2002 Ford Ranger fuel leak

2002 Ford Ranger 6 cyl Four Wheel Drive Automatic

fuel is leaking from the top of the gas tank, but only when the engine is running. I can't see where the fuel is actually leaking from, but when I reach up where the fuel lines are, it seems like there is a plug missing. I can't find any hoses that are disconnected, and there is a hole that doesn't feel threaded. I also haven't been able to find any schematics (online or at any parts store) to show wether or not there is something missing; any ideas or info would be very much appreciated.