KA24E Bad Mileage... Vacumn Problem?

I own an Axxess SE, AWD, Automatic with the KA24E and seem to be experiencing really back mileage. I'm estimating 17-20 L per 100 klms. Every time I turn off the car I get a loud hissing/air sucking sound for about 1-2 seconds. Also, when opening my gas cap I get what I would consider to be an abnormal air vacumn sound. Any Ideas???

do you have an aftermarket intake? mine does the same thing.
abnormal gas sound? explain. just so you know, all cars gas tank have a gas sound when you open the cap.