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Water Pump Diagnosis & Replacement

by Larry Carley copyright AA1Car.com

The water pump is the heart of the cooling system. The pump circulates coolant between the engine and radiator to keep the engine from overheating. Inside the pump is a metal or plastic impeller with blades that push the water through the pump. The impeller is mounted on a shaft that is supported by the pump housing with a bearing and seal assembly. The water pump is usually belt driven and is mounted on the front of the engine.


Water pumps typically fail one of two ways: the shaft seal starts to leak, or the impeller inside breaks, comes loose or the blades erode and wear down (which is more of a problem with pumps that have plastic impellers).

When a water pump starts to leak, the cooling system will lose coolant. If the leak is not discovered, the loss of coolant will eventually cause the engine to overheat. The drive may not realize anything is wrong until the temperature warning light comes on. If this happens to you, shut the engine off immediately. Severe engine damage can result if an overheating engine is driven too far.

If the engine has overheated, the entire cooling system (radiator, hoses, water pump and engine) must all be inspected to see if there are any coolant leaks. If coolant is leaking out of the water pump shaft or vent hole, the water pump needs to be replaced. Cooling system sealer cannot stop this kind of leak.


A seal on the water pump shaft prevents coolant from leaking past the bearing. Seal wear can be caused by rust, sediment or other contaminants that are circulating with the coolant inside the cooling system. The pump shaft and bearings are also under constant load not only from the drive belt or timing belt but also the fan on vehicles with pump-mounted mechanical cooling fans. Eventually the water pump shaft seal and/or bearing wears out and the pump begins to leak.

Most OEM water pumps are designed to go 100,000 miles or more, but they don't always go the distance. It's not unusual to see leaks occur after 50,000 or 60,000 miles. If the pump shaft shows any visible wobble or the bearings are making noise, the pump should be replaced even if it isn't leaking (because it soon will be!).


Sometimes a water pump will fail internally due to severe corrosion wearing away the impeller blades, or the impeller comes loose on the shaft, or the shaft itself may break from metal fatigue (caused by flexing due to an out-of-balance fan).

Many late model vehicles (Chrysler in particular) have plastic impellers to improve cooling efficiency and to reduce cavitation (drag). But the plastic can wear down quickly if the coolant is dirty and contains abrasives. The pump may not leak, but it may not circulate enough coolant through the engine to keep the engine at normal operating temperature.

squeeze radiator hose to check coolant flow

One way to test for a bad water pump is to squeeze the upper radiator hose when the engine is hot and idling. Careful, because the hose will be HOT! If you do not feel much coolant circulating through the hose when you rev the engine, the pump may be bad. The other cause might be a bad thermostat that is not opening properly (remove and inspect the thermostat), or a clogged radiator.


Replacement water pumps come in a wide variety of shapes and sizes as well as shaft lengths, and on some engines more than one style of pump may be used. Finding the right pump means matching not only the year, make, model and engine, but sometimes also the VIN or casting number. If possible, compare the replacement pump to the old pump to make sure it is the right one for the application. To consolidate applications, some replacement castings have extra outlets and mounting bosses or bolt holes so it can be used on a wider variety of engine applications. This is okay as long as there are enough outlets and mounting bosses to match the original casting.

To Replace Your Old Water Pump:

1. Drain the radiator. Open the petcock valve at the bottom of the radiator, or if there is no drain valve, loosen the lower radiator hose. Do this when the engine is COLD. Catch the coolant in a bucket for reuse later, or safe disposal if it is time to change the coolant. Use antifreeze can usually be flushed down the toilet. DO NOT dump it into a storm sewer or on the ground because it is toxic and will kill plants. It is also poisonous to animals and people.

2. Remove the fan belt. On engines with a serpentine belt drive, this means loosening the tension on the automatic tensioner. Note how the belt is routed BEFORE you remove it so you can reinstall it later correctly (draw a picture if there is not a belt decal under the hood that shows how the belt is routed around the pulleys).

3. Remove anything else that is the way. This may include the fan, fan shroud, timing belt cover if the pump is driven off the timing belt on an OHC engine instead of a serpentine belt, or any brackets or other engine-mounted accessories that are blocking access to the pump.

4. Unbolt the water pump from the engine.

5. Clean the pump mounting surface to remove all traces of old gaskets. The mounting surface must be clean and dry before you install the new pump.

6. Position the gasket on the new pump, using gasket sealer or adhesive as required, then bolt the new water pump to the engine. Use thread sealer on any threads that screw all the way through into open water jackets (not needed with blind bolt holes).

7.Refill the cooling system. Be patient because it may take some time for all the air to vent out. Many cooling systems have vent valves that can be opened during a refill to allow air a place to escape.

Cooling System Notes:

If the cooling system contains rust or sediment, the radiator and block should be cleaned and flushed BEFORE you remove the old pump to protect the new pump from possible damage.

If the coolant that you drained out of the radiator is rusty or dirty, do not reuse it. Refill the cooling system with a 50/50 mixture of new antifreeze and clean, distilled water. Do not use tap water or softened water because it contains minerals and salts that can cause corrosion inside the cooling system.

If your vehicle has a pump-mounted mechanical fan with a fan clutch, the fan clutch should also be replaced at the same time. The service life of the fan clutch is about the same as the pump. A slipping fan clutch will reduce radiator cooling and may cause the engine to overheat.

Other parts you may need to complete the job include a new thermostat (recommended if you are having an engine overheating problem), new radiator and heater hoses (hoses that are cracked, brittle or unusually soft should be replaced), and new hose clamps.

Electric Water Pumps

Some late model hybrid vehicles have a supplemental electric water pump that keeps the engine coolant circulating to the heater core when the engine is stopped. If the pump stops working, there will be a loss of heater output during cold weather when the engine is not running (as when stopped at a traffic light). If the supplemental electric pump develops a coolant leak, the loss of coolant will lead to engine overheating.

Common problems with a supplemental electric water pump include wiring faults, blown fuses and/or relays. If the pump is not coming on when it should, check the wiring for loose, damaged or corroded connectors. Use a digital voltmeter to check for voltage to the pump and a good ground connection.

Electric Water Pumps for Performance Applications

An electric water pump can also be used to replace a conventional mechanical belt-driven water pump on a performance engine or race car. The conversion saves horsepower that would otherwise be used to turn a mechanical pump (from 3 or 4 hp up to 15 hp or more!). An electric water pump may also provide more consistent cooling at various engine speeds and loads compared to a mechanical water pump. The pump can also be used to circulate coolant after a hot engine has been turned off at the end of a run to speed engine cool down between races.

water pump repair More Cooling System Articles

Finding & Fixing Coolant Leaks

Servicing Your Cooling System

How To Diagnose & Replace A Thermostat

Your Temperature Warning Lamp Is On. What Should You Do?

Engine Overheating: Causes & Cures

Troubleshoot Electric Cooling Fan

Troubleshoot Cooling Fan Clutch

Belt & hose service

Belts (Serpentine)

Belt Tensioners

Coolant Checks & Changes More Complicated These Days

Universal Coolant: One Antifreeze For All?

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