Coolant cap untwisting itself, why?

[Pushit2.0]

The cooling system is designed to be under pressure to raise the boiling point of the water in the cooling system. If the cap is loosening, this will in turn lower the pressure at which the cap will allow coolant into the over flow tank. I would try to secure the cap and see if you still push coolant. If I recall your car would hit MBT before the knock sensor would show anything threw LINK.

~John