Nissan Repair: Gas tank pressure, nissan d21, tank pressure

Question
I have a 1990 Nissan D21 pickup with a 2.4L. Whenever I take the gas cap off to put gas in it, there is a lot of pressure inside the gas tank. You can hear the pressure being released and the tank makes a popping sound like the walls are popping back into their original form. When I got the truck it had and still has an aftermarket gas cap. I assume that it is a non-vented cap. Do I need to see about a getting a vented cap, is the tank suppose to be pressurized, or could something else be the cause of all of the pressure? Any help would be appreciated. Thank you.

Answer
Alex,

The fuel system needs to be pressurized so it is not wrong, what is going on, and you might just need a factory style cap since it will bleed off pressure before it is opened.  You see, the fuel pressure is part of keeping smog under control.  But, rapid escape of fuel vapor is probably just the aftermarket cap.  The factory ones bleed off pressure as you begin to turn them.