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Old 12 June 2014, 18:08   #1
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Fuel Tank Breather

Bit confused here on a fuel tank breather...!

When my boat was surveyed prior to purchase the surveyor picked up that fuel tank was pressurising slightly. Never seemed to get too bad but when it went in for a service I asked them to routinely check the valve in the breather hose. It is an inboard petrol tank/Evinrude Etec 150.

The servicing guys said the valve was in the wrong way around so reversed it and all should be fine. But it is much worse..! Tank is building up serious air pressure in a fairly short run. The one way valve is now installed to let air out but not in. Originally it was letting air in but not out...the original set up resulted in a little bit of pressure, the new set up has resulted in a very pressurised tank.

So which way around should it be.. or am I missing something.!?

Cheers,

715
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Old 12 June 2014, 18:18   #2
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surely if the valve will let air out how is it holding pressure in the tank?
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Old 12 June 2014, 18:30   #3
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Yep that baffled me too...! Cue a big debate with Father and Father in law citing various different set ups on breathers.

I took the valve out to test whether it was working at all, and puffed through it and one way I can blow through it and the other way I can't. The way it was installed by the servicing guys would in theory let air out, but it obviously didn't.

Should valve let air in, to replace fuel as you use it, or let air out as fuel level drops and is replaced (in theory) with a vacuum/pressure?

Wish I had paid more attention in physics now ... :-)

Actually, thinking about it, in the way the valve was installed by the servicing guys maybe the tank is pressurising/contracting INWARDS due to the valve not letting air in... and so when I am opening the fuel filler the creaking of the tank is due to it expanding again.
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Old 12 June 2014, 18:37   #4
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i can only go on what i currently have seen.

my jetski has a one way valve that allows the tank to become pressurised. this is required to help the ski run correctly and if the valve let the air out or didn't work at all it causes major running issues. but this is a jetski

now on my bayliner and my dads bayliner it has no valve at all. all there is is a fitting on the tank and a pipe with a loop in it that goes to the vent on the side of the boat and no one way valve.

this was how i will be setting up the tank on my rib when it comes to it.

have you got any other detail of the setup you have? engine fuel type etc as someone may be able to help but from my personal experiences this is what i have seen
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Old 12 June 2014, 18:51   #5
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Quote:
Originally Posted by Revenger715 View Post
Should valve let air in, to replace fuel as you use it, or let air out as fuel level drops and is replaced (in theory) with a vacuum/pressure?
Its to let air into the tank as the fuel is consumed. If the vacuum is too great the engine will stall.

The one way part is to avoid vapors from escaping although most installed tanks do not have any valves on them at all since they change temperature all the time and "breathe" in and out over the course of a day/season.

I would remove the valve unless you store the boat indoors (e.g. garage attached to your house) or the engine specifically requires it for some reason.
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