why does my f150 have a large evap leak after running for 15 minutes

P0442 is a small leak in your evaporative system, something like a loose or worn gas cap can trigger this code. Loose or damaged vapor hoses, defective fuel tank pressure sensor, issues at the EVAP canister as well as any issues at evap solenoids. If you have a capless filler pipe, the seal may be poor. To diagnose this problem the proper "smoke" machine would come in very handy.