Your Vehicle's Water Pump

Team BlueStar Inspections

The water pump, also known as the coolant pump, is the heart of the engine’s cooling system. It is the water pump’s job to regulate the coolant flow rate, and to constantly circulate coolant throughout the engine and cooling system.

Most water pumps have seven basic components:

  1. HOUSING - The housing is the outer shell that encases the water pump. It is typically made of cast iron or aluminum, although many modern engines use stamped-steel housings. The housing has a weep hole: a small hole that allows any coolant that may be leaking from a faulty seal to escape, thereby preventing it from becoming trapped and forced into the water pump bearing assembly. The weep hole is just a few millimeters in diameter and is located on the top, side, or bottom of the housing unit between the hub and water pump seal.
  2. IMPELLER - The impeller is located inside the housing and is connected to the bottom of the shaft. It may be metal or plastic. The impeller spins and distributes coolant throughout the cooling system at a rate determined by the engine RPMs.
  3. SHAFT - The water pump shaft rides against the bearing with the impeller connected on the bottom and a hub or pulley connected to the top.
  4. BEARING - The bearing assembly, which rotates with the shaft, is the mechanical support that allows the stable, controlled, and continuous rotation of the shaft.
  5. HUB or PULLEY - Attached to the top of the shaft is the hub or pulley, which is the connecting source of power for impeller rotation. The engine’s serpentine, V-belt, or timing belt is typically connected to this.
  6. SEAL - The seal goes around the shaft and protects the bearing assembly from coolant and contaminates. If this seal fails, coolant will leak out of the weep hole.
  7. MOUNTING GASKET - The mounting gasket seals the water pump to the engine.

Amazingly, a typical automotive water pump can move up to 7,500 gallons of coolant per hour and can recirculate the coolant in the engine over 20 times per minute when operating at peak RPMs. This means your vehicle’s water pump operating at peak capacity could empty a typical private swimming pool in about an hour! The speed of the water pump is determined by the speed of the engine, therefore the slower the engine speed, the less it pumps. However, even at 35 mph, a typical water pump can still move about 2,000 gallons per hour!

As engine RPMs increase, more heat is produced by the engine and more cooling capacity is required. The water pump impeller speed increases as the engine RPMs increase to provide extra coolant flow as needed.

A failing water pump can cause an engine to overheat and leave you stranded. Be sure to inspect the water pump, coolant, and belt drive system that drives the water pump at each service interval to prevent these and other issues.

All mechanical water pumps work in basically the same way and are driven via the rotation of the engine’s crankshaft, although this can be accomplished in several ways. Mechanical energy is transferred from the crankshaft to the water pump, typically via an accessory or serpentine belt. In some cases, a water pump will be powered by a camshaft, although the camshaft itself is powered by the crankshaft via a belt or chain. A water pump may also be driven by a timing belt rather than an accessory or serpentine belt. In any case, the rotation of the water pump shaft is used to induce the circulation of coolant through the cooling system.  Coolant is typically drawn into the water pump from the engine block and directed into the radiator. There it passes through thin tubes that are designed to provide as much surface area as possible to cool the hot liquid. The liquid then travels back into the engine cylinder head and block where it can pull additional heat out of the engine. The water pump continues this process the entire time the engine is running. Water pumps will eventually wear out and fail. Whether you are purchasing a used vehicle or having a general inspection performed on your own vehicle, common points of failure should be inspected. These inspection points include an inspection for leaks (seal, gaskets, weep hole), checking for water pump bearing looseness or noise, a visual inspection and testing of the coolant condition, and inspecting the belt and tensioning system driving the pump.LEAKS (SEAL, GASKETS, WEEP HOLE)Leaks are the most common reason for water pump failure. The water pump seal protects the bearing assembly from being exposed to and contaminated by coolant, and it confines the coolant within the water pump housing and cooling system. The water pump mounting gasket ensures coolant transfers from the engine to the pump without external leakage.

There are three main conditions that can lead to water pump seal and gasket leakage and failure:

  1. OVERHEATING OF ENGINE - Excessively high operating temperature of the engine is a serious issue. An overheated engine operated with low levels of coolant can result in warping and heat damage to elastomeric seal components and gasket material. Boiling of fluid can also damage and distort seals and gaskets.
  2. COOLANT CONDITION - Abrasive or acidic contaminants in coolant can damage gaskets and seals. Contaminating factors include high acid content and debris such as rust or dissolved particles from system components.
  3. MECHANICAL CONDITIONS - These conditions may include excessive bearing play, incorrect seal installation, excessive vibration due to misalignment of belts, pulleys, or even a defective fan clutch or damaged fan blade.

If the water pump is visible, it can be easily inspected with a high-quality flashlight for evidence of leaks. Faulty water pump shaft seals will result in coolant leaking out of the weep hole on the water pump housing. The weep hole should be inspected for active or dried crusty coolant residue, and the water pump mounting gasket should be inspected for any evidence of leakage. If the water pump is timing belt or chain driven, inspect the timing cover area for any evidence of coolant leaks which would indicate water pump seal or gasket failure.


If the water pump bearing is defective, there will usually be some type of noise associated with bearing failure that changes depending on the engine RPMs. Before replacing a water pump that has failed due to a loose or noisy bearing, inspect the following:

  1. DRIVE BELT TENSION - If the water pump drive or serpentine belt is too tight, excessive force may have damaged the pump bearing and shaft.
  2. COOLING FAN or FAN CLUTCH - If there is a cooling fan or fan clutch attached to the water pump hub, it should be inspected to determine if it is damaged, bent, or out of balance. The resulting vibration can damage the water pump bearing and shaft.

Water pump bearings are the second-most common reason for water pump failure. Most shaft bearings fail due to faulty bearing seals allowing grease to come out of the bearing, or due to the normal oxidation of lubricant on the bearing surfaces. In some cases, bearing failure results from coolant getting into the bearing.


Since you cannot physically inspect the impeller and inside of a water pump without removing the pump, the best way to know if it is functioning properly is to visually inspect the coolant condition, test the coolant pH and protection levels, test drive the vehicle to verify normal operating temperature, and squeeze the upper radiator hose (careful - it will be hot) once the engine is at operating temperature to confirm coolant flow and pressure.

The main killer of water pump gaskets, seals, and internal components is acidic and contaminated coolant. Fresh new coolant includes buffers that can control the pH level. Neglecting the coolant replacement intervals can deplete the coolant’s additives that keep gasket and seal materials conditioned and protect the working components of the water pump.  All metals in a cooling system will corrode under certain conditions. Some metals are more sensitive than others. Corrosion weakens metals, and the component will eventually deteriorate into tiny abrasive particles constantly flowing through the cooling system. These particles can erode and eat away cooling system components.

One major corrosion rate factor is the coolant’s pH. Shifts in coolant pH will affect the metals that corrode and the rate of each metal’s corrosion. The pH scale runs from 0 to 14. Coolant becomes more acidic closer to zero; and more alkaline towards 14. Coolant pH should always be maintained between 8.5 and 11. If a coolant’s pH drops below 8.5, it becomes aggressive to ferrous metals (cast iron and steel), aluminum, copper, and brass.

Coolant should be tested for pH and temperature protection levels. These two tangible measurements will provide valuable information about the current condition of the coolant. The coolant should also be visually inspected for debris, contaminants, and proper color.


Belt tension is vital to the proper operation of the accessory belt drive system. An automatic tensioner constantly maintains the correct amount of tension on the serpentine belt while the engine is running. It also helps protect the water pump and other components from undue stress and premature failure. A seized belt tensioner can cause premature bearing and shaft failure, and as such, can drastically reduce the life of the water pump. A loose drive belt tensioner can cause the belt to slip, which will impact components such as the water pump and can cause problems such as overheating.

A good rule of thumb for inspecting serpentine belts is to try to twist the belt 90 degrees in the middle of the longest span of the belt. It should twist about 90 degrees, but no more. Drive belts that have actual adjustments to tighten or loosen the belt should have approximately one-half inch of deflection in the middle of the longest span if tightened correctly. There should be no squealing, rubbing, or grinding type noises when the engine is running.

Water pumps are vital cooling system components that perform a great deal of work each time you drive your vehicle. If you are buying a used vehicle or planning a long road trip, have an ASE certified technician visually inspect the water pump, belt and tensioner system, inspect the coolant condition, and test and record the pH and protection level measurements to ensure you and your vehicle stay cool on the road – pun intended.