So... I gave this coolant flush a try and to my disappointment, I was unsuccessful in accomplishing a full flush.
1. I cannot locate the engine block drain.
2. I do not know the volume of the whole coolant system.
What I bought:
Two gallons of BMW antifreeze/coolant (one shown), 2 gallons of distilled water (one extra), new radiator drain plug - part#17117530902
What I did:
1. Jack up the car and level. Remove the felt panels underneath the bumper (Left, Center and Right). They are mounted with 8mm screws and 3x 10mm screws on the back of the center piece connected to the metal skid plate.
(L piece - part# 51757896419, R piece - part# - 51757896420 )
- Unhooked (twist and slide to bigger hole) the hangers holding the center piece on both sides - part# 51757899820
2. Found the radiator drain plug (left side underneath the radiator).
3. Used a large phillips to unscrew it and drain.
4. Replugged the radiator drain hole with the old plug.
5. Filled the reservoir to about halfway with %50 BMW coolant / %50 distilled H20 (made separately in a container). The ball on the floating stick sat above the fill line.
6. Removed the bleed screw and fill cap. Start the engine and turn heater on highest temp without putting the fan on high.
7. I let the engine get to about 140 degrees F (took about 7-8 mins and heat starts coming out of cabin vents) and then shut down. Repeated steps 3.-6. three times using a new drain plug when done.
8. On the final fill, I filled to full and closed the bleed screw, opening and closing to let the air bubbles out while the engine is on. When there are no more bubbles, I used a turkey baster to remove some coolant from the reservoir.
Since I did not know the volume of the coolant system, I did not try to flush the entire system using pure distilled water. The ratio of coolant to water would've been unknown when it came time to fill. I made a premix of 50/50 coolant/H20 and filled with that after each drain. Each drain yielded about 1 gallon of coolant.
What my method hopes to accomplish is to dilute old coolant with new coolant. This is why I mentioned this is NOT a flush but a way to get keep the coolant somewhat fresh. My car is just under 2 years old and I have about 28,000 miles.
If anyone knows a full flush method please let us know. For the time being, if anyone sees other problems with my method in addition to what I mentioned or has a better method than mine please let me and the rest of us know.