Question on fuel tank venting as gas is drawn

[AlanT]

I have the old style 6 gallon metal fuel tanks attached to my 2-stroke evinrude. The fuel tanks have caps that are NOT vented. The engine fuel hose is connected via the 2-prong quick disconnect shown below. If I disconnect the hose, the tank will build pressure, so it obviously seals tight - very tight. Everything works fine and the engine gets fuel and has for many years. My question is this......how does this setup vent air into the tank as gas is drawn to prevent a vacuum in the tank and cause fuel starvation? It must be done through the quick disconnect, but how?

I have another reason for asking, but I don't want to derail the simple question.........I know how you guys are with shiny objects. LOL

 

[DaleH]

The ol' metal or plastic connectors were sized to push in a 'pin' that on the tank connector boss itself, like see photo.

 

[DaleH]

Another picture … just ran out back to take this photo of a metal OMC tank someone gave me ….

 

[Ronbedard57]

You can buy the parts to re-hab that venting push-pin mechanism.

 

[AlanT]

OK. Makes sense. So if I switch to a tank in which the hose is direct connected (no quick disconnect), then the tank would need to be vented. So is that the purpose for the fuel demand valves on tanks without a manual vent? To allow the pressure to release when heated AND to allow air to flow back into the tank when drawing fuel? Or am I confused about the purpose of these valves.

 

[LDUBS]

I don't think that is what the fuel demand valve will do. They allow fuel to flow when the motor is running and there is demand for fuel. Otherwise, if the motor is off, they prevent fuel from being pushed through the line due to pressure from the tank.

 

[AlanT]

So venting the tank is to avoid a tank swelling and to allow air to fill the tank when gas is withdrawn, but is completely separate from the problems solved by a demand fuel valve. I've researched the demand fuel valves and the explanations how they work are very poor. I see references to tank pressure in the explanations, but I guess it only protects the engine from getting gas due to high pressure in the tank. It seems they only solve for one problem of a tank building pressure due to heat expansion and not the root cause.

 

[AlanT]

Can you buy that same type of connector that would fit a 1/2" connector on an aluminum tank? I'm not seeing one.

 

[LDUBS]

A simple solution might be to get vented caps for your tanks. If you can find an older one that would be best. With the relatively newer EPA vented caps you still might need the fuel demand valve. Reason being they don't start to vent until something like 5 psi.

To take a slight detour, I've spilt more gaso since the "improved" EPA sealed portable gas cans were adopted than I did in the 50 years previously. This might be a good idea, but the execution simply fails. I use a 5 gallon gaso can to fill my boat's tank. I got tired of the constant spillage. I replaced the "EPA" spout and installed a vent cap. Essentially went old school (thank you Tractor Supply). Haven't spilled a drop since.

 

[AlanT]

A simple solution might be to get vented caps for your tanks. If you can find an older one that would be best. With the relatively newer EPA vented caps you still might need the fuel demand valve. Reason being they don't start to vent until something like 5 psi.

To take a slight detour, I've spilt more gaso since the "improved" EPA sealed portable gas cans were adopted than I did in the 50 years previously. This might be a good idea, but the execution simply fails. I use a 5 gallon gaso can to fill my boat's tank. I got tired of the constant spillage. I replaced the "EPA" spout and installed a vent cap. Essentially went old school (thank you Tractor Supply). Haven't spilled a drop since.

Yes, LDUBS, I've converted my gas cans to vented to avoid spills. Ridiculous. I believe you are correct that I will still need a vented tank because I haven't found documentation that states a demand fuel valve will also allow air into the tank. It seems to me the only purpose of a demand fuel valve is to prevent a siphon effect caused by a pressurized tank resulting in a large fuel spill in or outside the boat. I'd like to know the statistics on that. I'm pursuing the vented cap, but doesn't seem like the demand valve is needed with a vented tank. I dunno. This stuff makes no sense. The EPA police will need to come get me.

 

[LDUBS]

Yes, LDUBS, I've converted my gas cans to vented to avoid spills. Ridiculous. I believe you are correct that I will still need a vented tank because I haven't found documentation that states a demand fuel valve will also allow air into the tank. It seems to me the only purpose of a demand fuel valve is to prevent a siphon effect caused by a pressurized tank resulting in a large fuel spill in or outside the boat. I'd like to know the statistics on that. I'm pursuing the vented cap, but doesn't seem like the demand valve is needed with a vented tank. I dunno. This stuff makes no sense. The EPA police will need to come get me.

No, the fuel demand valve will not vent the tank. I used one on my previous boat because the new EPA tanks build up quite a bit of pressure before they vent. My Merc manual suggested use of the demand valve, so I did. I don't use one on the current boat (Honda outboard). The fuel line is hose clamped to a fitting at the tank (no quick connect). So far I have not had any issues. I don't know the statistics. But my boat's 12 gallon plastic gas tank really starts to swell up on a hot day.

I've heard that there are instructions on the internet about how to turn those EPA portable gas tank caps into vented caps. Of course, doing that would be highly illegal.

 

[airshot]

In the old days when I had my Lyman, the spiders loved building nests under the cover. An old timer mentioned leaving the vent open on the tank before putting the cover on. Yep, worked great...the gas/ oil fumes kept the spiders away !! And yes my portable tanks have been converted to manual vents. No idea why it switched to bold type !!

 

[thill]

IF you don't have a vented cap, just crack the lid a little before running...

 

[AlanT]

Thanks, guys. I now know what needs to be done when I switch gas tanks.