The Liquid/Fuel Separator separates the gasoline you put into your car from the vapor the engine uses for power. If you overfill your gas tank, you run the risk of letting liquid get into the compartment designed for vapor, which will create an overly rich fuel-air mixture and a partially flooded condition at start-up. Typically, the engine will run poorly and roughly for a few moments after start-up until the purge valve closes and the engine “clears” the excess fuel.