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Water Pump Know-How

The water pump on an engine is unnoticed until it either leaks or needs to be replaced due to a bearing failure. If it does require replacement, the task is usually unpleasant because of its location on most engines.  

Most water pumps are engine-driven and of a centrifugal design. The pump has an inlet, outlet, and an impeller, along with a cavity for the impeller to reside in.

The separate inlet and outlet sides are referred to as the pressure and suction sides, respectively. The pressure side of the pump directs the coolant to the radiator; the suction side sends it back into the engine. The direction of coolant flow in and out of the pump is also used to identify the engine’s thermostat location and temperature rating. If the thermostat is in line with the top radiator hose, this is pressure side placement. If it is connected to the bottom radiator hose, it is suction side.

how operating temperature is set

A radiator under full engine load is designed to drop the liquid temperature approximately 20°F. If the operating temperature is set to not drop below 180°F., a pressure side thermostat would be calibrated for 180°F., and a suction side thermostat would be calibrated 160°F. This is important to note since coolant temperature will be influenced more by the higher rated temperature of the suction side thermostat. 

Most, if not all, engines use a bypass circuit. Its purpose is to allow movement (flow) of the coolant through the engine when it is cold and the thermostat does not allow travel to the radiator. The bypass is meant to facilitate movement of coolant and, by its design, is flow-limited. 

The seal on the water pump keeps the coolant from exiting the impeller cavity from around the shaft and leak from the weep hole in the casting where the bearing and seal reside. If the engine rpm is continually brought to a high level when the thermostat is closed or partially closed, the suction side of the pump will become starved for liquid and create a vacuum that, over time, will impact the integrity of the shaft seal.

In this case, the pump will begin to leak coolant from the weep hole. This is due to the lack of flow through the bypass circuit when the thermostat is closed.

Water pump life can be dramatically increased if you do not overspeed the engine while the thermostat is still closed. The engine can be put under load and used. Water pump speed needs to be kept under the cavitation rpm. This is frustrating since no speed guidelines are provided by engine manufacturers. A rule I use is to not exceed one half the engine’s maximum speed while the coolant is cold. 

Keep in mind that the thermostat works in a linear fashion once its crack-open rating is reached. Then its influence on the pump seal at higher speeds diminishes. This is not as burdensome as it first sounds since the proper warm-up procedure for any engine is to place it under light load.

coolant impact

Keeping the coolant fresh and properly additized goes a long way in extending water pump life as does servicing the system’s filter (if the engine is so equipped).

Some people claim overtightening a drive (fan) belt will put undue stress on the water pump bearing. In practice, that is not possible. It is advisable to keep the belt tight for water pump efficiency. The shaft and bearing are quite robust and are indifferent to belt preload.  

If the time comes when you need to replace the water pump, always choose a pump from the original manufacturer. If that source is not available and an aftermarket equivalent must be used, be warned. While I haven’t seen a replacement water pump not bolt up properly to the engine, I have experienced a situation where a will-fit pump causes the engine to run hot under certain conditions when the engine never had this problem with its original water pump.

The flow of a pump is determined by the speed and design of the impeller, the shape of the cavity the impeller operates in, along with the quality of the suction and pressure ports. The most common explanation for the problem described above is that the impeller and cavity design were altered by the aftermarket company from the original manufacturer’s design. This difference affects flow through the engine and radiator.

Ray Bohacz

For example, a pump impeller and cavity for a 4.6-liter engine in a Ford car may be different than that used in a 4.6-liter engine installed in a pickup or an irrigation pump. An impeller fin’s design and the material it is made of all impact the flow. For example, the least efficient impellers are star styles made by sheet metal with one fin left open to make it easier to assemble. The most efficient pumps feature cleanly cast scroll impellers. Such impellers provide the most flow and least tendency for cavitation, especially when the thermostat is closed and coolant is going through the bypass circuit.

A water pump will have a flow curve that is predicated not only on its design but also on its operating speed. That speed is the result of the crankshaft rpm and the ratio of the crankshaft and water pump pulleys.

It is important to note that if the pump is spun too slowly or too quickly, flow drops off. If it is driven too slowly, then there is not enough energy to move the coolant. When the pump is spun too fast, cavitation occurs (air bubbles) and flow drops dramatically.

Most engines that use serpentine instead of V-belts have the water pump spinning in the opposite direction. These have a mirror-image impeller and cavity design. They are identified as reverse rotation water pumps. If such a pump is installed on a standard rotation application, the engine will overheat almost immediately since there will be little to no coolant flow. 

pulley ratio

In most truck or irrigation engines, the pulley ratio will be higher for more flow under load. To determine the ratio, divide the drive pulley diameter by the water pump pulley diameter. The measurement should be taken as close to where the belt rides as possible. If the engine has an 8-inch crankshaft pulley and a 6-inch water pump pulley, the equation is 8÷6, which equals 1.33. Thus, the water pump is spinning 1.33 times the crankshaft pulley speed. At 3,000 engine rpm, the water pump would spin at 3,990 rpm. 

For most engines, a pulley ratio of between 1.25:1 and 1.4:1 is considered desirable.

I have seen some low rpm, high-load engines (such as those in use on irrigation pumps) employ a pulley ratio of 2:1. This is due to the fixed engine speed, constant load, and the need to move coolant quickly through both radiator and engine. 

Turbulent flow through the radiator needs to be increased since a stationary engine does not benefit from the higher airflow that a vehicle does. The same can be said for an engine on a combine, forage harvester, or most other machines.

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