What is the function of a vented gas cap on our vintage trucks?
There has been allot of information/misinformation, both on this forum and on the internet on the function of a vented gas cap on our vintage trucks. I’m not talking about a charcoal canister vent or the vent along the gas filler neck that helps vent the air when filling the tank, but just the cap itself.
It has always been my understanding, that a vented gas cap is vented in one direction, “in” to allow air makeup as the fuel pump empty’s the tank. In addition, the cap seals sloshing fuel and vapors from exiting the tank. This theory is supported by some vented caps that give a warning printed on their cap “Fuel Spray Many Cause Injury, Release Slowly”.
If you do a internet search you will get answers from so called experts that are all over the place. From my theory, to venting equally in both directions, to venting “in“ for air replacement and venting “out” for pressure release after a few PSI, like a pressure release valve.
Many of these posts originate from smelling gas vapors inside the cab or outside by the gas cap or a failed gas cap causing a vacuum in the tank, starving the engine from gas. Some ill advice is the drill a hole in the cap so it will vent in both directions which to me, skirts the real issue.
So getting back to my question, what is the function of a vented gas cap and then specifically how it applies go our beloved trucks.
