Brand new Yanmar 2TNV70 keeps boiling the coolant

Hello!
I've bought a Yanmar 2TNV70 to replace a broken one inside a Thermo King reefer. I installed it, it starts up, but the coolant begins to spray out through the safety valve right away (it boils). I checked and cleaned the whole cooling system, made sure there were no bubbles or obstructions left. Everything seems to be in working order, so I have no ideas what could be the issue. I'd be glad for any help.
(PS: The "I" who installed it is my dad, who doesn't speak english, while I don't speak diesel engine, so please excuse the lack of expertise)