what is this?

[bishbashbosh]

the black rubber stopper?

What is it's function?

54889

[Stropp]

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.

[BigRAWesty]

As above but pretty sure it's a cap

[bishbashbosh]

i won't fiddle with it..

[lhurley]

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