Wow ChrisFix has come a long way in terms of video quality. LOL I had never seen that one before. That poor E39.

I agree, that coolant was ridiculous. I'm betting there was a leak at one point and water had been continually added until it was almost all water. Then over time electrolysis did it's thing and you ended up with rust colored water in the system.

The way I flush is the remove the bottom rad drain as shown in the vid, then start the engine and keep filling with filtered water (or distilled). I do that for up to 30 minutes usually... have a lot of water on hand. On neglected cars it takes the full 30 minutes and well maintained ones, much less. If the car was like the one in the above video... I'll also do the cooling system flush/cleaner that he did with distilled water. Then shut the car off, let the distilled water drain out. Fill with BMW (or equivalent) coolant (50/50 mix) and then bleed the system.

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