P0440 Chevrolet 2001 S-10

Question

New OEM cap, no hiss sound, raw gas seen on fuel filler cap contact surface

Answer 1

P0440 Evaporative emission control system malfunction

The way  the EVAP system works you have a canister vent valve, a purge valve and a FTP (fuel tank pressure ) sensor, when you turn the vehicle off the the fuel tank vent valve stays in the open position which allow fuel fumes to collect in the charcoal cansiter, and it is stored until it is consumed by a purge command which allows the collected fumes to be utilized by means of a purge valve. When the vehicle is turn on the vent valve is commanded to close. hving the hiss sound that a good indication you have a EVAP leak in the system , this  can be from the fuel tank filler clamps being loose to a torn EVAP hose or a faulty evap vent valve which stays in the open position. , inspect the EVAP system look for any torn or ruptured hose that can cause P0440