Here are the procedures I used to completely flush and refill the cooling system on my car. BMW recommends that this be done every 2 years. I used BMW anti-freeze (at a hefty $14.75 per gallon) as this is highly recommended and I'm not one to be penny wise and pound foolish on these matters.

This information applies to a 1993 325iC E30 with a M20/B25 engine.

Tool's required: a 8mm wrench, a 10mm wrench, a 19mm wrench, a screwdriver, 2 gallons of anti-freeze, and a shallow pan that'll hold at least 4 gallons. (you'll have right much anti-freeze left over. 1 gallon is not _quite_ enough for a 50/50 mix, and 2 gallons is way too much)

You can make it easier if you have a 10mm socket on a ratchet with a 6" extension or a 10mm nutdriver.

Note: To drain the block you need to start with an engine that's cool to the touch. ;-)

Step 1: Remove the plastic piece that covers the bottom of the radiator. It's held in place by 2 8mm screws and 2 10mm screws on each side. The easiest way to do this is remove the 2 8mm screws first, and gently free the ducts that direct air to the brakes. The duct on the driver side has an electrical connection that doesn't have to be disconnected.

Step 2: Remove the drain plug from the radiator (screwdriver) and drain the fluid into your pan. It's going to make quite a mess so be sure you're working in an area you can hose down, spread kitty litter, etc.

Step 3: Drain block. On my engine, this is a 19mm screw located just under the exhaust manifold on the passenger side. Be sure to move your pan to catch the coolant that comes out of the block, although I guess you'll only get 60-70% of it (and I was using a pretty big pan).

Step 4: Flush. After everything finished draining I filled the expansion tank repeatedly until the water coming out of the block was clear.

Step 5: Put the plugs back in and tighten to spec. If I had it to do over again, I'd replace the radiator drain plug with a new one since the rubber washer looked a bit on the aged side.

Step 6: Loosen the bleeder screw on top of the thermostat. On my engine, this was an 8mm brass screw that stripped when I looked at it. I sprayed it with WD40, played with the kids for a few minutes and was able to gently loosen it with a pair of vise grips. Pour in coolant/water mix until it comes out the bleeder screw opening. I started with 4 quarts of coolant and then a gallon of water.

Since I had an empty 1 gallon container, I then prepared a gallon consisting of 1/2 gallon antifreeze and 1/2 water for step 7.

Step 7: Crank the engine, turn the heat wide open, and run the engine at fast idle (I did 1500 rpm) until the thermostat opens. At that point, the coolant level will drop and I started adding the 50/50 mix I made in step 6. Keep an eye on the opening at the bleeder screw. It'll spit and sputter as the air works out of the system but in short order it'll clear the air out and a steady stream of coolant will be coming out. BTW, I enlisted my wife to be the one who sat in the car with the heat wide open and the engine revved up so I could watch the coolant level, etc. I owe here dearly for that 10 minutes of purgatory. ;-)

Step 8: Tighten the bleeder screw back down and turn the car off, and make sure the coolant level in the expansion tank is at the proper level.

Step 9: Put the plastic cover back on. This is where you'll appreciate the 10mm socket w/ 6" extension. Since you have to have the brake ducts back in place during reassemble, there are 2 10mm screws that are almost impossible to tighten w/o the socket.

Step 10: Let the engine cool, and check the coolant level again, making sure it's full to the "Cold" mark.

After all this was completed I drove the car approx. 20 miles running various errands keeping a close eye on the temperature gauge and all was normal. The next morning I checked the coolant level again and everything was right-on.