I had a small gas tank leak which I repaired. This is a non vented tank with a new vented gas cap. I guess my question is how does the gas cap work, does it let air into the tank, or pressure out? When i open cap I hear pressure release, is that normal? I don't recall it doing this with the old cap, but maybe I'm just noticing it now. Thanks It's a 53 Studebaker, but i don't think that matters much.
Yes, that's normal. The fuel cap should have a set pressure at which it "opens" and releases the built up pressure behind it.
The early caps vented freely, without any regulation. Later (smog) caps have regulated venting, usually a spring loaded valve that works on expansion and vacuum. These types will allow some pressure build, then when opened will cause some pressure release. These regulated caps don't always work well on gravity feed systems, as they can restrict flow. H-D learned this the hard way back in the early eighties. Their caps wouldn't reliably open to vent when the fuel was used from the tank. That left bikes sputtering to a stop. The rider, wondering WTF?, would remove the cap to see in the tank, try to start his bike and it would fire up...go several miles and it would happen again.
WHEN it is sitting out in the sun this is true. Actually in the real world the venting allows air to be drawn into the tank to replace the fuel that the fuel pump draws out. If the tank isn't vented it creates a vacuum in the tank. On some tanks It will actually **** the sides of the tank in in the process. The fuel pump will create a vacuum in the tank that it finally can't overcome and draw fuel out and this kills the engine. When you open the tank the wosh of air going in because of the vacuum may seem like air rushing out. On my 51 Merc I could hear the bottom of the tank pop back out when I opened the cap, it took three new "vented" stant caps to get one that was actually vented.
To my knowledge. A good, vented cap allows pressure release both inside and outside of the tank. Both from like mentioned vacuum build up inside the tank but also for evaporative fumes to vent outside of the tank. With the design for evap emission systems most caps should be non-vented in most cars now. It mostly comes down to your cars evaporative emission and fuel feed and return system to determine what's best. Edit: I do believe around the 70's certain fuel tanks had built in relief valves as well.
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