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post #1 of 5 Old 09-27-2012 Thread Starter
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External fuel tank air vent

I've got a 6HP Nissan with an external tank. The manual says to close air vent when the engine is stopped. The concern I have is with the pressure that builds up in the tank with the air vent closed. What do you recommend while the boat is moored/docked/underway?
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post #2 of 5 Old 09-27-2012
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Re: External fuel tank air vent

When stored, you keep the tank vent closed to prevent the absorption of atmospheric water into the fuel; keep the tank in a shaded place out of the sun to lessen the build-up of tank pressure.
Long term heat/pressure build up inside these rotomolded ext. tanks will eventually destroy the tank .... @ ~$70.00 per 3 gal tank. If you store these on deck, get or make a 'suncover' to help shade them, etc.

Best is to not long term store any fuel in these 3 gal. tanks; but rather, dump the unused fuel into your automobile and store the tank empty and with the vent valve open. Gasoline only has a shelf life of 30-45 days without stabilizer, a little bit longer with stabilizer. Empty tanks dont need stabilizer.

Last edited by RichH; 09-27-2012 at 03:02 PM.
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post #3 of 5 Old 09-27-2012 Thread Starter
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Re: External fuel tank air vent

Thank you for the amazingly quick response RichH. I hadden't thought of the water aspect and it makes perfect sense. The external tank is stowed under the port locker so it's out of direct sunshine. This is our first sailboat so naturally we're learning as we go. I plan on yanking as much out of the boat as possible for winter storage, including the fuel tank, and will dump whats left after the season into the sacrificial lamb, er my lawn tractor.
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Re: External fuel tank air vent

The latest tanks (as of last year, I think) have a pressure relief valve at 5 PSI. So most of the time they stay sealed, unless you leave it out in the sun when the valve will vent the excess.

Note that they also tell you to leave an air gap when filling, this allows for the fuel to expand without creating excess pressure. That's why the tanks are 3.2 gals but the max fill line is at 3 gallons, for example.

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Re: External fuel tank air vent

Great, thanks MarkSF. I've learned my lesson from overfilling some 5 gal tanks in my garage. ;-) This is a '08 that was bought in '10 so I'm guessing the tank was produced somewhere in there, although I'll only be able to check once I'm back on board.
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