![]() Since its new, I said what the hell and put my mouth on it to see what happens when I try to move air through it. So today I received my new vented gas cap, which has the plastic venting piece on it. When I replaced the tank for the efi I connected the vent port to a hose that runs up near the filler neck and ends in one of those tank vent canisters. And when using a vented gas i would sometimes get liquid out the filler neck when the tank was full. ![]() I had the same venting issue with my 68 elco. Drive around a little bit in 85 degree weather and that change in temp of the gasoline was enough to put some serious pressure in the tank. I have seen the issue in my car where you would go get gas, and out of the ground tank it would be 58 degrees. The coefficient of expansion of gasoline is relatively high, so it would not take much heat input to see a noticeable pressure increase, even without creating any vapor, which would compound the issue. It is also not just the vapor creation but the amount of expansion of the liquid and the gas as they both heat up. If the level is high it takes much less vapor to pressurize to the same level than if the tank is almost empty, as the liquid is non-compressible. To an extent it will also depend on how much gas is in the tank. I'm going to open the gas cap and see if it's a vacuum or pressure situation as soon as I get to my stop. I have about a 20-minute drive tomorrow morning. I should have a vacuum condition when I open the gas cap! But in my case with the Elco, I'm running the car - taking fuel out of the tank - now requiring air to enter into the tank, of which it cannot with a closed-off gas cap. I see this with my non-vented lawn mower gas cans in storage. I understand fuel heats up and causes pressure. This anti-splash mechanism cannot be relied on to never let fuel exit out on a passenger car - hence the non-vented cap on a rear exit filler neck car. The little white thing on the inside of a vented cap is an anti-splash mechanism that is supposed to allow air to enter and exit, but if fuel sloshes up into it the white thing has a valve to block the fuel from exiting. The Elco filler neck exits out the side and is higher than the gas tank. Since the cars filler neck exits out back - it needs a non-vented cap or else you'd lose gas when accelerating. The Elco requires a vented cap, whereas the car has a tank venting system, so it requires a non-vented cap. 69 El Caminos have a different situation than the 69 Chevelle passenger car.
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