Make sure to use distilled water. It is okay to not know the capacity. Just keep mixing up your own 50/50 blend a little at a time and only add that mix. Keep in mind that there will be residual fluid left in the block after you do these water flushes unless BMW has a plug on the side of the block to remove to get that out. Toyota gives you a block plug to remove. So if you cannot do that, err on the strong side when blend up your coolant and water to end up at 50/50. If you truly knew the capacity, that would help. I would fill and drain 3x with distilled water if I was doing it.
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