When does the water pump need to be replaced?

The water pump creates the flow in the cooling system of a car engine. It's called a water pump because in the early days of automobiles an engine cooling system was filled with water. Modern cars use a special fluid called antifreeze or coolant.
Water pump in a car engine Water pump creates the flow in the cooling system of a car engine. See more illustrations: Fig 1, Fig 2

In the majority of today's cars, a water pump is a mechanical device rotated by a drive belt that is driven by the crankshaft pulley (see photo below). In some cars, a water pump is driven by a timing belt or chain. Some cars have an electric water pump that is operated by an electric motor.

A working water pump is vital for the engine; if it doesn't work, the engine will overheat. A modern car engine can survive mild overheating but severe overheating can cause damage. For this reason, if your engine temperature starts rising above normal, have the cooling system checked as soon as possible.
In most cars, a water pump replacement is not included in the maintenance schedule, but it doesn't mean that the water pump cannot fail. When should it be replaced? There are several reasons:

One reason is replacing a water pump as a precaution, for example when it's suspected to cause overheating or if a water pump failure is a known common problem for this car or when installing a used engine.

A water pump that is driven by a timing belt is also often replaced as a precaution during the timing belt replacement.
Water pump In this Volkswagen engine, a water pump is driven by a timing belt.
This is because it takes a little extra labor to replace the water pump when doing the timing belt, since it's in the same location.

Is it necessary to replace a water pump with the timing belt? No, but it's an option worth considering. For example, if you check an owners manual for one of the Honda vehicles with a V6 engine, it says Replace timing belt and inspect water pump. Many Honda owners, however, opt to replace a water pump together with a timing belt because if the water pump fails later it will cost the same labor to replace. Plus, a water pump is not a very expensive part ($45-$170).

For the same reason, if a water pump is driven by a timing chain, a mechanic may recommend replacing it as a precaution if some work is done with the timing chain in the same area.
Water pump Water pump.
Again, it's not necessary, but it will be a lot more expensive doing it later, since many parts will have to come off to get to the water pump.

Another reason to replace a water pump is when it fails or shows early signs of failing. What are the signs of a failing water pump?

A failing water pump often shows a coolant leak. A water pump has a weep hole in the lower portion. A small amount of dry coolant residue around the weep hole is normal, but leaking coolant from the weep hole is a sign that the pump is on its way. Car makers provide guidelines for the mechanics on when a water pump is considered bad and need replacing. Sometimes coolant leaks from the pump seal, or O-ring. A coolant leak from the pump usually shows as a pinkish or greenish residue, depending on the color of the coolant. Often, coolant is splashed by a drive belt, and it looks like a whitish residue around the drive belt and surrounding area.

In some cars, a failing water pump can cause intermittent grinding or rumbling noise. In this case, a mechanic will have to diagnose the source of the noise, before condemning the water pump. For example, a mechanic may find that the water pump shaft feels loose.

In some cars, the impeller inside the water pump is made out of plastic and can crack and come loose on the shaft.
Water pump A water pump in this Ford engine is installed outside and is easy to replace.
Corrosion can damage a metal impeller too. In either case, the flow of coolant will be reduced and the engine will start overheating.

If an engine has been overheated, there is always a chance that it might have suffered some other damage as a result. It all depends on the severity of overheating. The problem is that a mechanic may not be able to check for further damage, until the engine is assembled back and started. In cases like this, your mechanic may recommend considering all of your options before doing repairs. In some cases, one of the options might be to replace the entire engine.

How much does it cost to replace a water pump? It depends on the water pump location. In most cars, a water pump is located outside the engine and is not very difficult to replace.
For example, replacing a water pump in a 2009-2013 Toyota Corolla is fairly easy and costs around #300-$450 parts and labor. On the other hand, changing a water pump in the 2007-2014 Ford Edge V6 engine might cost over $2,000, since the water pump is driven by a timing chain and is located inside the engine. Often, parts like a thermostat or a drive belt are replaced together with a water pump. These parts are not very expensive.

After the water pump is replaced, the cooling system needs to be bled properly to remove air from the system. One of the signs of air in the cooling system is when there is no heat from the vents at idle with the heater on.