To flush a system I like to remove the lower radiator drain and run the vehicle while continuously feeding it distilled water as the it goes down in the expansion tank. Also give the engine an occasional rev and have the heat set to full with the blower on. Takes a long time to flush (about 30 minutes) but it should get most of the old gunk out. I'm sure there are better/quicker ways, but this has always done the trick for me and gotten even the most corroded and rusty system to pass clear water by the time I am done.

Like AM, I also only use BMW blue in my cars unless doing a flush or test of the cooling system's health.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)