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Water Pumps Keep It Cool

The water pump circulates coolant between the engine and radiator to manage the engine’s waste heat and prevent the engine from overheating.


When a water pump reaches the end of its road, don’t hesitate to replace it.


The water pump circulates coolant between the engine and radiator to manage the engine’s waste heat and prevent the engine from overheating. Though the pump turns continuously while the engine is running, coolant flow is actually controlled by the thermostat. A thermostat that is stuck shut will block coolant flow and cause the engine to overheat. A good thermostat also can be damaged by overheating, so if a customer is replacing a water pump because the engine overheated, I recommend replacing the thermostat as well.

A water pump is fairly simple and consists of a cast iron, aluminum or stamped steel housing, a shaft mounted “impeller” that moves the coolant through the pump, a shaft seal (usually ceramic) and ball or roller bearings to support the shaft.  Seal failure will allow coolant to leak out of the pump, while bearing failure will often make the pump noisy.


Most water pumps are belt-driven off the crankshaft, but on some engines, the pump is driven by the timing belt. On most engines, the pump pulls coolant in through the lower radiator hose and routes it into the block and heads. On “reverse flow” systems, the pump first routes the coolant into the head(s) and then to the block. Some pumps have additional inlet and outlet ports for heater hose and bypass connections.

Many OEM water pumps are capable of going 100K to 150K miles or more, but may fail sooner for a variety of reasons. Cooling system neglect can shorten the life of the shaft seal. Fan imbalance on applications where a mechanical cooling fan is mounted to the front of the water pump also can shorten the life of the water pump shaft bearings and seal.


Because of the many differences in OEM water pump designs, make sure the replacement pump has the same mounting configuration, bolt locations and hose connections as the original. Likewise, it is important to compare pump heights, as these may also vary depending on the dimensions of the timing cover or other belt-driven engine accessories.

When a water pump is replaced, the cooling system should be drained, flushed and refilled with a fresh mixture of antifreeze and water to restore proper cooling performance and corrosion protection. Merely draining the radiator can leave up to half of the old coolant in the block.


Belts and hoses should all be carefully inspected and replaced if any are found to be worn, cracked, damaged or in poor condition. Hoses that are brittle, aged, cracked, bulging or chaffed must be replaced. New hose clamps are also recommended. High-mileage belts should also be replaced, regardless of their appearance.

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