Vented Vs Non Vented Fuel Cap

Both styles of caps provide vacuum relief.

Vented vs non vented fuel cap.

The vented gas cap is designed to vent small amounts of air into the car gas tank line. Emissions gas caps have a built in pressure relief valve that will only vent to the atmosphere if the pressure gets too high. 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. If your tank has a vent then you will need 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 vented gas cap has a one way release valve that is pressure activated. Gas fireplaces come in two configurations. Gas caps have not been vented since the introduction of evaporative emission controls smog control on cars many years ago.

Vented gas logs produce carbon monoxide emissions so your fireplace must have a chimney or flue. The non emissions caps freely vent the tank to the atmosphere which can waste gas through evaporation. All modern vehicles use a non vented cap. If that tube is not present then you need a vented gas cap.

Vented caps only vent one way into the tank to allow air in as the fuel is used to prevent a vacuum from occurring. 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. Ventless logs do not put off these emissions so no chimney or flue is needed and the heat is more likely to stay in your space.