Gas Cap vents
 

My boat has a perfect vent, but when the tank is full and I have the trailer up a little on the trailer to keep the water draining, the vent gets blocked by gas, The result is leaking gas out the vent.

I have thought about adding a second vent in the front of the tank so the air will vent there when boat is tilted. But that means drilling a vent hole in front of the tank?
So i was thinking of a vented gas cap, but all the ones I see still needs the small hose connected, which means still drilling a hole for a vent.

I know i can't be the only one with this problem, so what have you guys done??

Thanks

My boat has a vented cap, it has a small line running next to the fill hose, that connects in the cap assembly, but I still get a little leakage when it starts getting warm out. I think that is just from expansion.

Could that be the only issue?
Can you just not "top %$%$%$%$, leaving a little room for expansion?

Just a thought, personally I would not want to drill a hole in a gas tank.
But thats just me.

Putting another vent in is just going to make the problem worse! :smash: You're best bet would be to just fill the tank a little less so the gas level doesn't reach the vent line.

Thanks Van!
Yes it is all from expansion, and the fact that when the boat is tilted up it blocks the vent hole in the tank as the vent is in the rear of the tank.
The vented gas cap, does it only work when there is a hose from the tank to the cap, or does it vent its self?
I agree drilling the tank os not the thing to do!

Thanks Mike, Why would adding a 2nd vent be worse?? If I added one to the front it would not be blocked when boat is tilted allowing for it to vent, right?

I know that it being to full is the case, but even if I don't fill till it tops off and tilt the boat up it blocks the vent in the rear.
Why they ever vented it in the rear is beyond me!

Just like the old fruit punch cans, the punch would flow out slow with one hole but add the second and it flows like Tiger Woods, because you release the vacuum..

If the main shape of the tank is fore and aft the vent really should be coming from the forward end of the tank now. Hopefully it is.
check to see if the vent hose is hanging low in the bilge causing a "slug" of fuel. If so run it as to not sag. Can the vent be moved higher? If this happens on the trailer is it going to happen out at sea when the boat is trimmed up?

The vent is in the rear of the tank, not forward. and the hose runs along the top of the tank, up the side and out under the rub rail.

I have no use for 100 gal of gas, I have been out 35 miles off Chatam and back cod fishing and only used 35 gal gas. Now with the 4 stroke I am sure I will not use any more.
I just like keeping it full all the time so gas is not a issue were ever I want to go!!

I am going to pull the deck plates when the rain stops and see again if there is something I can do??

[QUOTE=macojoe;758512]
The vented gas cap, does it only work when there is a hose from the tank to the cap, or does it vent its self?
I agree drilling the tank os not the thing to do!
QUOTE]

I am nearly 100% sure there is a second small line that goes into the tank right next to the fill. I think these are welded to the tank. I'm not sure where it really vents to within the cap assembly. I will uncovering soon (I HOPE---back issues) and I will get a good look and let you know.

Right now with the vent blocked the gas is being held in the tank (somewhat) by the vacuum between the gas and the tank, if you put another vent in the gas will flow freely, via atmospheric pressure from the tank. In other words, you'll spill the excess out of the tank much faster if you put another vent in.

If you go to a vented cap, you don't have to drill another hole. Just move the vent line from the current vent to the cap. done. Then you will have a vent on the side of the boat that doesn't do anything...

:grins:

thanks zacs, but the issue is the back vent gets blocked by gas when boat is tilted up, so I need a second vent up front so when tilted it vents.
I am waiting for a new cap i ordered on line and see if that is going to help

Believe me, you're going to regret it if you put in a second vent. all you're going to accomplish is spilling the gas out of the boat even quicker!

I can't see how another vent would make the problem worse, as long as both are above the level of the fuel in the tank.

But both vents are not above the level of the fuel, that's his problem. The one vent he has now is below the fuel level in the tank, so gas "weeps" out of the vent line. Adding another vent above the fuel level is just going to force more gas out of the vent that is below the fuel level. Physics 101.

I think you need to revisit your "Physics 101" because you couldn't be more wrong. By your explanation, gas should shoot out every time he opens the fill cap since he is in effect adding a vent by doing that.

Fuel weeps out because there is an air pocket that warms, thus expanding. Since his one vent is below the fuel level, the air pressure builds up and forces fuel out. If that air pocket was vented the pressure would equal the outside air pressure and there would be nothing to force the fuel out.

From the way he described it that is exactly what would happen is he removed the fuel cap when the boat was jacked up as he described. Your description of why the fuel weeps is absolutely correct, but what you didn't say is that when the air pocket is cool the VACUUM behind the fuel is what is holding the fuel in the tank. If he breaks that vacuum and allows atmospheric pressure into the tank the fuel is going to come shooting out until the fuel level is below the vent.

Try this little experiment. Take a can of liquid (beer soda juice or water) and poke a little hole in the side. The liquid will run out but only slowly. Now do the same thing but open the top, the liquid will shoot out of the hole. Same thing applies to his fuel tank, no atmospheric pressure on the top and it only leaks slowly. Add atmospheric pressure and the fuel is going to run out much much quicker.

Vacuum holds the fuel in???:confused: If that was the case the motor would not run, It'd be fuel starved, like when the vent is clogged. Try this experiment - seal your vents and take your boat for a ride.

You're beer can example is also flawed because the hole in the side is below the liquid level, so of course it runs out due to gravity. The "atmosphere" end of the vent hoses are above the fuel level, so fuel stays in the tank thanks to gravity.

If you still don't get it I have to give up.

Wow!!

Just to be clear, when cap is sealed the only way out is thur the vent, vent is blocked by fuel when boat is tilted up, allowing the expanded fuel to push up the vent hose and out.

Now Maybe I am wrong? But adding a vent in front so the vent is now above the fuel will allow the air to escape and not push gas up and out the front vent right??

Well I just ordered a new vented gas cap and will find out soon.

Thanks guys for your in put!! and we all have a opinion on how it should work, right or wrong.
But we will see soon and for those who are right congrats, for those that are wrong, consider it as a learning lesson and be glad you learned something new today! I like to learn new things all the time!

I give up! If you go back and read the thread it just might become clear.

MJ:

I pulled the cover this week and took a look at what I have.
My fill and vent are very close to the front of the tank. The hoses run up together and connect to the gas cap/fill assembly. All integral.

Still when shes covered and the weather warms, I get a bit of gas coming out and evaporating.

On another note. Damn good thing I looked at the system. The gas fill line (about 2" dia.) is showing some age damage, surface cracking and feels real dry. So its time to replace that line and the vent line. too. But I won't do it until the tank is 1/2 to 1/4 full. (That means I'll be doing it at the slip) Right now its topped off. The supply line to the power has been replaced already.

Thanks Van,

I have added the new vented gas cap, so far nothing has leaked out either vent, (YEA!!)
3 years ago when I bought the boat the gas tank was not working cause of blocked vent, (Found only after removing the floor tank cove (: ) and at that time replaced all hoses vent and fill so i am all set there.

As been pretty cool out and I think the tank is down a few gals from the test run I did with the boat last week, so Iwill need to fill to the top to see what happens. But when it was leaking before and I removed the cover it would blow the cover off in my hand when the pressure was released, so I think it will work now??

I will let you guys know after I fill it next week, again Thanks all!!