Q. Why do engines need a water pump?
A. Because internal combustion engines produce a LOT of waste heat. Almost a third of the heat energy that is produced by the burning air/fuel mixture inside the cylinders is absorbed by the engine block and cylinder heads. It’s like a fire burning inside a stove. The metal around the fire absorbs heat and gets hot. So some type of cooling is required to keep the engine from overheating and self-destructing.
All automotive engines today have liquid cooling systems, though years ago some small cars such as Volkswagens Beetles, Corvairs and a few others had air-cooled engines. Air-cooling eliminated the weight, bulk and expense of a heavier block and heads, radiator and external cooling system. That’s why many motorcycles still have air-cooled engines (though most bigger bikes now have liquid cooled engines).
The problem with air cooling is that it is not very efficient, it allows hot spots in the upper cylinder area, and too much fluctuation in operating temperature. A liquid-cooled engine is much more stable thermally, which improves fuel economy, performance and emissions while also reducing ring blowby and internal engine wear.
The liquid coolant inside the engine block and heads acts like a heat sink to absorb and dissipate the heat. This produces a more consistent operating temperature, and a more uniform temperature gradient from the top to the bottom of the cylinders, eliminating hot spots. Liquid coolant also retains heat, which allows an engine that’s been driven to get back to normal operating temperature much more quickly following a restart (assuming it is still warm). The coolant can also be used to carry heat to the heater in the passenger compartment during cold weather.
The water pump’s role in all of this is to circulate the coolant between the engine, radiator and heater core. The pump is usually located on the front of the engine and is driven by either the serpentine belt or the overhead cam timing belt. On some hybrid vehicles, an additional electric coolant pump is used to cool the hybrid electronics and to continue circulating coolant to the heater when the engine stops running.
Q. Why do water pumps fail?
A. “Common symptoms of water pump failure are leaks, noise and lack of cooling,” according to Joe DiCara, technical services manager for CARDONE. The most common cause is normal wear, though internal corrosion and/or cavitation can also be factors that lead to early failure. Most original equipment water pumps are designed to last upwards of 100,000 miles. Most do, but some don’t.
The pump shaft is supported by bearings, and there is usually a ceramic seal around the shaft. “Seal failure can also be caused by incorrect coolant fluid or running only water,” according to DiCara. “Be sure the technician is using the correct coolant in the correct proportions specified by the OEM.” Antifreeze contains lubricants for the shaft bearing, and corrosion inhibitors to prevent internal rust and corrosion. But these additives eventually wear out over time. If the coolant is worn out or dirty, it will shorten the life of the pump’s bearings and seals, as well as other cooling system components.
A water pump with bad bearings may make noise and/or wobble as it turns. There should be no play or movement in the pump shaft if the pump pulley is wiggled sideways by hand when the engine is off.
Coolant leaking out of the vent hole on the housing means the shaft seal has failed and the pump needs to be replaced. Most water pumps have metal impellers, but some are plastic.
Cavitation erosion can slowly wear away the vanes on a plastic impeller, causing it to pump less efficiently. This may lead to engine overheating on hot days or when the engine is working hard.
Corrosion due to worn-out coolant can also eat away metal impellers. In some cases, the impeller may also come loose on its shaft and stop turning, causing the engine to overheat very quickly.