Hello Peter,

Good to hear from you again.

If the engine does not overheat when full of coolant- then lose of coolant would be the problem to address.

I would recommend a new pressure cap and that you add a coolant recovery tank to the system.

The addition of the coolant recovery tank will prevent air from entering the cooling system. Air in the cooling system can sometimes create an air lock at the top of the engine and lead to intermittent overheating as you describe. This can be more of an issue if the engine is set into the vessel at a steep angle, placing the thermostat at the highest point.

We keep a stock of both these items.

Please call if we can assist you further.