The pressure in the tank should be negative (vacuum) if you've been driving a while. If not, you may have knocked off a vacuum or EVAP hose somewhere. That would likely make it idle lower and/or poorly, and EVAP system problems also usually set off the check engine light (like when you leave the gas tank cap off).

'97 4Runner, '06 F350, '86 4Runner, '05 WR450
http://home.4x4wire.com/erik