when you remove the head bolts from that engine you are supposed to install new ones, you also need a diesel engine coolant like BASF Glysantin G-05 or G-30 to prevent cavitation.
Please also check the fan clutch for correct operation, it should couple up when the radiator is hot.
btw - pure coolant has very very poor heat transfer capability. The water mixed in the coolant does the heat transfer while the chemical make up prevents corrossion, raises boiling point, lowers freezing point and keeps it stable.