Excessive Gas Tank Pressure when it's hot outside

[2erobrd]

Hi Guys,

I just returned from a local car show here in the San Fernando Valley where the temperature topped 105. Once I got home I pulled in to the garage and was doing my normal walk around the vehicle. I noticed a steady drip of gasoline coming from right behind the bumper on the right side. My first thought was that something pierced the tank on the way home. After spending a few minutes under the car, it looks like the gas was coming from some sort over "over flow or vent tube". On a hunch I removed the gas cap (which has been mounted inside the trunk for well over 3 years now) and when I gave it the final turn to the cap to remove it I heard a healthy "whoosh" of air come from inside the tank as it had ballooned (due to too much the pressure in the tank) and then the tank went back flat in the trunk. This is the first time I have seen anything like this.

Any suggestions what I should look at or replace? It's a new tank, new floater and new mechanical pump. The Morot is a 170, nothing fancy on here just headers and an alternator conversion. Other than that she drove great but I had never driven her in this type of excessive heat.

What do you guys think?

[ew1usnr]

I noticed a steady drip of gasoline coming from right behind the bumper on the right side. After spending a few minutes under the car, it looks like the gas was coming from some sort over "over flow or vent tube". On a hunch I removed the gas cap (which has been mounted inside the trunk for well over 3 years now) and when I gave it the final turn to the cap to remove it I heard a healthy "whoosh" of air come from inside the tank as it had ballooned (due to too much the pressure in the tank) and then the tank went back flat in the trunk.

That is really weird. Hot pressurized gasoline would scare the heck out me. I would leave the trunk open, the gas cap loose, and roll the car out of the garage until you figure out what is going on.

On a stock Falcon, I do not know how the gas tank would be ballooned out unless the vent tube was blocked. I do not know how gasoline would be dripping out of the vent tube because the vent tube rises higher than the filler spout. The gas tank will overflow from the fill spout before overflowing from the vent tube.

I cannot explain how the symptoms that you have described could occur.

[2erobrd]

That is my concern as well. In the mean time, yes the cap is off, the trunk is open and all is well. For now.....

[2erobrd]

And if the vent tube was blocked then how did it start leaking through the tube. No explanation for that. Maybe I need a gas cap that allows the tank to vent? But that is the purpose of the tube. Well, for the time being this is only happening when it's really hot so I plan not to drive it that much since It does not have any AC, but, leaking gas and a bloated gas tank are things that need to be addressed and not forgotten.

[PhilC]

And if the vent tube was blocked then how did it start leaking through the tube. No explanation for that. Maybe I need a gas cap that allows the tank to vent? But that is the purpose of the tube. Well, for the time being this is only happening when it's really hot so I plan not to drive it that much since It does not have any AC, but, leaking gas and a bloated gas tank are things that need to be addressed and not forgotten.

You have the cap off and trunk open to dissipate fumes. Rhetorical question, where will the fumes go if you put a vented cap on the filler neck in your trunk?

Liquids expand when heated, fuel does too and that's why you got some fuel out the vent tube. There was enough pressure in your tank to force fuel through a restricted vent tube - not good.

Ensure your vent tube is free of any obstructions. Also verify the vent tube has not been kinked or pinched anywhere from the tank to where it exits the car.

Please, please, DO NOT use a vented cap on the filler neck in your trunk.

[2erobrd]

I have looked at the tube and for the fact that it permits the gas to flow out I have to imagine it is not kinked or pinched.

The cap is a solid cap and has no venting mechanism whats ever.

[ew1usnr]

I have looked at the tube and for the fact that it permits the gas to flow out I have to imagine it is not kinked or pinched.

Try blowing some compressed air through the vent tube. If it is not visibly kinked anywhere, maybe it is blocked with mud.

Was the gasoline "flowing" out, or was it dripping? Maybe mud would block most of the vapor from escaping but would still allow liquid to drip though once the mud plug was saturated.

It is just a theory.

[2erobrd]

the Gas was dripping out.

Today its pretty hot (93) so waiting for the sun to go down before I disconnect the tube and put some pressure into it to blow it out.

[2erobrd]

Well, Happy 4th of July guys !!!!

so in-between chores today I disconnected the "vent" tube from the gas tank and blew it out with about 80lbs of compressed air.

And the result was as I figured, nothing came out other than 80lbs of air pressure. I also inspected the vent tube as it's not s simple as you would think, it goes up from the tank to the top edge of the trunk, then travels all the way under the inside of the right her fender. Then it makes a crazy hairpin turn back towards the rear of the vehicle and down right behind the bumper.

All in all, the tube is a factory part, not kinked, not bent, not rusted and certainly not obstructed by any means.

another observation I made today is that even though it was only about 85 degrees today it was no where near as hot as it had been the last few days. Anyhow, I pulled the car in reverse up the driveway instead of down the driveway in hopes of seeing if it would leak today after I blew out the tube. we got up to 91 today and nothing happened today, no excessive pressure and no leak out of the vent tube.........

any other suggestions ?

[Luva65wagon]

There's really no way for the tank to have bloated if the vent tube was in-fact clear. Maybe 80 PSI dislodged something, or shifted it, but it is still inside the tube and now is finally letting air pass by. Hard to say, but you can't blow up a balloon with a hole in it. I'd pull that tube loose and chase a wire though it, if possible, to see if a lump of something doesn't come out.

Agree with Phil regarding the internally located cap. Some cars, like my 65 wagon and Gene's 65 Ranchero, have no vent tubes and used a vented cap. I think most early cars did. My Ranchero, though, did have a vent tube just like you describe. Gene's doesn't have a vented cap now and he lamented a week ago about hearing a big woosh when he released the cap. No surprise. He's thinking of adding a vent tube because he's relocated his filler into the bed under his bed cap.

The design of the vent tube is such that fuel should never get pushed up that far. You would have to have a full tank to the cap level and tilt the car on its side. But if you had pressure like you said, it might have occasionally purged the pressure through that tube with enough velocity to where some raw fuel came along with.

What happened happened, clearly, but I would verify all is clear and go back to normal driving to see if you fixed/changed anything. Should be quickly obvious and you'll never know until you try.

Too bad it is gas fumes you are having to experiment with, though...

[ew1usnr]

I disconnected the "vent" tube from the gas tank and blew it out with about 80lbs of compressed air. And the result was as I figured, nothing came out other than 80lbs of air pressure.

Maybe the blockage is upstream of where you blew air through. Look in the nipple where the hose clamps onto the gas tank.