Radiator over flow ????

I have an 89, did have a closed system I changed it over to open. Things were fine and then all of a sudden system builds pressure and fills the overflow bottle. Doesn't seem to over heat just builds a lot of pressure (through the overflow in the cap). Where does all the pressure come from???

Thanks
Kenny

If you haven't done so already, put on a 16 psi cap. I know the guys at the parts place will tell you 13 psi, but get a 16 psi with a pressure release lever.

x2 ^^