Tech Tip: Water Pumps - Why They Fail and When to Replace Them

Tech Tip: Water Pumps – Why They Fail and When to Replace Them

If you've wondered how much work a water pump must do, remember that only about 30 percent of the heat energy produced by combustion results in mechanical energy. That estimate, of course, is a mathematical comparison between the heat value of the gasoline going into the engine, and the heat value of the mechanical energy coming out of the engine. The remainder of combustion heat must then be dissipated into the atmosphere through the exhaust, lubrication and cooling systems.

by Gary Goms, Import Specialist Contributor

If you’ve wondered how much work a water pump must do, remember that only about 30 percent of the heat energy produced by combustion results in mechanical energy. That estimate, of course, is a mathematical comparison between the heat value of the gasoline going into the engine, and the heat value of the mechanical energy coming out of the engine. The remainder of combustion heat must then be dissipated into the atmosphere through the exhaust, lubrication and cooling systems.

Water pump operation is important because, if the coolant’s circulation rate is too slow, the coolant will begin to boil in the engine’s water jackets. Unfortunately, when coolant boils, it becomes gaseous and can no longer cool hot cylinder walls and cylinder heads. The result is overheating, which is a primary cause of catastrophic engine failure.

To prevent the coolant from overheating, a water pump is used to circulate the coolant through the engine’s cylinder block, heads and radiator. As it circulates, the coolant absorbs heat from the engine block and dissipates it into the atmosphere through the radiator.

Of course, coolant temperatures may vary according to ambient temperature, engine load and engine speed. The modern import engine must operate between 185° and 195° F coolant temperature to evaporate volatile combustion by-products from the lubricating oil, achieve correct dimensional fits between aluminum pistons and cast-iron cylinder walls, and to achieve optimum fuel vaporization in the cylinder head ports. To maintain a constant coolant temperature, a thermostat is usually placed at the coolant outlet on the engine assembly where it controls coolant flow into the radiator.

Although the modern import water pump may appear simple in design, the actual coolant circulating capacity of the pump is based on mathematical models that take into account the amount of heat generated by the engine under various driving conditions. Severe operating conditions, for example, may range from stop-and-go driving on a hot day to pulling a camping trailer over a high mountain pass on summer vacation.

Water pump components include the pump housing, impeller, seal, bearing, pump shaft and drive pulley hub. The water pump housing also incorporates one or more vent holes located between the bearing and seal to protect the bearing from coolant contamination if the seal begins leaking coolant.

The impeller is designed to circulate coolant by using centrifugal force to impart motion to the coolant. Impellers are designed for either clockwise or counter-clockwise rotation so, for all practical purposes, an impeller driven in opposite rotation becomes a very inefficient coolant pump. In addition, an impeller must fit the water pump housing and engine block cavity perfectly to achieve optimum efficiency.

In the modern import, water pumps are designed to consume as little engine torque as possible while achieving the greatest possible coolant circulation. Consequently, the water pump is a compromise between circulating too little coolant at engine idle and too much coolant at maximum engine speed. Too little coolant circulation obviously causes overheating, while too much circulation wastes gasoline and aggravates water jacket erosion. Most passenger car water pumps may be designed to deliver approximately 10-gallons-per-minute flow at normal engine speeds. Here again, the flow depends largely upon the engine’s size and average load. Many import manufacturers also boost water pump performance by installing molded plastic impellers, which may operate more efficiently than stamped alloy steel versions.

Leaking shaft seals, which are the most common water pump failure, usually reveal themselves by leaving a coolant stain around the vent area. Shaft seal leaks can be difficult to diagnose because they can be intermittently temperature and pressure sensitive, and can be aggravated by rust and other particulate contamination in the system.

Shaft seal leaks can often be diagnosed by using a cooling system pressure tester to pressurize the cooling system. In most cases, however, a visual inspection is the most reliable method simply because most intermittent shaft seal leaks are detectable only by the traces of coolant around the vent area and surrounding parts.

Noisy shaft bearings are usually the second-most common water pump failure. Most shaft bearings fail due to normal wear in the bearing or due to the normal oxidation of lubricant on the bearing surfaces. In rare cases, bearing failure can be hastened by over-tightening conventional accessory drive belts. In many cases, a water pump bearing also fails because it supports an unbalanced fan assembly that may also have bent or misaligned blades. Water pump bearings can also fail because an amateur mechanic diluted the lubricant by washing the pump in a solvent tank!

The third and most rare water pump failure is the impeller slipping on the water pump shaft. Since the impeller and shaft is a press-fit assembly, slippage occurs most frequently on remanufactured water pumps. In other cases where a plastic impeller is used, the plastic material can degrade through sustained heat and age. In any case, slippage can be intermittent in nature and can depend greatly upon the temperature and speed of the engine.

Last, some replacement impellers can be manufactured from inferior metals that are susceptible to rust corrosion. In most of these cases, impeller blades begin to break away from the impeller due to a rust-through condition, which results in decreasing pumping capacity and an increasing presence of rust contamination in the cooling system.

Water pumps have become an important routine replacement item largely because the water pump may act to help tension timing belts or because it may be accessible only after the timing belts are removed. Since a water pump replacement essentially requires the same labor operations as a timing belt replacement on many imports, it’s more cost-effective to invest in a new water pump when timing belts are replaced. Because the service life of most water pumps averages between 100,000 and 150,000 miles, it’s good to recommend replacing any pump in that mileage range, especially if it’s part of a repair or new engine installation service.

Since most water pump failures are caused by leaking seals, it’s important to inspect the cooling system for the presence of abrasive rust or sand particles. Rust, in particular, will cause early seal failure because of its abrasive qualities. Although rust is difficult to remove, it should be flushed from the system as thoroughly as possible before the old water pump is removed. Adequate flushing is aided by removal of the thermostat, which allows for a maximum water pump circulation rate.

In rare cases, the presence of dirt or loose core sand left over from casting the engine block may also cause repeated seal failure on water pump replacements, especially if the vehicle is driven on extended trips. Last, but not least, remember that many import vehicles require application-specific antifreeze to prevent corrosion in the cooling system and prevent premature water pump seal wear.

Before installing a new water pump, always compare the fan or belt pulley flange height with that of the old pump. If the height isn’t to specification, belt alignment may be adversely affected. To ensure the pump has the correct rotation, compare the impeller for similarity of configuration and size. Also, make sure that the old gasket has been completely removed from the engine block in order to maintain correct tolerances between the pump impeller and engine block mount and, of course, to prevent leaks. Before bolting the pump to the block, test for insufficient block clearance by holding the pump against the block and turning the impeller. When all dimensions and clearances have been checked, the water pump is ready for installation.

To prevent damaging the new water pump seal, make sure that the engine is completely filled with new coolant before starting the engine. With that done, always warm the engine until coolant circulates freely through the radiator and all air is bled from the system. Last, inspect for leaks and check the coolant level before releasing the vehicle to your customer.

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