Bleed all three points with heater valve fully open and run up to temp again if it still does it change expansion cap and try again, if that doesn't help I would suspect head gasket and system pressurising from exhaust gasses.Hi my mgf has just starting blowing steam through the expansion tank top fans all cut in when engine temp is at right temp heater only blows cold air it's pushing water into expansion tank but after a while seems like somewhere there's a block in it and just falls up and then steam erupts I have put a new cap on the bottle but it still does it any ideas please