The water pump is one of the more critical components of a vehicle. While years ago it circulated water to cool the engine, today it usually circulates specially formulated liquid called anti-freeze and usually referred to by professionals as "coolant".
What the Water Pump Does
The water pump circulates coolant through the engine to pull heat from the core and dissipate it via the radiator at the front of the vehicle. This keeps the engine at operating temperature and prevent overheating and serious problems. Most engines use anti-freeze in the coolant lines through which the water pump pushes liquid in order to avoid winter freezing of the coolant ñ which usually leads to serious engine problems.
The water pump is usually located at the front of the engine and is operated by the timing belt, which spins the pump's pulley, providing spin to the internal pump, circulating coolant. The pump itself is a relatively simple machine and looks like a fan inside a specially-shaped container. Coolant comes in one side and fan blades push it through and out the other side. The speed of the pump is regulated by the speed of the engine via the timing belt's movement.
How To Tell If the Water Pump Repair is Needed
The most common problem needing is leaking. The seal around the pump will break, allowing coolant (which is somewhat pressurized in the pump) to be pushed out, leaking onto the ground. A leak from the water pump will put yellow or green anti-freeze (depending on that being used) on the ground at the front of the engine. It will often be dripping from the inlet (at bottom) of the pump, but could be leaking from anywhere. A leak usually is not an indication of failure of the pump, but only of a broken seal.
If the engine overheats or the dashboard gauge showing the water level drops, then it's likely that the pump is not circulating coolant and needs repair. Let the vehicle cool all the way down and check the anti-freeze level in the radiator. If it's low, add some (of the proper mixture) until it's full, then run the engine until it's warm. If it keeps warming up and gets hot (close to overheating), shut it off. Your water pump is likely not working.
Most often, a broken water pump is actually suffering from a bad pulley. Only some models allow replacement and most mechanics advise replacing the entire pump either way ñ it's often roughly the same price regardless.
Timing Belt Replacement and Water Pump Repair
Most mechanics consider replacing the water pump when the timing belt is being replaced as being good preventive maintenance before major water pump repair is needed. This is because when the timing belt is replaced, all of the work removing the components required to get to and remove the water pump is already done. This is a huge amount of labor that you don't have to pay for again when your water pump wears out. Read more about timing belt replacement here