At the heart of the cooling system is the water pump. It spins as does the engine in order to continuously circulate engine coolant through the automotive cooling system. Out from the engine, into the radiator, and then back again. So goes the water-cooled internal combustion engine. If the water pump fails in its task the cooling system will have the automotive equivalent of a heart attack. A few miles outside of Baker, California on a 107-degree day is usually where a water pump clutches its chest and quits. Steam pouring from under the hood and an engine clattering from detonation are sure signs of heart failure.
The water pump takes its circulating power from the engine. Most operate through a belt and pulley and others via a gear or chain. The power spins a shaft on which there is an impeller. As the impeller spins it circulates coolant in much the same way a propeller on a boat works to tow drunken water skiers. The shaft and impeller spin on sealed bearing. Sealed, in order to prevent coolant from escaping the confines of the cooling system. This bearing is the part of a water pump that usually wears out first - followed by the shaft chewing up the pump housing.
A water pump that is about to give up the ghost will bleed or leak coolant. Coolant weaping from the underside of the water pump is the beginning of the end. The bearing inside the pump may also howl before it comes apart. Tell tale signs of a failing water pump are coolant leaks originating from the water pump, or if you're really out of luck, the water pump behind the timing cover of the engine. Coolant leaks can be tricky things to isolate. Check all the hoses and clamps and radiator itself before digging into the water pump and finding a heater hose was leaking coolant thanks to a broken 17-cent hose clamp.
Eat Your Vegetables
Since a water pump is either working or not replacement is a matter of necessity or preventive maintenance. If the water pump is leaking or puking coolant through a an unsealed bearing or cracked housing then it obviously needs to be replaced. Preventative maintenance is a good plan if the cooling system is half apart anyway, or if the water pump lives behind the timing cover or some more nefarious location. Water pumps that get their power from the timing belt or chain are often located inside the engine, and best replaced when servicing either component, or vice versa.
With any luck the water pump will be mounted to the engine in such a way that it can be easily removed and replaced. This is true of the good lot of water pumps. If leaking coolant is obvious but the location of the water pump is a source of great mystery, then the engineer that designed the cooling system has played a cruel joke on the DIY greasemonkey. Once again we cannot stress enough that the most important tool in the toolbox is a service manual. Search the message boards for digital copies if driving a Meapsy or a Citro n Ami. If the water pump is easy to find then it should be fairly easy to remove and replace. If not then some reading may be required. The following should supply a basic idea of water pump removal and replacement.