what is this?

the black rubber stopper?

What is it's function?

Attachment 54889

Not sure as I am fortunate enough to have a 4.2 :) I'm sure someone can help you out. If I had to guess I would say it's to check the inlet manifold vacuum.

As above but pretty sure it's a cap

i won't fiddle with it..

It's an unused vacuum port. No need to worry about it unless it's damaged