1989 318 coolant issue

Don't miss post 17

But if the hose I see in that pic, that is
attached to that plastic straw;
if that straw goes to the bottom of the tank
on the outside of the tank
like mine does, then
it is a coolant recovery tank, and the cooling coolant in the rad, will in fact suck it back into the top of the rad, as long as the cap is working. And you will know the cap is working by checking the coolant level in the rad, when cold, after having gone thru at least one hot/cold cycle, and finding the rad as full as it gets.
In this type of system, a pressure leak into the cooling system will usually manifest as an oily brown frothy mess in the recovery tank.