Fuel Vent Question

Would someone be able to give me some insight into how the vent from the fuel tank is suppose to work?

In the process of troubleshooting some of my fuel problems, I was double checking that the vent line from the fuel tank was clear. The vent line from the fuel tank (under the ski locker) goes up to the fuel filler neck where it has a separate little hole next to the fuel fill hole. I took the line off from the tank and blew through it (toward the gas cap) to make sure there where no obstructions. It seemed to be fine. Obviously when the tank is decreasing in fuel, air would be coming the opposite direction of my test so there wouldn't be a vacuum in the tank. What I don't understand is how the vent line can pull air when the gas cap is on since it's located under the gas cap. It seems like a tight fit and there's no visible way for air to be pulled in through the cap itself. On my last boat the fuel vent had a separate port right next to the gas cap. Dirt-dobbers like to build there nest in there and that was another problem. Any enlightenment would be appreciated. I want to make sure I'm not getting a vacuum in my tank when the engine is running.

2006 Outback DD Carb

Sometimes the caps are vented even though they look like they aren't. Not sure on these though. Anybody?

The caps are vented. If I fill mine too full and let it sit out in the sun, the expansion effect causes fuel to purge out of the gas cap and run down the back corner of the boat. I have had to siphon out about 2-3 gallons last year when I over filled it one time. Now I have to patch the driveway where the boat sits...

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Originally Posted by aerolland

The caps are vented. If I fill mine too full and let it sit out in the sun, the expansion effect causes fuel to purge out of the gas cap and run down the back corner of the boat. I have had to siphon out about 2-3 gallons last year when I over filled it one time. Now I have to patch the driveway where the boat sits...

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Thanks. That's kinda what I thought. If I blow through the vent hose toward the cap, while keeping the vent on top of the tank covered, I can hear air trying to escape from under the cap. It's a little restrictive when I do it that way, but I guess it's enough flow to equalize the pressures. I can't really test this while sucking (that sounds bad) since I don't want to inhale fumes or any gas that might be in the vent line. I suppose it would be working equally well in the opposite direction.

I just seem to recall a post where someone had to keep their gas cap lose to make keep a vacuum from building in the tank. A lose gas cap is obviously not a good idea and would indicate that the cap wasn't venting in properly. I just wish I could test it for that direction.