The Stovebolt Site Forums The Shop Area General Truck Talk Gasoline dripping from filler cap (51 1/2 ton)

My gas gauge is broken (along with a bunch of other stuff) so I put some gas in, not filling until the pump auto-shutoff.

A couple days later I parked next to a curb with the typical street sloping toward the curb, had breakfast, came out and discovered that gasoline was dripping from the filler cap.

It was a pretty steady drip, about one every 5-10 seconds. I removed the cap and gasoline was indeed up to the rim of the filler pipe. I wiped up the gas, left, parked on a level surface without further incident.

What could cause this? Isn't there some sort of venting system that prevents pressure from building in the tank to force liquid out the filler tube?

I have no idea if the filler cap is vented or not. Should it be?

I'm not quite comfortabe with the thought of sitting within inches of a malfunctioning gas tank... if you know what I mean

Woody

Those were vented at the gas cap. If the gas was up to the edge you have too much gas in it. You will have to park differently when the tank is full.

It wasn't parked on that steep of an angle where the left side of the tank was higher that the top of the filler opening. That I could understand.

Let's assume that the level of fuel in the tank did cover the opening where the filler pipe enters the actual tank... what would cause it to rise the extra what, 8 inches (?), to reach the top of the filler?

Seems odd that the tank would leak like that unless there was pressure building up inside the tank.

Yes it can happen. It happened to me a while back. Parked on a very slight slope, hot day, not only did it drip, but when I took the cap off, I got a spray of gas. I don't park on hills anymore when it's hot out.

Im missing most of the paint down below the cap. They arent vented. Even on flat ground, on a slightly warm day itll puke gas all over the place. Ive gotten in the habit of getting gas at the station farthest from my house before heading home, or getting it as I leave my house. A vented cap will help, but not much.

Heat and expansion.

I always fill to the first 'click' on the gas pump. One reason is as stated above the second is that if I fill it too much the float in the tank bangs on the top.

I ran into the same thing last year and asked everybody's opinion.
The fact is the tank is in the cab. Hot day the gas expands. El-drippo !
So as they say, stop filling after the first click. Park in the shade with the windows down.
And I like the one where you fill up at a station far away from home. I do that too!
Jeff

Thanks to everyone for responding... now I don't feel so bad.

Gotta get that gauge fixed.

I seem to recall terms like vapor pressure and thermo expansion and a bunch of other term that relate to gasoline. The common sense approach would be the gas comes from tanks below that is more than likely 70 degrees or so. It goes into your tank and it expands due to the temperature rise and the heat also causes a vapor to be produced. As the vapor excapes throught the cap, some condenses and falls out as a liquid. I think the trick is to not fill up all the way and after you have filled up, don't park in the direct sun until you have had time to burn off some of the gas. My two cents!

Even something small like which direction your parked truck faces the sun makes a difference. If the back is toward the sun when parked I smell vapours in the cab. Turn it around and there are none. That back wall of the cab must absorb a lot of heat and of course it is against the tank.