Water pumps are often likened to the heart of the cooling system. When the pump fails, the results are not pleasant. Most water pump failure can be prevented with a few simple measures.
Water pumps come in a vast variety of shapes and sizes but they all share similar design. There is an impeller driven by a shaft that is supported by bearings. These bearings are protected by grease seals. On newer vehicles, there is also a very complex seal that seals the shaft from the engine coolant.
Modern water pump seals are not like most automotive seals. They no longer use rubber or even ceramic. Most use rings, made of silicon and other hard materials and pushed together by a spring. The primary ring is attached to the seal body and is stationary. The mating ring is pressed onto the driveshaft and rotates.
This type seal depends on proper coolant for lubrication. Engine coolant comes in a vast array of mixtures. While most is ethylene glycol base, the corrosion protection is vastly different. Older vehicles used silicates to prevent corrosion. Newer vehicles rely on phosphates or organic acid. The wrong coolant type will not properly lubricate the water pump seal.
Water pump seals also become damaged in other ways. Coolant depletes over time and turns acidic. If it is not properly replaced, the coolant attacks the metal in the system. Cooling systems today are largely aluminum and acidic coolant forms aluminum oxide, by galvanic action.
Galvanic action occurs when metals are exposed to acidic liquids. Basically the cooling system becomes a battery and metal is eaten away. This can be measured by placing the positive lead of a volt meter in the coolant and touching the negative lead to ground. Galvanic action is different from electrolysis and transient current flow, but can be just as damaging.
Aluminum oxide is an abrasive and can quickly damage water pump seals as well as radiators, heater cores and even engines. The impeller on this water pump was eaten away by galvanic action and caused a severe engine overheat problem.
When engines overheat the water pump seal may also be damage. Adding cool water to a hot system or running the engine without coolant can cause the water pump seal to crack.
Detecting a leaking water pump is not always easy and many good water pumps are needlessly replaced. A slight dampness or stain at the water pump weep port is NOT necessarily a sign of a bad water pump. Dripping water from the weep port means the internal seal has failed.
On vehicles with timing belts, the water pump is normally under the timing cover. Because of the difficulty in replacing such a pump, it is wise to replace it when the timing belt is replaced. Failure of the pump after belt replacement usually means re-doing the timing belt and can cause the belt to break.
While it is unusual, it is also possible to mis-route a serpentine belt on certain vehicles. Such a mistake can cause the water pump to rotate backwards and overheat the engine. Best is to always refer to the routing diagram when replacing a serpentine belt.