Vented Vs Non Vented Fuel Cap

Emissions gas caps have a built in pressure relief valve that will only vent to the atmosphere if the pressure gets too high.

Vented vs non vented fuel cap.

All modern vehicles use a non vented cap. Look at the front of tank at the axle for a u shaped metal vent tube about one inch in diameter. The non emissions caps freely vent the tank to the atmosphere which can waste gas through evaporation. Gas caps have not been vented since the introduction of evaporative emission controls smog control on cars many years ago.

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 these systems the tank incorporates a third vent line that is routed to a cannister inside the engine compartment that is designed to capture and recycle gases formed in the fuel system. Gasoline systems have been running non vented caps for quite a while due to the emissions requirements. Gas fireplaces come in two configurations.

Using a non vented cap on a car from the 1960 s that required a vented cap can cause the car to run for a while until the fuel system draws a vacuum in the gas tank. If your tank has a vent then you will need a non vented cap. Both styles of caps provide vacuum relief. Vented caps only vent one way into the tank to allow air in as the fuel is used to prevent a vacuum from occurring.

A non vented cap is just that no venting at all and will not let air in or out so unless you have an eec tank or other vent setup elsewhere you ll have some issues. 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.