How does the gas cap work?

[TruckintheBoro]

This will sound dumb, but I'm new to trucks. My prior post about why does my carb flood feeds this post. THe mechanics I have been using wondered if there was a chance that pressure had built up between the fuel tank and the carb as temperatures here in North Carolina got up into the 90s... Obviously, with the gas cap in place, as the air above the fuel in the gas tank expands with the heat, pressure does build up. they looked for some sort of venting path to discharge that pressure (they made it sound like that's a normal feature of modern cars/trucks?) but couldn't find one. I definitely find that if I open the gas cap on a hot afternoon, I can hear and feel a good bit of pressure getting released.

There is a pressure valve in the gas cap, but my mechanics were under the impression that that just lets air into the gas tank so that you don't get a negative pressure situation as the fuel level drops, and that it doesn't let pressure back out. Another mechanic suggested one remedy was to drill a tiny hole (1/8") in the bottom of the flange the cap screws onto. Does anyone know how things are supposed to work?

[old Chevy guy]

Depends on the truck. Mostly year and to some extent location of origin. I'm no expert but I've read enough info on this site to know some are vented and some are not. Some vehicles have some form of emissions on the west coast. Post more info on your truck,year model engine and maybe the spid sticker from the glovebox door. With that info someone who is more expert will probably chime in. However if the needle and seat is working properly I'm not sure that a gas cap issue would cause your flooding . Post more info.

[GR8-68]

I understand what you are saying but like oldchevyguy said I am not sure that would cause a flooding issue. Sounds more to me like you have a carb issue. Trash in the float, gummed up, piece of dirt in the needle, there is numerous possibilities.

[joeydurango]

If it's a pre-smog truck (could be wrong but I believe smog controls began on '72 model year C10's nationally and maybe a little earlier in California), it will have what's known as a non-EEC (Evaporative Emissions Control) tank. These tanks should have a vented cap to regulate pressure. If it's a truck with smog controls, it will have the EEC tank, which had a non-vented cap but a vent system on the passenger side of the tank that ultimately led to a charcoal canister in the engine bay. Often someone will put a non-vented cap on a non-EEC tank, and then you have issues when the temperature fluctuates significantly.

I believe that too much pressure in the tank can indeed cause to much fuel pressure and carb issues. So can vacuum in the tank. (That said, you should check the other potential issues too!) A true vented cap - one that lets pressure flow both directions - can be hard to find nowadays. Maybe someone can chip in with a link if they know of a true vented cap source?

[GRX]

Any fuel system must be able to equalize pressure in the tank somehow. Otherwise when gas is pumped out the vacuum would build and eventually counteract the pump creating a "stale mate" situation. This is what happens when a non-vented cap is used in a non-vented fuel system. Vehicle eventually stops running, and a great sucking sound is heard when cap is removed.