Pontoon Boat & Deck Boat Forum

Newbie, Installed 12 gallon above deck plastic gas tank. Filled and came back 3
days later gas tank had expanded a lot. close vent on gas cap when not using
the boat (keeps water out) open when using. Is this right? I see ones guy in marina has what looks like tin foil under his gas cap while his boat is in its slip. Any info,
Thank you

always left the vent open on my old boat. My new boat has the tank in the transom with a vent hose (always vented).

My tank say right on it - Close vent when not in use. Why?? I don't know. If I leave it closed it'll blow up like a balloon so I always leave it cracked open.

Not 100% posative but I think you may have the new & improved (BS) EPA aproved
tanks. The tanks are not supposed to let any fumes into the atmosphere sp you know
global warming. old jim

Mine does the same thing, I just vent it once a day so it doesn't blow up...of course, now the gas gauge is extremely inaccurate due to the tank's shape being warped

I hunted real hard to find the old style tanks last year to avoid that problem, the new style vented tank caps simply dont work, depending on the tank you may be able to find a old style cap to fit it and yes the old style tanks do lose some fuel in vapor form but so what, Its been like that for 50 years and it works just fine. If it is the older style cap leave it open it will not hurt a thing and no it wont get water in it. I have used that style for 30 years with no problems.

I keep a 5 gallon can in my truck all year for 2 stroke gas. Weed wips and blowers in the summer , snow blower in the winter. Anyways with the new style cans you can't even vent them. My can goes from a bloated huge can in the heat of the day to a shriveled up can on a cool morning. I always wondered about the can cracking and leaking. It lasted three years beofe it got a pin hole. If I had a vent that I could open on a boat tank I would just leave it cracked open a bit. Yeah you might have a little bit of gas evaporate but it will be very minor especially if you use your boat often. They probably tell you to close it due to some epa regulation. My boat vents through the cap so I don't see why it would hurt to open it a bit.

My Premier did the same thing. It had a small check valve (hole with a spring and ball in it about the size of a BB) in the cap, under the screw holding the tether on. It stuck and didn't vent. I just took the damn check valve out and its been fine, since. The hole is about the size of a pencil lead.
It works fine now. But don't tell the EPA I did it! I'll deny any wrong doing that may harm my political well being.
(just heard a Senator say that on TV, stuck in my mind... Sorry!)
Something to try or at least look for.

jcollins wrote:Newbie, Installed 12 gallon above deck plastic gas tank. Filled and came back 3
days later gas tank had expanded a lot. close vent on gas cap when not using
the boat (keeps water out) open when using. Is this right? I see ones guy in marina has what looks like tin foil under his gas cap while his boat is in its slip. Any info,
Thank you

since ethanol fuel absorbs water rather quickly, you may want to keep the vent closed when not in use.

Boat tanks have open tank vents since the first outboards were made. and yes the newer E 10 fuels will absorb a very small amount of water over a long time but it isn't enough to cause any problems, the big problem is phase separation if left untreated and exposed to air The answer is go to the lake and use it or buy real gas or treat it.