Trouble shoot 270 crusator marine engins?

Question

anti freeze will not stop boiling out over flow

Answer 1

Heat exchanger check the ends to see if the zinc's have plugged them over the yrs>Clean the tubes out>Sea strainer full>

Answer 2

if your engine is overheating it will cause overflow. more info would be helpful in answering. your thermostat coud be stuck in a closed position. your water pump may be bad. your exhaust system may be clogged.you may need new coolant. coolant should be replaced every two years.