The Stovebolt Site Forums The Shop Area General Truck Talk Pressure build up in gas tank?

So my '51 3100 has been running wonderfully. Been driving it everywhere. ...until yesterday.

The gf and I went out for brunch, I filled up on the way and parked in a surface lot in the sun. Temp was probably in the upper 80's or so. When we got back to the truck there was gas flowing out of the filler neck, running down the side of the cab and pooling on the running board and ground. When I loosened the cap there was a fair amount of pressure causing gas to spray. So, I removed it completely and just let it empty what it needed. When it finished the gas was to the top of the filler neck. After putting the cap back on, I cleaned up the mess with an old towel and started it. The level went back down and everything seems fine.

My first thought was that the heat caused the fuel to expand but while the back of the cab was hot from being in the sun, the tank felt cool.

...any ideas?

I beleive the cap should be vented. If it's building up pressure, the vent is not working.

On the flip side, filling up and parking just right on a hill will cause some gas to leak out the vent! argh.

Hope this helps.

The vented cap only works one way. It lets air in to replace used fuel but does not allow fumes out.

What happened is very common. The fuel came from underground tanks and is a lot cooler than the surface temperature. It is going to expand as it warms. Nothing to be concerned about. Just learn not to fill up right before parking.

You could add a vent to your filler tube at the top of the neck and run a hose/tube up & then down & out of the cab w/a filter in line to keep dirt out of the tank. I did this to a friends truck many years ago & they never had problems w/a pressurized tank again.

Thanks guys!

DMGfifty - Its a brand new cap. ...but I'm thinking about replacing it with a locking one anyways.

Cletis - That's what I thought, the cap vent only working one way. Didn't realize the pressure could build up that much tho.

80_Grit - thats not a bad idea. I have an extra filler neck in the parts bin. I might solder on a nipple and attach a line.

I had this problem with a Ford van with dual tanks. There was a blockage in the valve between the tanks. It was weird to take the cap off and have a gallon of gas gushing out. At $3.60 a gallon it was a bit more than weird.

What they said, and: When I bought a new gas cap for my truck, I drilled a small hole into the plastic vent on the cap, so it will vent fumes, with no pressure build up. I believe modern gas caps (repops) won't vent out until they hit 3 psi, to protect you in the event of a tip-over. 3psi is allot of pressure in my books, so out came the little drill.

I always fill up on the way out, or just what you say happens!

the tank pressurized because of the heat.Don't fill the tank full on a hot day if you are going to park the truck.I had a few spills before I caught on.On a hot day I usually don't fill above 3/4 full

Good advice. I remember my dad doing this on his '72, back in the day, for the same reason and never again had the pressure build up problem.

Pressure build up problem? How is it a problem? That is how it is designed to work. That is not what pushed fuel out of his tank.

Cletis is right. As he mentioned in his first response, the gas was cool when it went in and then expanded. The fix---don't fill it all the way full unless you're going to drive it right away.

Guess this means I'll need to fix my fuel gauge/sending unit soon.

My old boss said his truck got such good fuel economy that every once in a while he would have to pull over to the side of the road and let some of the fuel out! I thought he was spinning tall tales, but now I wonder. . .

if you have no fuel gauge,feed the fuel in slower.Listen at the fuel neck,you will hear the sound change at the tank fills.Stop filling when you hear the change.

^ good idea!