I understand before refilling your system after a coolant flush you should refill and drain the system several times with water.

This is fine however last time I refilled and drained over three times and there was still colour in the fluid. I don't have the time to keep flushing over and over until it is pure clean because I need the car for work.

I was wondering would it do any harm to flush out most of the fluid but not all and refill to correct 50/50 mixture using a hydrometer?

I would also like to ask from your experiences what is the rough number of times you drain and refill your fluid until it is pure water as maybe I am doing something wrong?

  • 2
    I don't drain and refill, I flush water constantly until it is clear.
    – Moab
    Commented Jun 18, 2017 at 3:55
  • IMO if you are doing a routine scheduled refill there is no need to be retentive about purging every last drop of the old fluid. If you have some contamination in there that's another matter.
    – agentp
    Commented Sep 12, 2017 at 15:14

1 Answer 1


You were probably hoping to not hear this, but it depends. Some engines simply have more twists and turns and possibly areas that don't see flow except under certain conditions or until the cooling system is properly burped. I've had some cars that drained clear after about the third refill, and I've had others that still have a tinge of color after the sixth refill! Unless the coolant is seriously grungy for some reason, I'd say once it starts looking like extremely watered down soda, you're probably fine calling it a day.

Make sure you run the heater when running the engine to get it good and mixed up, and that run the engine at least a few minutes beyond warmed up. Also, keep a bottle of mixed coolant in the car for the first few days and check on the coolant level regularly. As you drive the car, bubbles will work their way out and you'll end up with a low reservoir level.

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