Vented Vs Non Vented Fuel Cap

Both styles of caps provide vacuum relief.

Vented vs non vented fuel cap.

Gasoline systems have been running non vented caps for quite a while due to the emissions requirements. Gas fireplaces come in two configurations. Vented caps only vent one way into the tank to allow air in as the fuel is used to prevent a vacuum from occurring. 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.

All modern vehicles use a non vented cap. If your tank has a vent then you will need a non vented cap. If that tube is not present then you need a vented gas cap. The vented gas cap is designed to vent small amounts of air into the car gas tank line.

Look at the front of tank at the axle for a u shaped metal vent tube about one inch in diameter. 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. 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. Gas caps have not been vented since the introduction of evaporative emission controls smog control on cars many years ago.

Emissions gas caps have a built in pressure relief valve that will only vent to the atmosphere if the pressure gets too high. The non emissions caps freely vent the tank to the atmosphere which can waste gas through evaporation. 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.