I have a Volvo D2-75, and it has the small coolant expansion tank mounted above the engine. So why do I still have a vent on the coolant cap on the engine itself?

Re rute it from engine under the coolant cap to underneath the expansion tank

I guess the adapter above the cooling cap was connected but then the thinner hose on other side of adapter that supposed to go to the engine vent 

That’s how I would do 

I also meant to say that the coolant in the bottle of FalParsi’s second post looks very weak. Ideally coolant should be a 50:50 antifreeze and water mix and preferably using OAT rated antifreeze (longer coolant life). Apart from the obvious freezing prevention, the reason for this is primarily for corrosion protection and the benefit of a higher boiling temperature of the coolant. This higher boiling temperature adds a considerable margin of safety for an engine that works in a hot environment and in the event of partial coolant loss situations the staining from the antifreeze will be more visible to identify repair or maintenance issues.

Hope this is helpful 

Simon 

Since it does spit out a bit of coolant through the hose barb when I run the engine I am considering putting a short (10 cm) hose on it with an end cap.