There is no line that goes to it. It's a port that's supposed to be vented to atmosphere.
I've never seen one just open like that. The ones that have it also have a black protector like is shown in your other photo.
I'd look for the cap. It may have fallen down on top of your intake or something. Otherwise I'd stick a vacuum hose on it, and run it towards the rear of the engine, leaving that end open. This wil prevent dirt, grime, and water from getting into the valve. (same thing is done on differentials)
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