Head gasket is a major possibility, but first check
raw water flow through
heat exchanger. If plenty of water coming out the
exhaust, that will probably be OK. If not , then the
engine may be
overheating and boiling the
coolant out. If the head gasket is gone, the sump
oil may be milky, and there will be gas bubbles coming through your header tank, esp with the
engine under load. There is a tester for combustion gas in the
cooling system for petrol engines, not sure about diesels.Regards, Richard.