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.
Check leakage like this: Water pumps usually start leaking slowly: occaisional need to add water and spots of coolant on your driveway after a hot day can be from the water pump, especially when you can see no obvious other leak (like a hose or the radiator). When you can get to it to see it, there is always a little hole behind the pulley which drives the pump. This hole is to allow the tiny amount of water which normally comes past a seal (a few drops per year) to get out rather than wash through the water pump bearings. When seals fail, water pours from this hole. Often you’ll see stains on the pump housing by this hole on a leaky pump.
Many autos today use the timing belt to drive the water pump. If you have one of those cars, it’s a good idea to replace the water pump when you replace the timing belt as a maintenance item.
To avoid running into the problem of a failed water pump, there are a couple warning signs that you can look for. The first is if coolant is leaking around the water pump. Weep holes located on the casing of the water pump will leak coolant when the pump is failing. The second red flag is if the water pump is making more noise than usual. This may be from a defective impeller or an impeller that’s no longer properly attached to its drive shaft.
It’s unlikely that you’ll have to diagnose your car’s water pump problems yourself, but just remember it’s always a good idea to have your water pump replaced when your vehicle’s manufacturer recommends changing the timing belt.