Fuel Tank Isolation Valve (FTIV)
WARNING: This page is about a different car, the 2007 Mercury Mariner and 2007 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.
The FTIV is a PCM-controlled solenoid that isolates the fuel tank from the rest of the EVAP system. The FTIV is a normally open valve allowing the flow of vapors from the fuel tank to the electronic EVAP canister purge valve and the EVAP canister. Whenever it is desired to isolate the fuel tank from the rest of the EVAP system, the PCM provides a variable duty cycle signal (between 0% and 100%) to the solenoid which controls the FTIV operation.