Gas Tank Pressure

[Mark & Carla Meswarb <mmeswarb@itexas.net>]

I was working around the trunk on the Tiger yesterday and noticed gas fumes. 
I tracked it down to the passenger side.  Everything looked secure, but when I
popped open the gas cap, it released a lot of air pressure.  The car had not
been driven for days, but the ambient temps here in Texas are on the rise. 
Almost 100 last three days.  Is the cap or tank supposed to be vented somehow?

Thanks,
Mark