Australian Car Repair Issues? Ask an Auto Mechanic for Answers ASAP
Hello, the coolant cap is the pressure regulator also.
If you look, the cap isn't just a plain cap, it's job it to allow excess pressure out when the system is heated and to release vacuum as the system cools, it's a quite involved piece of equipment in the cooling system.
If there is no leaks on the engine that you can find, I would consider whether this cap needs replacing.
Think back to old pressure cookers, the pressure was maintained so that the water wouldn't boil until a higher temperature, if the pressure cap is faulty the water boils at a lower temperature.
I hope this helps ;)