[Vwdiesel] Simple little TDI question

[LBaird119 at aol.com]

  Dad's been checking a few little things on the Passat and came
across two things.  First, the air filter had gotten wet at some
time and had been sucked partly inside out.  It was sucked up
against the outlet from the air box and was restricting air flow.
The next thing he found was a lot of air in the fuel line.  He put
on a new fuel filter only to have even more air!
  I checked a few things this morning and it seems that it's just
plain slow to bleed all the air out what with the recirculating arrangement
they use.  Does anybody know just why they do that?  I thought I'd
figured things out when I found that most of the return was going to
the filter instead of the tank.  If you turn that little plastic thing around
then most of it goes to the tank.  Thing is the check engine light now
comes on.  I'm wondering if it's causal or coincidence?
  Could someone take a look at that little plastic thingy that fits into
the fuel filter?  There's a little dot toward one inlet/outlet.  I'm
wondering
which end gets the dot, the tank end or the pump end?
  It just doesn't make sense for the fuel to return heavily to the pump.
There's no place for the air to go quickly!
  Thanks,
     Loren