I have that plug in mine as well. I think that would normally be where the fill pipe is vented. With all the emmissions laws they probably can't allow that any longer. You need to get a good picture/diagram of how the venting works.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)