Water is boiling out of the overfliw
If coolant is actually blowing out the reserve tank, it's being pressurized by compression from the engine. You probably have a blown head gasket or a crack/leak in the head. Have a compression test done to find out what the exact problem is.
Head gaskets seem to be a common problem with the PT.
Have a chemical block test done to see if you are getting exhaust fumes in the cooling system. You should not be using just water in the cooling system