to contaminated fluid, excessive vibration or unbalanced loads. Contaminated fluid can contain abrasive materials that cause premature failure of the water pump shaft seal. Vibration or unbalanced loads can include improperly torqued or loose mounting bolts, bent or distorted pulleys, bent, cracked or improperly mounted fan assembly, defective fan clutch (if applicable) or improperly torqued belts.
Failures from vibration or unbalanced loads can often be catastrophic, usually ending in shaft failures. When shaft failures occur, the fan/pulley assemblies can cause extensive damage to surrounding components.
Shaft failures can be put into two categories: heat stress fractures and clean instantaneous fractures. Heat stress fractures indicate excessively high temperatures generated by centrifugal forces due to unbalance. The shaft overheats and eventually fails. Clean instantaneous fractures occur from the engine being rapidly accelerated with an unbalanced load. The combination of these two forces shears the shaft in half.
Always carefully inspect all moving components for cracks, looseness, or missing components prior to installation of the replacement water pump. Check the alignment of the pulley assemblies and torque the drive belts to the correct specifications. Replace any component suspected of problems. Thoroughly flush the cooling system prior to replacement of the water pump and install new antifreeze from a sealed container. Fill and bleed the system according to the manufacturer's service procedure.