gas tanks venting

[RICH MUISE]

I'd like to have a dedicated discussion on venting our gas tanks. I've always had an issue with gas getting into my Custom's vent line and spitting gas , sometimes a lot of gas, out the vent tube above the license plate.
My vent line, a second trial, is actually up into the trunk from the in-tank fuel pump flange, mounted on the top backside of the tank. There is no place in the line that is lower than the top of the tank. It is connected to a tip-over valve, but I'm not sure that is relevant. What is relevant, and I'm aware of, is that the vent is in a recessed area so when it is near full, the vent is actually under gas level. So, yes gas can slosh up into the tube, but why doesn't it drain back since it's uphill from the tank?
Picking up bits and pieces of info, I've become confused (as usual), in that recently Gary and a Hamb member talked about vacuum inside the tank. Makes sense since there would be a void/vacuum as gas is being consumed, but if it is a vacuum, why is the gas being spit out of the vent tube? Seems like it would be sucked back in!?? I guess I always thought of the problem in terms of pressure inside the tank forcing gas into the line, not vacuum.
Am I right or wrong, in that there is both vacuum OR pressure depending on the circumstances....ambient temperatures, etc?
One of the guys on the Hamb brought up the fact that many new cars have a check valve in the vent line. What are your thoughts on that? I just ordered one to give it a try. The valve I ordered has a directional arrow molded onto it, any thoughts as to which way the arrow should point when I install it in the line? Towards the tank, or towards the exit.
All opinions appreciated.

[rmk57]

 
  What happens when you remove the gas cap? A big whoosh or nothing?

Mine has the stock vent tube up to the inside quarter panel and I'm not sure but an original looking fuel cap and it never has a whisper of pressure when I take the cap off.

  Are the gas caps in these supposed to be vented or non-vented?

[59meteor]

Randy, at least on the passenger cars, the gas tank is vented externally, with a non vented cap. On my 59 Tudor, there is a nipple on the side of the gas tank, near the top, and a short section of rubber hose, which is connected to a 5/16" steel tube that goes up to the top of the LR 1/4 panel, and then down thru the trunk floor, with a short section of the tube sticking out below the trunk floor. With the gas filler located below the trunk lid area, a vented gas cap would allow gas to leak out thru the cap on a hill or under hard acceleration if the tank is full. I seem to recall my 57 Ford Sedan Deliveries, as well as the station wagons, Rancheros, and likely Skyliners, had a different vent setup, since they have the filler located much higher, on the left 1/4 panels. With a vent tank, if the tank has pressure, and "whooses" when you remove the cap, the vent line must be plugged somewhere. As for Richs case, it does seem odd that the fuel could rise up high enough in the vent tube to actually spill out onto the ground.

[lalessi1]

Rich if your vent line port is submerged there is a chance that there is an air pocket in the tank somewhere. As the air and the gas expand with heat that pushes gas up into the vent which may get over the highest point in the vent line? I don't know anything about a roll over valve but I suspect that is a possible contributing factor. If the tank had no vent it will pressurize with expansion and go into a vacuum when pumping out. A check valve in the vent line will let air in but not let it out? Don't want gas fumes in  the atmosphere. A check valve might solve your issue. This is a thinking out loud exercise.

[Ford Blue blood]

The Sky Liner is vented through the cap as the fill is high on the drivers side.  The "vent" tube comes from the top of the tank next to the fill tube and terminates at the top of the fill tube.

[RICH MUISE]

When I am having the vent tube issue, there is a "whoosh" when I undo the cap, but I'm not sure if it's pressure escaping, or a whoosh of air going in. I'll have to pay more attention next time.

[59meteor]

Rich, if there is a whoosh of air, either from vacuum or pressure inside the tank, I would have to think that there must be a kink or blockage in the vent line. When I remove the gas cap on my 59, even after a long hot,highway trip, there is never any pressure or vacuum, and with a properly functioning vent system, nor should there be. The vent is there to prevent the tank from trying to collapse as the fuel level goes down, or expand in high temperature conditions. I would take a good look at your vent line and hose for restriction.

[rmk57]

  Find the end of the vent tube and blow in compressed air with the gas cap off. I guess if it's spewing fuel out the vent tube it isn't plugged though or partially plugged.

[hiball3985]

I agree with Lynn, I think an air pocket is causing pressure to build. Just a thought but your car being lowered in the front may be causing an air pocket in the rear of the tank? Take your jack and level the car next time you fill it 

[RICH MUISE]

My vent line is new and clear. The only time it doesn't function as it should is when gasoline gets in there.
I thought vented gas caps were illegal unless the car was originally equipt with one.  ??
I did once have a mud dauber in it.
I think the problem is worse in the summer.
I do have a locking gas cap that has a heavy rubber seal. I could cut a few notches in the seal and see if it vents without spilling gas with a full tank.

Yes, my rake on the car adds to the problem, as does the fact I have a 5/8 x 2  nylon bar in the back end of the tank between the trunk floor and the tank top. That was added to clear the floor support with the rubber fuel lines that connect the hard lines to the tank. Also, as mentioned, my fuel pump and vent line are mounted in a recessed "box"...all adding up to the vent line outlet being "under water" for a longer time than oem. With that mentioned, I might just have to live with not being able to fill the tank in the summertime at least.
If anybody is planning on using Tank Inc's recessed mounting box, I recommend not doing as I did and using the provided tapped hole for the vent line. My tank, when new, came with the vent tube outlet in the oem side position, but I welded it closed.

[hiball3985]

Vented out to the atmosphere caps have been illegal since the early 70's. Caps today marked vented only let air in. This is a problem with the ranchero/wagon type tanks that have no vent tube and require a vented cap, I like others just drill a small 1/16 hole in it 

[RICH MUISE]

I installed the check valve (arrow pointing towards the tank) and filled the tank. I took it for a 60-mile drive, stopping to check several times. The vent tube remained dry as a bone. It was not too hot today, 70's, but there was no "whoosh" of air either, when I undid the gas cap at home. I'm not anticipating an issue with pressure build-up until the weather gets hot again.
My thinking is when you fill a tank while you're on the road, the vacuum created by depleting the gas from usage, neutralizes the pressure build up that could result from thermal gas expansion. Accordingly, I would think the biggest issue with gas expansion would come from parking on a hot day. I'll keep an eye on it.
On my next road trip, I may keep a battery-powered drill with a 1/16th bit in the car just in case.
Haha....already changed my mind. I got to thinking closing up a 1/16 hole is a lot easier than repairing potential damage from a high pressure build-up, so I went ahead and drilled the hole.

[59meteor]

Was it the gas cap that you drilled? If so, if you want to plug the hole in the future, you could just drill the hole to 1/8", and use a pop rivet to close the hole. What did you use for a check valve? On my 59, there is no valve, just tubing from the top of the tank, up thru the trunk floor, which loops up to the top of the LR 1/4 panel, and then back down thru the trunk floor to the atmosphere .

[hiball3985]

Rich, Just out of curiosity is the valve a one way valve? When you fill the tank the vent line allows air to be pushed out as the tank is filled. As the fuel level drops it has to allow air back in to avoid a vacuum. That would happen on a stock system with a pump that sucks. You have an electric pump that pushes? it shouldn't be effected by a vacuum condition? or is my thinking wrong? Either way you are safer with a hole in the cap..

[59meteor]

highball, wouldn`t the act of filling up the tank, allow any pressure during the filling process to just come out the filler tube ? After all, the gas pumps nozzle is considerably smaller than the opening in the filler tube.