Draining radiator only removes about 1/2 of the coolant. ONly way to do a complete job is with a flush machine or by pulling the block drains.

Open up the radiator petcock valve and let it drain. Safely dispose of the old coolant of course. Refill with a 50/50 solution of water and coolant.

If you have an air compressor, use it to flush the system out. But like they said above just drain and refill

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