Coolant spraying... What is going on?

[Waterloo]

I have not tried with the cap off. I start the motor cold, cap on, she sprays, I shut her down, and immediately restart, no issue on the restart with cap on or off. The system itself does not seem to be building any pressure other than what would normally expect, a bit of water cavataion in the bottle. I put my hand over the coolant jug top for a few minutes, I felt no pressure on the palm of my bare hand at all.