Modern day engines have the water pump mounted on the front of the cylinder block and it is usually driven by the fan belt. In some cases it is driven off the camshaft or the gear train. Its function is to deliver coolant from the bottom of the radiator pan into the engine's water jacket efficiently. This water, after absorbing heat from the engine, circulates back to the top of the radiator.

The impeller of the pump is a rotating disc carrying vanes, which fling the water outwards against the pump's casing by centrifugal force and impel it forward into the water jacket. The shaft is mounted on the water pump casing and rotates on bearings. A seal prevents coolant from escaping along the impeller shaft. At the drive end a pulley driven by the camshaft is mounted to accept the fan belt.

When the engine is cold the thermostat is closed and coolant does not reach the top of the radiator. In order to circulate coolant through the engine during warm up, a bypass pipe is provided below the thermostat that leads back to the water pump. The pipe also allows hot coolant to pass through the valve, which will open the thermostat when it reaches the requisite temperature.

A small hole at the bottom of the water pump housing allows coolant to pass through if the seal leaks. In such circumstances it is easier to simply replace the damaged water pump with a rebuilt or new pump.