Fuel tank suction

[bigmac]

I'm pretty sure (can't find the reference) that NMMA rules require fuel tanks be vented to the outside. Obviously they also have to vent in to keep negative pressure from building in the tank. USCG and NMMA do require that all fuel tank venting be to the outside of the boat - not into any closed compartment.

As I understand posts about problems with these Perko "gas fill and vent" fittings, that two way venting is done through the filler cap.

[rudaire]

nice work bigmac... That is exactly my filler assembly. That cap is different than mine however. And while I have a segment of broken chain on my cap, and also a segment of broken chain on my boat, I believe someone along the way may have dropped my cap and replaced it with one with identical size and threads, but no venting mechanism... Seems like if they rely on than cap to allow air in, and if it does so through the tool holes used to remove the cap, that water would easily collect in the holes and be sucked in the fuel system as air was drawn in to replace fuel..

To those that have responded that they get a sound when popping the cap, do you still have the original cap on the boat as far as you know?

Big mac, all the research I've done seems to agree with what you've stated about fuel systems being required to vent to the atmosphere. But I cannot find a provision on my boat for this to happen if it isn't in the cap itself...

dave

[bigmac]

Originally Posted by rudaire

Seems like if they rely on than cap to allow air in, and if it does so through the tool holes used to remove the cap, that water would easily collect in the holes and be sucked in the fuel system as air was drawn in to replace fuel..

That's been exactly the complaint against those Perko systems....when they malfunction, they can suck a significant amount of water into the fuel tank.

[JimN]

Originally Posted by rudaire

It would be nice to know that, I agree. But it's pretty hard to determine that from just a sound coming from the fuel cap. It isn't like a gush of air you can feel with your hand. It appears like my system doesn't have a provision to let air out of the tank, like if things heat up and expand.. But I refuse to believe there isn't a provision to let air into the tank to replace spent fuel...

hmmm.
d

If the sound occurs and the smell of gas in really strong immediately after, it's positive pressure. If it was suction, it wouldn't be so strong. With a pressurized tank, you could easily feel the air rushing out.

[bigmac]

Originally Posted by JimN

If the sound occurs and the smell of gas in really strong immediately after, it's positive pressure. If it was suction, it wouldn't be so strong. With a pressurized tank, you could easily feel the air rushing out.

I guess the question is "is a marine fuel tank supposed to EVER have pressure in it, either positive pressure or negative"? I suspect his fuel cap, which is supposedly where his tank is supposed to vent, is malfunctioning and accumulating negative pressure in it as the fuel in the tank draws down.

[rudaire]

Originally Posted by JimN

If the sound occurs and the smell of gas in really strong immediately after, it's positive pressure. If it was suction, it wouldn't be so strong. With a pressurized tank, you could easily feel the air rushing out.

I agree the smell test may be valid, but easily feeling the air escape around the cap is harder to be certain of...

I've exhaustively researched the perko fill/vent systems today.. Even called perko although they were closed. They are open until 4:20. What's up with that? Smoke break? Anyway, Seems they call it a 'fill/vent' system only because the vent tube, which allows for more rapid filling, is contained in the unit, but behind the sealed cap that is sealed with an o-ring I might add. They also sell 'fill' systems that expect the vent to be a through hull fitting elsewhere. With the 'fill/vent' system, it seems it really is a closed system. It's hard to imagine the cap is designed to vent, and if it is, where? Again I'm not at my boat, but I believe all the holes in the cap are blind and only there to allow the cap tool to have a place to grab the cap. And if the cap were indeed 'vented', funny they'd add an o-ring to seal the circumference...

Bigmac, you've pointed out that you have the same vill/vent perko inlet. I realize you have one and not two orange hoses as I do, but have you found a vent on your boat that doesn't lead to the area just BEHIND your sealed fuel cap? Because that really isn't vented if the cap is tight and sealed..

I cannot believe they'd design the system without a provision to at least allow air to be drawn in somwhere. And at the same time cannot find such a location on my boat where air could be drawn in, unless it's something on top of the fuel pump cartridge itself..

I'm not having any problems with my 'ventless' system as far as driving goes..
But if something is plugged or malfunctioning, I'd like to know and fix it..

thanks for helping me track this down guys..

dave

[bigmac]

Originally Posted by rudaire

I agree the smell test may be valid, but easily feeling the air escape around the cap is harder to be certain of...

I've exhaustively researched the perko fill/vent systems today.. Even called perko although they were closed. They are open until 4:20. What's up with that? Smoke break? Anyway, Seems they call it a 'fill/vent' system only because the vent tube, which allows for more rapid filling, is contained in the unit, but behind the sealed cap that is sealed with an o-ring I might add. They also sell 'fill' systems that expect the vent to be a through hull fitting elsewhere. With the 'fill/vent' system, it seems it really is a closed system. It's hard to imagine the cap is designed to vent, and if it is, where? Again I'm not at my boat, but I believe all the holes in the cap are blind and only there to allow the cap tool to have a place to grab the cap. And if the cap were indeed 'vented', funny they'd add an o-ring to seal the circumference...

Bigmac, you've pointed out that you have the same vill/vent perko inlet. I realize you have one and not two orange hoses as I do, but have you found a vent on your boat that doesn't lead to the area just BEHIND your sealed fuel cap? Because that really isn't vented if the cap is tight and sealed..

I cannot believe they'd design the system without a provision to at least allow air to be drawn in somwhere. And at the same time cannot find such a location on my boat where air could be drawn in, unless it's something on top of the fuel pump cartridge itself..

I'm not having any problems with my 'ventless' system as far as driving goes..
But if something is plugged or malfunctioning, I'd like to know and fix it..

thanks for helping me track this down guys..

dave

I've never heard any whooshing when loosening my cap. The black vent line from the top of my fuel pump goes to the filler neck, and I don't see any other vent lines, just the filler hose. I am puzzled by your situation too.

[rudaire]

If there is no other vent other than the one just inside of the o-ringed and sealed fuel filler cap, then neither of us have a vented system.. But I still can't believe there wouldn't be a provision for air to enter the system to prevent a vacuum situation. I'm going to pull a vacuum on the system, maybe this weekend, and see what I learn.

dave

[treptowr]

FYI, I have a 2002 X-star. Third summer and have never heard it under pressure when removing the gas cap. I have the same tank configuration as yours and the gas cap like Big Mac posted.

[shunra]

Originally Posted by rudaire

To those that have responded that they get a sound when popping the cap, do you still have the original cap on the boat as far as you know?
dave

As far as I know I still have the original cap. My cap is sligtly different than the one pictured above. I bet mine is the same as yours. Mine does not have a grip round the Circumference. It is almost impossible to turn by hand, you pretty much need to use the tool.

I am not near the boat to get a picture of the cap. Do you have a picture?

As far as I can tell my cap wasn't designed to be a vent.

I filled up again this weekend and tried to pay more attention to the sound. I belive it is suction. If you only loosen the cap sligtly the sound can last for several seconds.