The water pump on my car developed a leak in its rotating seal and started to hemorrhage coolant. Here's how I replaced the water pump.
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Step 1: Disconnect the Battery
Disconnecting the battery is a good first step for almost any car repair. Now you don't have to worry about shorting/frying anything expensive or pesky radiator fans turning on and chopping your hands off.
Step 2: Find the Leak
There was obviously a substantial leak. Every time I parked the car, a pool of bright green liquid formed underneath. However, the leak hadn't quite gotten to that easy-to-find spraying, steaming, hissing, girlfriend-yelling-at-you stage.
First step in finding a leak is to get under the car and follow the bright green coolant trail. Unfortunately, in this case the trail quickly hid behind a pulley.
Second step in finding a leak is to just take everything off that might be in your way off. Don't be timid here; if it looks like it might come off, take it off! I ended up taking off the coolant reservoir, wheel, belt, alternator, idler pulley, water pump pulley, etc... And I still didn't find an obvious leak! Though, I did find a suspicious puddle of coolant on the crankshaft pulley.
Step 3: Really Find the Leak
The leak still wasn't in plain view, but I was pretty sure it was coming from the rotating seal of the water pump. Before I took of the water pump, which is kind of a pain, i wanted to make sure this was the source.
So, I pressurized the coolant system by running a hose into it. To make a seal between the hose and coolant system, I used an old bicycle inner tube (Anyone who's hung around TimAnderson knows that these are the solution to all of your problems).
The area behind the water pump pulley starting spraying water. My mission was clear: I had to replace the water pump.
Step 4: Drain Coolant
Before you take off the old water pump, you have to drain all the coolant out of the system. You'll find the drain nozzle on the bottom of the radiator. Open up the nozzle, and drain all the coolant into a bucket. Don't touch/breath/drink the stuff; it's not very good for you.
Step 5: Remove Everything to Get to Water Pump
I already took off almost everything to find the leak, but if you haven't now is the time.
To remove the serpentine belt, you'll need a breaker bar (i.e. lever arm). There's a notch in the back of the automatic tensioner that you attach the breaker bar too. Pull the tensioner up and remove the belt once it's loose.
Everything else is just removing screws and hose clamps. Be sure to take a bunch of pictures because you'll probably forget where everything goes when you try to put it all back together.
Step 6: Remove Broken Water Pump.
Take off the old water pump. Mine came off pretty easily, but sometimes you have to pry it off by gently hammering a chisel into the gasket area. But be careful not to scratch the gasket area or you won't be able to make a good seal later!
After the water pump is off, scrape off any old gasket that's still hanging on (but again don't scratch the metal).
Step 7: Prepare New Water Pump
There were pretty good deals for new water pumps on eBay, but I was in a hurry, so I bought one from the local autoparts store.
Apply some RTV silicone to one side of the gasket. I'm not sure exactly how much to put on. The internet says that you should not put "too much" or "too little" on, without really going into more detail. Put the gasket on the water pump. Then, apply the silicone to the other side.
Step 8: Install New Water Pump
Put the new water pump back on the car.
Tighten the bolts. Whenever you tighten a bunch of bolts, it's generally a good idea to follow some sort of "star" pattern rather than a "circle" pattern (e.g. top,bottom,left,right rather than top,right,bottom,left).
Step 9: Replace Everything
Put everything back together in reverse order of how you took it apart.
Step 10: Fill With Coolant
Fill it up with coolant (50/50 anti-freeze/water).