Fuel tank under pressure. Should air exit out gas line.

[bdblazer1978]

I?m getting ready to put my fuel tank back in and wanted to check it before covering it with the deck. I put a pressure gauge on the hose that would run to my engine. I then attached my air compressor to the vent tube and plugged the fill tube with a golf ball. I started to add air to the tank and could see it expanding but never got any pressure on the gauge. I then removed the gauge so it was just the hose coming out of the tank towards the engine and no air escaped. When I removed the air compressor, air flowed out meaning there was pressure in the tank. My question, is this correct? Is there something in the fitting going to the engine that works on a negative(sucking) pressure and would prevent air from flowing from a positive pressure, or is it most likely clogged?
Thanks,
Bill

 

[roscoe]

Re: Fuel tank under pressure. Should air exit out gas line.

anti siphon valve ?

 

[bdblazer1978]

Re: Fuel tank under pressure. Should air exit out gas line.

As I've been doing some more reading on here that was another thought. Right now there is an aluminum square that the feed tube connects to. Not really sure what an anti-siphon valve looks like or how to check it? Thanks for the help. I figure its easier to fix now with the tank out then once the deck is in.

 

[metriccrescentwrench]

Re: Fuel tank under pressure. Should air exit out gas line.

An anti-siphon valve looks like a normal hose barb, but it's got a spring and ball inside.

 

[bdblazer1978]

Re: Fuel tank under pressure. Should air exit out gas line.

Thanks for the info. I took the tank to the local marina and they recommended pulling the anti-siphon valve and try blowing through it. Gave it a good blow and the ball moved, also the pick-up line was clear, gave that the blow test too. Went ahead and replaced the anti-siphon valve. Installed the new valve and still have the same issue, but I think I have figured it out.

I don't think I was putting enough pressure in the tank to make the ball move. Every time I saw the tank expand I would stop adding air since I read it would go BOOM if I added to much air. Since I can suck on the fuel line and get air out, I think I'm good.

 

[Silvertip]

Re: Fuel tank under pressure. Should air exit out gas line.

What are you trying to prove -- other than trying to blow up your fuel tank? Fuel tanks should NOT be under pressure which is what the VENT line is for. The anti-siphon valve is required and prevent fuel from draining from the tank when the engine is lower than the tank outlet. If the fuel line has a quick connect fitting on the engine end, that also has a valve in it that is forced open when the connection is made. Putting more than a couple pounds of air in the tank is a good way to rupture it. If the tank is holding air, the only thing you've proven is that it DOESN'T LEAK. Yes -- fuel is SUCKED from the tank, SUCKED through the anti siphon valve, SUCKED through the primer bulb if you have one, SUCKED though the fuel line fitting if there is one, SUCKED through the fuel filter if you have one, SUCKED into the fuel pump and then PUSHED for FORCED under pressure to the carbs or fuel injection system. So no -- fuel is not forced through the system which is what you are trying to do -- it is sucked.

 

[metriccrescentwrench]

Re: Fuel tank under pressure. Should air exit out gas line.

Well..... applying suction on one side is really no different than applying positive pressure on the other side. You've created a pressure differentiial, which is what is needed to make a fluid flow. In the case of a fuel pump, you've got atmospheric pressure inside the tank (assuming it's properly vented), and maybe one or two psi less than atmospheric pressure on the far end of the fuel line. There's really no difference (as far as moving fluid) between that and having the far end of the fuel line open to the atmosphere and putting a couple of psi of pressure into the tank (which is how Johnson outboards did it years ago - but of course we know which method is safer). All the fluid sees is a difference in pressure from one end to the other, it doesn't matter how that pressure differential is generated.

bdb, if you're intention is to test the tank for leaks before reinstalling it (which I infer that's what this is all about), temporarily remove the anti-siphon valve and go from there. Don't put much pressure on the tank, as silvertip said modern fuel tanks aren't really made to hold pressure.

 

[NYBo]

Re: Fuel tank under pressure. Should air exit out gas line.

Yep, something on the order of 3 or 4 psi is all the manufacturer has to use for certification.

 

[bdblazer1978]

Re: Fuel tank under pressure. Should air exit out gas line.

My primary reason for testing the tank was to make sure there were no leaks. I figured it was a lot easier to test a 20 year old tank when it wasn't full of fuel, in the boat, with a brand new deck over it. Since I had read on here that more then a couple pounds was BAD, that's why I stopped adding air when I saw the tank expand a little. I did find that my sending unit wasn't on tight enough and was blowing bubble once I put on some soapy water. But since I didn't know about the anti-siphon ball, I became concerned when air pressure didn't come trough the fuel line. Wanted to make sure it wasn't clogged.

Thanks for all the advice.

 

[Silvertip]

Re: Fuel tank under pressure. Should air exit out gas line.

What was the range of the pressure gauge. If you had a 0 - 100 psi gauge, chances are 2 or 3 psi would hardly make it register if air did reach it.

 

[jimbo_jwc]

Re: Fuel tank under pressure. Should air exit out gas line.

A couple of pounds of air Ps is capable of finding leaks in a fuel tank that only has gravity with weight of fuel as pressure as source . Add more pressure and you find out that the anti-siphon check vale is holding gas from draining out , And tank can expand/explode with vent pluged and fill line as air supply . Good you found sending unit leaked but not major prob as it is at top of tank. This is my best explanition of what I learned with my first I/O motor ,fuel pump pulses anti-siphon to open long enough to flood fuel pump suction pick-up
line with no leaks what so ever to carb to work . Makes all the difference in the world