coolant boiling out of the expansion tank means that somehow the coolant is boiling inside the engine/radiator - boil should not occur as the radiator cap pressurizes it hence increasing boiling point, AND there is no air in the system, boil can only occur in the presence of air, you might have trapped some air in the system - when filling up the system open the heater to max, park the car nose up and fill.If you suspect the engine is running hot take it to someone who has a live OBD scanner, not a fault reader but live scanner - ask them to display engine temperature in centrigrade, anything below 98*C and above 80*C is good. These modern engines are made to run hot to increase fuel efficiency and reduce emmissions.