How Do Vented Gas Caps Work?by Steve Smith
Vented Gas Cap Function
The vented gas cap is designed to vent small amounts of air into the car gas tank line. The vented gas cap has a one-way release valve that is pressure-activated. When the pressure reaches a certain point (defined by so many pounds per square inch) the valve opens by a very small amount, relieving some of the pressure. In this case, the pressure is formed on the outside of the tank, due to the vacuum formed from the displacement of fuel on the inside. After the pressure inside the line equalizes, the pressure valve closes. The valve is set to balance the pressure with the surrounding atmosphere. Air is allowed in, but no fumes are allowed out of the gas cap, because of environmental concerns.
The vented gas cap has a chamber under the plastic cap handle that inserts into the fuel fill line. The cap screws shut, and this chamber fits relatively close inside the intake tube of the fuel line running to the tank. On each side of the chamber are small openings where air is able to enter. These chambers lead to the pressure valve, which, as the level in the tank decreases, allows air inside to remove any possibility of a vacuum forming.
How it All Works
Because the fuel tank is pressure-driven from a fuel pump and gas naturally expands and evaporates, a large amount of pressure can build up inside the fuel system over time. This second kind of pressure is taken care of through the operation of the fuel pump mechanism, as the vented gas cap needs to be the one-way flap to keep evaporated gas fumes in.