I repeat again: to drain ALL the coolant, you need to remove the engine block drain plug. Item #4 in the diagram below (p/n 07119904539) and replace the gasket ring item #5 (p/n :07119963200). Also, you need to drain the heater core. Ask me how I know (AMHIK).

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.