Gas Tank Pressure Question.

TwoBallScrewBall

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Sep 14, 2003
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88 Bayliner with an aluminum tank....<br /><br />If I remove the fuel line going from my tank to the fuel pump at the pump, and apply slight pressure to the line, should the tank vent this pressure? I'm asking because when I did this, even with the filler cap off the transom, it seemed to build pressure in the tank and cause fuel to come back out the line when the pressure was removed from it. <br /><br />Does not make sense to me, I would think that even if the vent was clogged or had a check valve, the tank should vent with the filler cap off. <br /><br />Any ideas??
 

vinney

Chief Petty Officer
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Oct 29, 2002
Messages
437
Re: Gas Tank Pressure Question.

There is an anti-siphon valve in the fuel line at the top of the fuel tank. The valve allows fuel out not in and has to have a vacuum pull to remove the fuel from the fuel tank. Thats why you can't blow back in the tank thru the fuel line.
 

TwoBallScrewBall

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Re: Gas Tank Pressure Question.

I know about that one. I bypassed it for this test. Again, with the filler open, if I apply pressure into the tank, it does not vent. Is this normal?
 

18rabbit

Captain
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Nov 14, 2003
Messages
3,202
Re: Gas Tank Pressure Question.

Steve – Blow harder!!! :) Just kidding. The anti-siphon valve is NOT in any way related to the filler cap. It is installed on the “out” port of the fuel tank, immediately on the tank. The engine’s fuel line connects to it AT THE TANK. Often you cannot tell the difference between the anti-siphon valve and a normal brass fuel tank fitting if just looking casually at it.<br /><br />When you are applying pressure to the fuel line, the pressure is building in the line up to, but not in, the fuel tank.<br /><br />If you have something that looks like a value on the fuel tank vent line, that is an anti-backflow valve that allows air, but not fuel, to pass out of the tank. It is there to stop fuel from spilling out the vent when filling the tank. It is not an anti-siphon valve.
 

TwoBallScrewBall

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Re: Gas Tank Pressure Question.

Hey Rabbit,<br /><br />You guys are confused, let me try to be more clear. <br /><br />I have the check valve off the tank line going to the filter, so it's just straight hose into the tank. I can blow INTO the tank fine, but, when I release that pressure, fuel comes back out of the line for a long time, as if there is pressure building in the area above the fuel in the tank. The tank is almost full if that helps. When I removed the filler cap to try to vent the tank, nothing happened, and the fuel continued to flow as if under tank pressure. <br /><br />Now that I think about it, all of my hoses enter the tank on the top at the very back. Since the boat was nose high, and the tank was almost full, I bet that the fuel had submerged all of the hose ends, including the filler and vent. I bet if I tried this with the boat level it would vent fine. I think I just answered my own question. <br /><br />Any opinions on that?
 

Boatin Bob

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Sep 24, 2001
Messages
1,858
Re: Gas Tank Pressure Question.

Steve...I think you are experiencing a syphon effect, the exact thing that the valve you removed is supposed to prevent. Why are you doing this in the first place? Do you have some kind of a fuel supply problem?
 

TwoBallScrewBall

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Sep 14, 2003
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Re: Gas Tank Pressure Question.

I thought I might have, but I don't. Problem seems to ultimately have been ignition. I only removed the check valve to see if the pickup in the tank was clogged or not, I put it right back on after I saw that the tank flowed fuel OK. Tank seems to flow fine, I was just wondering about the situation I mentioned above, but I think I figured it out as I posted.
 

Bondo

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Staff member
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Apr 17, 2002
Messages
71,293
Re: Gas Tank Pressure Question.

I think I just answered my own question. <br />
Yep.................<br />Again......................... :D
 
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