The bearing in the water pump on my wife's car (2002 Vauxhall Corsa C, 1.2 Ecotec engine) failed - so I decided to make this my first Instructable! I'm keen to point out that I'm not a professional mechanic, and although this isn't the first time I've tackled a job of this nature, I'm sure there are better tools or techniques for elements of this task (and if you look closely at the background of some of the photos you might notice that I didn't necessarily carry out the steps in the order that they're published here!).
However, since replacing the water pump the vehicle has covered 500 miles or so without losing any water - so the fix appears to have worked so far.
I hope this is helpful to somebody.
Step 1: Diagnose / confirm the fault
The car had developed a rattling / grinding noise, which didn't seem to vary in pitch or speed with acceleration. The obvious thing to do here was to listen closely with the bonnet (hood) open while the engine was running. Having established roughly where the noise was coming from, I employed one of the mechanic's favorite diagnostic tools - the Listening Stick. Everyone has one of these in their tool box - even if they don't know it. It could be a length of wood, piece of plastic / metal - basically a stick of some sort which will allow you to place one end against your ear, and the other end against (in this case) the engine, in order to pinpoint noise / vibration. I used a long reach screwdriver, with a soft plastic handle. Placing the tip against various points of the engine, and the handle against my ear I was able to establish that the source of the noise was the water pump. BE CAREFUL what you're poking here, as there are moving parts nearby - in this case I placed the tip of the screwdriver against the engine block, very close to the moving water pump pulley in order to establish that this was the faulty bearing. If working under the bonnet with the engine running is unavoidable, make sure that any loose clothing is tucked in, and long hair tied back.
The water pump is driven by the top pulley of the auxiliary drive belt (the only moving belt you can see with the bonnet open), and is situated at the LH end of the engine block. On many engines, the water pump is driven by the timing belt, making it quite tricky to get at and involving a lot of work to replace (timing belt / tensioner are usually replaced at the same time). Luckily this Corsa's engine is an exception and the pump is easy to get at once you've removed a few bits.