[lunarhighway]

perhaps there's not enough pressure difference to draw the air out of the vent.

i'm not sure what the setup looks like but if you place a small dam in front of the air exhaust this will create a low pressure area behind it wich will draw the air out.

this will work especially well if the undertray causes the air to flow faster in that area.

this setup might add a little drag but it will be marginal and you might not have to run the fan as much