- Do not ever mess with the engine block drain, it is always a pain to deal with.
- I have done complete flush and this is what I do: d/c at the lower rad hose and drain it, then reconnect the hose gently.
- Fill system with distilled water. Run engine around the block.

Repeat the cycles a few times, now you only have distilled water in the engine.

- Now, calculate system capacity, drain the water, fill with pure coolant such that you get around 50-50 mixture.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT