A wierd gas tank vent line, maybe?

[HenryB]

I have a 30+-year-old 18? CC and the gas tank vent is acting like a pickup tube. If I try blowing into the vent line it has a stop and no air will pass. If I apply a vacuum only fuel comes out through the tube. I?m thinking the tank was built with two pickup tubes; one in front and one in the rear, and one used for a vent.

The barb at the tank fitting is for a 3/8 I.D. hose (a 5/8 I.D. hose is clamped to it).

A problem happened recently when the fuel began belching out the filler tube during the fill, like the check valve in the vent line has gone bad and is partially closed.

If it is a pickup/vent tube gone bad it is going to stay that way and be plugged off, and a new fitting installed (one that will only extend a very short distance into the tank). It would be almost impossible to get a wrench on the existing elbow.

It sounds strange but that is where I am at the moment.

 

[JoLin]

Re: A wierd gas tank vent line, maybe?

There's no check valve in the vent line. A 30-year-old boat isn't old enough to have been built before universal standards were in place. Depending on the shape of the tank, you have either one or two 5/8" vents. The only boat i've seen with 2 vents is my 16' Sylvan- the tank is long and wide but very shallow- there's a vent at the front and another at the rear.

If the barb at the tank fitting is for a 3/8" hose, you're looking at a fuel pickup point that's supposed to run to the engine, not to a vent... which would explain why you're sucking fuel out of it- the pickup tube runs all the way to the bottom of the tank.

Something's screwed up in your arrangement. The tank should have - one 1.5" fill hose fitting, one (could be 2?) fuel pickup fitting (1/4", 3/8"- something like that), and one or two 5/8" vent host fittings.

My .02

 

[HenryB]

Re: A wierd gas tank vent line, maybe?

With my fuel tank ? full, if I draw a vacuum on the vent line fuel (not air) comes out.

If I blow into the vent line it is like blowing into a restricted line 12 inches deep in liquid.

The vent line fitting at the tank is a 90-degree elbow with a 3/8 hose barb (same fitting arrangement as the fuel pickup fitting). A 5/8 hose is pinched down on the 3/8 barb. I’ve had the boat for over twenty years and the vent arrangement has never been changed.

The boat runs fine, otherwise. The fuel burping out of the filler tube when filling the tank is new.

It behaves like two pickup tubes came from the factory and the one used for the vent is now partially restricted. I cannot think of any other reason for the burping fuel.

 

[JoLin]

Re: A wierd gas tank vent line, maybe?

If somebody else has a possible explanation for this I'd love to hear it. Until then, I stand by my original 'diagnosis'. Your first 2 sentences in the above post confirm for me that you're blowing and sucking into a fuel pickup tube, not a tank vent. And, tank vent fittings are NOT 3/8". They're 5/8" (larger in some boats). The hose coming off that fitting might be connected to an outside vent, but it isn't supposed to be... because it isn't a vent.

My .02

 

[JoLin]

Re: A wierd gas tank vent line, maybe?

It behaves like two pickup tubes came from the factory and the one used for the vent is now partially restricted. I cannot think of any other reason for the burping fuel.

Sorry- missed this sentence. Yes, it does sound like you have 2 fuel pickup tubes... but a fuel pickup tube CANNOT and WILL NOT vent the tank. The tank vent evacuates fumes that lie in the 'dead space' between the fuel in the tank, and the top of the tank. A tube that extends down INTO the fuel isn't venting anything.

That begs the question- where's the vent fitting on your tank and where does it lead? Is it disconnected, venting fumes directly into your bilge? Some P.O. rigged a 5/8" hose, from a 3/8" fitting, to an outside vent that doesn't do anything but push fuel out of the boat into the water.

My .02

 

[HenryB]

Re: A wierd gas tank vent line, maybe?

I'm not sure yet if I actually do have two pickup tubes but if I do have a pickup tube for a vent then it has vented just fine for over thirty years. If the vent is a pickup tube it will vent. As the fuel rises in the tank it also will rise in the pickup tube and the air would exhaust out. If the air would not exhaust through the pickup tube I’m stumped.

The vent line vents outside and not to the bilge.

Squeezing down on a 5/8 tube on to a 3/8 barb at the OEM is definitely strange, but it ,too, has endured more than thirty years.

 

[JoLin]

Re: A wierd gas tank vent line, maybe?

No sense in continuing to go around in circles. Read my last paragraph again. Your tank is obviously venting somewhere, but not via a 3/8" fuel pickup tube.

Take care.

 

[saumon]

Re: A wierd gas tank vent line, maybe?

Maybe a couple of pictures could help. Here's what JoLin's trying to explain. As you see, fumes cannot exit through the pickup tube cause there's fuel in between...

 

[HenryB]

Re: A wierd gas tank vent line, maybe?

I am just about convinced that I have a pick up style vent (stopped up now) in my gas tank.

I’ve had no luck finding complete gas tank vent design information. I think that what I have now may make more sense than having a 5/8 hole in the tank with a hose venting overboard. I believe the boat manufacturer deliberately made my vent system on purpose.

The vent that I have now may just be for equalizing the pressure in the tank. As long as the pressure in the tank is equal to outside pressure there should be no burping through the fill pipe.

A pick up tube style vent like mine will keep the pressure in the tank stabilized. A straight hole in the tank may lead to sloshing gas out the vent, especially in speed turns.

Fumes in the tank are not a problem, but overpressurization is.

 

[JoLin]

Re: A wierd gas tank vent line, maybe?

I am just about convinced that I have a pick up style vent (stopped up now) in my gas tank.

I’ve had no luck finding complete gas tank vent design information. I think that what I have now may make more sense than having a 5/8 hole in the tank with a hose venting overboard. I believe the boat manufacturer deliberately made my vent system on purpose.

The vent that I have now may just be for equalizing the pressure in the tank. As long as the pressure in the tank is equal to outside pressure there should be no burping through the fill pipe.

A pick up tube style vent like mine will keep the pressure in the tank stabilized. A straight hole in the tank may lead to sloshing gas out the vent, especially in speed turns.

Fumes in the tank are not a problem, but overpressurization is.

Sorry- I gotta try one more time. Henry, there's so much incorrect about your scenario that I barely know where to start.

1. There's no such thing a 'pickup style vent'. As fuel enters the tank, the air being displaced needs to escape. If a TUBE is inserted into the tank, air can only escape until the end of the tube is covered by fuel. Once that happens, the air can no longer escape that way, because that way is blocked by fuel. That's when things start 'belching' and 'spitting back'. Air can then only escape through the fuel FILL (along with some fuel), or by pushing fuel out your so-called 'vent tube'.

2. A 'straight hole in the tank' is EXACTLY how a proper fuel vent is designed. The hose leading up from the tank should have a 'vented loop' in it. The hose is run from the tank to a point higher than the exterior vent fitting, then angled down to that fitting. That keeps fuel from sloshing out and water from entering the tank through the vent. At the same time, it doesn't interfere at all with the displaced air that needs to leave the tank.

If the fitting you're referring to as a vent is REALLY acting as a vent, then it has NO TUBE on the inside of the fitting. The only explanation I can think of is that for some crazy reason, the original 5/8" elbow was replaced with a 3/8" elbow, and the 5/8" hose was cranked down onto it. In that case, the connection is NOT vapor-tight, and does not adhere to marine specs.

If the fitting you're referring to as a vent really HAS a tube on the other side of it, then you're flat out wrong. It CANNOT vent the tank once the end of the tube is covered by fuel.

 

[dmccaffrey]

Re: A wierd gas tank vent line, maybe?

You are over-thinking this.

The simplest explanation for this is that you have a low spot in your vent line which has recently become filled with gas, like a P-trap under your sink. This would account for every symptom you are seeing: fuel spitting back during filling, unable to blow through the vent line, gas being sucked out under vacuum, and the fact that this is a new problem.

To fix it, remove the low spot or get the gas out. That may be easier said than done.

 

[HenryB]

Re: A wierd gas tank vent line, maybe?

Seems my gas tank vent problems might be behind me. The old tank vent fitting did have an anti siphon barb, which apparently gummed up, and it was causing gas to blowback out of the fill tube. It had to be removed and replaced, but neither the old elbow nor the barb fitting would budge, so I drilled a hole in my tank and installed a new vent line.

But the gremlins were not done with me, because then my pickup tube failed. At ? gas tank load it failed to pickup.

It had me stumped for a while. Then it struck me to just switch the lines, so I fashioned a new pickup tube made of copper tubing, and installed it in the new tank vent fitting.

Then I installed the 5/8-inch vent line on to the pickup tube 3/8 barb and squeezed it tight.

The old pickup tube is working just fine as a vent line. I filled the tank from dead empty to full, just like always, no Belching gas.