Tech Tip: Diagnosing a Faulty Water Pump
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Tech Tip: Diagnosing a Faulty Water Pump

The water pump is one of those parts that no engine can run long without. If the water pump shaft seal leaks, the cooling system will lose coolant and the engine will overheat. That means a breakdown – and a sale of a replacement water pump. The water pump circulates coolant between the engine and radiator to keep the engine from running too hot. Almost a third of the heat produced by combustion ends up as waste heat absorbed by the ….

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By Larry Carley
Technical Editor

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The water pump is one of those parts that no engine can run long without. If the water pump shaft seal leaks, the cooling system will lose coolant and the engine will overheat. That means a breakdown — and a sale of a replacement water pump.

The water pump circulates coolant between the engine and radiator to keep the engine from running too hot. Almost a third of the heat produced by combustion ends up as waste heat absorbed by the engine. If not carried away, it would soon melt the engine!

The water pump is a fairly simple component, consisting of a cast gray iron, aluminum or stamped steel housing, a shaft mounted “impeller” that moves the coolant through the pump, a ceramic shaft seal and ball or roller shaft bearings.

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Most water pumps are belt driven, 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 routes the coolant into the head(s) first, and then to the block.

Because of the continuous load on the water pump, the seal and shaft bearings eventually wear out. The first sign of trouble may be coolant leaking from the pump shaft weep hole. More than a few drops of coolant coming out of this little vent means the seal is worn and the pump is failing. Other symptoms may include bearing noise (rumbling, chirping or growling), loss of coolant (through the leaky shaft seal), overheating (from coolant loss or separation of the impeller from its shaft), or fan wobble (if the engine has a mechanical fan mounted on the water pump shaft).

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ASSESS ITS CONDITION
One way to spot a water pump with bad shaft bearings is to check pulley or fan play with the engine off. The pulley or fan should not wobble or show any visible play when it is tugged on by hand.

Sometimes a water pump will fail internally. The pump may not be leaking, but the engine is overheating. If a cooling system pressure test shows the system is holding pressure (no internal leaks), and the problem is not a bad thermostat, a slipping fan clutch, or dead electric cooling fan, the water pump may not be pumping enough coolant. Some late-model water pumps have plastic impellers that are designed to reduce drag and noise. But over time, the fins on the plastic impeller can wear down, reducing the pump’s ability to circulate coolant through the engine. If the coolant contains any rust or sediment, it will accelerate the wear process. The same thing can happen to the blades on a metal impeller.

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WIDE RANGE OF APPLICATIONS
If a customer needs a replacement water pump, you can sell him a remanufactured pump or a new pump. Aftermarket pumps are often consolidated to fit a wider range of applications, so compare the mounting configuration, bolt and hose fitting locations, and length of the pump shaft on the old pump to make sure the replacement pump will fit.

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 corrosion protection.

Belts and hoses should all be carefully inspected, and replaced if any are found to be in less than perfect condition. Hoses that are brittle, aged, cracked, bulging or chaffed must be replaced. New clamps are also recommended. Belts that are frayed, cracked or glazed should also be replaced. If the engine has overheated, you should recommend a new thermostat, too.

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If the customer’s vehicle has a fan clutch, the lifespan of the clutch and water pump are typically about the same. Many experts recommend replacing both components if either one fails to reduce the risk of future problems. Most water pumps come with gaskets. If not, be sure to include the required gaskets. Some also have installation instructions (if anybody will read them!). One common installation mistake is not getting removing all the old gasket residue from the pump mounting surface on the engine (which results in leaks). If the pump mounts with a gasket, both sides of the gasket should be coated with sealer.

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If an anaerobic sealer or RTV silicone is used instead of a gasket, the sealer must be applied all the way around bolt holes. Use only a 1/8th-inch bead of RTV and allow it to cure 30 minutes or more before refilling the cooling system with coolant. Water pump bolt threads that extend into the water jacket must also be coated with sealer to prevent leaks.

Fan belts should be tensioned to recommended specifications with a gauge. Too much tension can shorten the life of the water pump’s shaft bearings. If the engine has a serpentine belt and automatic tensioner, the tensioner should be checked to make sure it rotates properly and provides proper tension.

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