The fuel tank will have a line out to allow air to be pushed out as fuel goes in.. Place be never had a problem fueling my X so I haven't looked into this but my VW Passat had a button just inside the fuel filler hole that could be pressed to vent the air out quicker.. It's a common 'mod' to remove the valve to save pressing the button, it's called a ventectomy.. Some say this also allowed more fuel to be put in..