This happens when red & green coolants are mixed together.They do not mix that is at least what I was told. You have to use new. I'm guessing here that you will likely use a different type that's already in the system.
(As Mika described drain the whole system if possible plain water)
... When mixed together they can form a thick, jelly-like substance that can completely stop all coolant flow which can lead to overheating.
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