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I completed my first coolant flush on my 2007 M Coupe today and beforehand had a difficult time finding an in depth DIY so I thought I'd post up some tips for anyone wanting to do this for the first time in the future. I won't go into any detail about the entire process because there are plenty of good write-ups and videos out there for coolant flush procedures (i.e. Bav Auto's Coolant Flush video on YouTube). My biggest issue was how to access the engine block drain plug. There are non-M DIYs that make it look like there is significantly more room to get around and M DIYs that suggest accessing from the top although they lack photos and details on exactly how to do this. Of course you're using these tips at your own risk and I take no responsibility for any mishaps that may result from using these tips. I'm not too savvy in the image posting department so I'll just number and explain the pictures here (EDIT: On mobile the pictures appear out of order. My apologies.).

Pic 1: This first pic is just of the little foil spillway I made to direct flow from the radiator drain over my APR splitter and into a drip pan. Didn't spill a drop. If you don't have a splitter this won't be necessary.

Pic 2: After removing the undertray and draining the radiator I removed the 13mm bolt connecting the ground wire pictured then removed the 12mm Vanos Pressure Accumulator (black cylinder) clamp bolt. Removing the ground wire allows room to push the Pressure Accumulator up and out of the way.

Pic 3: After pushing the Pressure Accumulator up out of the way remove the two 13mm bracket bolts and remove the bracket. This creates a tiny bit of valuable space. Do be careful moving the Pressure Accumulator, though. I don't know how sturdy the hose attached to the top is.

Pic 4: After removing the bracket I dropped the Pressure Accumulator back down to allow better access to the drain plug. I used a 3/8" Tite-Reach wrench and 6" extension. Piece of cake. If you don't have a set of Tite-Reach wrenches you won't regret investing in them. After breaking the plug loose it comes right out by hand easily.

Pic 5: Before fully removing the plug other DIYs recommend placing paper towels below the plug to catch the drainage and route it to your drip pan. Both times I flushed I tried to limit the drainage to a trickle but ended up with flood. Nothing escaped until the plug was fully removed and it made quite a mess. If others have a good technique for minimizing the mess I'd love to hear it but even so it wasn't that terrible. Just difficult to manage in such a tight space.

Pic 6: Here are the plug and gasket part numbers. To illustrate the importance of using a new gasket each time observe the new gasket on the left and the used, compressed and expanded (circumferentially) gasket on the right.

When you're finished just reassemble in reverse order. I put everything back together loosely before flushing with distilled water just for my peace of mind though the ground wire is probably the only thing necessary to reinstall for that step.

I would also highly recommend replacing the plastic OEM bleed screws with the brass bleed screws available at most of the BMW parts suppliers online. After bleeding the system the last time my radiator bleed screw was looking pretty rallied so I'll have a couple on hand to replace them next time around. As other DIYs suggest it's also a good idea to have a spare block drain plug and radiator drain plug on hand.

Hope this is helpful and if anyone has any other suggestions I'd be happy to modify this.