A car's water pump is "critical" to keeping your engine running cool. Without a properly working water pump the vehicle would over heat very quickly and quite possibly, if left unattended to, actually ruin the entire engine.
Identification
The water pump is attached to the engine block and operates through centrifugal force. You can easily locate it by following your lower radiator hose to it since it is connected to the pump. The heater hoses are also connected to the pump. Within the pump housing are fan like blades, or fins, that constantly turn, pushing the coolant from the radiator through the cooling system channels in the engine block and cylinder heads and back to the radiator again for re cooling the fluid or antifreeze. This is a continous cycle. There is a pulley on the pump, turned by a drive belt, which turns or propels the pump fins within the housing to keep the coolant moving.
Effects
Without this turning motion, the coolant would remain stagnant and not be able to flow, and the engine would overheat. In order for a car engines to remain cool, it relies on fluid cooling for the engine and also fan cooling to keep the coolant cool within the radiator as it recycles back into the hot engine. A working water pump is imperative to a vehicle's operation, along with other parts within the cooling system -- for instance, the hoses that carry the fluid, the thermostat which opens and closes, and of course a leak- and clog-free radiator, including a clutch or electric fan that air cools the coolant in the radiator. They all must work together.
Risk Factors
Water pumps have a small opening called a "weep hole" which is a way to detect if the pump is leaking. If the pump is leaking, coolant will leak out of this small hole to inform you that the pump has developed a leak. Usually by this time the pump is on the critical list and should really be replaced right away. In other cases, the pump may not leak but still be ineffective in moving the coolant properly if the internal fins or blades are worn or corroded. Over time, the edges of the fins may wear away leaving only thin blades turning which are not collecting enough coolant to push along with proper force and pressure to cool the engine block and cylinders.
Risk Factors
In the case of a skipping or defective pulley belt not turning the pump effectively, the pump itself may be fine, but the problem may be with the pulley belt or possibly the pulley. In some cases the pump is driven by the same pulley that drives the timing chain or belt. Often times the bearing in the pump will wear out, and in some cases the pump may just have a faulty gasket, which sits between the pump and engine. On some cars, it is very difficult to remove and replace a pump, and it only makes sense to replace the whole pump since the labor to get to it was so intensive, rather than just replacing a bad gasket. It's always a good idea to change your coolant, check your thermostat, all hoses, pulley belt, radiator and radiator cap when you change a water pump to be sure that all is well.
Misconceptions
Many times an amateur mechanic or a lay person may automatically assume that it must be that the water pump is bad if a car overheats. However, many times you may have a clogged radiator, low coolant level, a bad thermostat not opening at the proper temperatures, a faulty radiator cap not holding proper pressure, a small leak in a hose carrying coolant, or even bad ignition timing. These are all possibilities for an overheating situation. The water pump may even end up as the last item on the list to troubleshoot, unless you can literally see the coolant leaking from the weep hole or possibly the gasket.
