If your vehicle is showing car is hard to start immediately after filling the gas tank, here are the most likely causes and the parts you'll need to fix each one.
The purge valve routes fuel vapors into the engine to be burned while driving. If it is stuck open, the act of filling the tank pushes a massive amount of vapor directly into the intake manifold, flooding the engine and causing a hard start.
Regularly 'topping off' the gas tank after the pump clicks off forces liquid fuel into the vapor canister, which is only designed for gases. This saturates the charcoal, causing raw fuel to be sucked into the engine upon startup.
The vent valve allows air to escape the EVAP system while refueling. If it is stuck closed, pressure builds up in the tank and can force fuel vapors past the purge valve into the engine intake.
In rare cases, a weak or failing fuel pump can temporarily bind or lose pressure when a large volume of cold gasoline from the station hits the hot pump motor inside the tank.