Gastank waterfall. What in the world?

[qslim]

So I fill up on my way home from the store (4.2 gallons, riding on 'E') and make it home about 15 miles later. Park, center stand it, get inside, go back out about 30 mins later to get stuff from my bags and freakin fuel is POURING out from under the cap. Panic ensues.
I popped the cap and gas is ALL the way up to the tank lip. I pulled the fuel line and flipped the valve to prime until the level went down to where it was when I filled it. As I was washing it off, MORE gas starts escaping! I pulled the cap and the whole process repeated itself. Now it looks like it's staying put. What the hell?
I fill up about 2 inches from the bottom of the lip, it's only about 80F here, and I always gas up while on the bike, so the level is flat. I'm no gas tank scientist, so does anyone have an idea for me?

[semerm]

liquid expands as it heats up. The temp in the gas station (underground) tank is much cooler- 80 degrees in the direct sun will heat the inside of a car alot- Just a guess!

[qslim]

That is what I know the case must be, but the only thing that really shocked me was when after draining it overflowed again within two minutes!

[onchiman]

Had the same thing happen to me once. I kept squeezing and squeezing the gas in, forcing it because I just wasn't satisfied with there being unused space in there (just young and stupid I guess). I learned pretty quickly why the filler neck extends into the tank a couple of inches. Gasoline expands when it get's warm and that neck gives it some room! It was a hot sunny day and my tank is black. Well in a few minutes gas was just running all over the place (fortunately gas was a lot cheaper then). Since then I fill it only until the pump nozzle clicks off then out it comes. As Mark Twain once said, "youth is wasted on the young".

[Matt Vann]

I just went through something similar two days ago.

I filled my 80 GS1100E and made a short ride home. I never thought about the expansion. I parked it in my shop. The temp was near 100 the next day when I came home for lunch and I could smell gas everywhere.

Fuel had seeped through the fuel gauge sender seal and was raining all over the motor. It was actually condensing all over the bottom of the tank and raining everywhere.

Not completely understanding how the sender seal and drain worked, I emptied the tank and disassembled everything. All of the rubber was in good shape, but the bolts felt extremely loose. There was no mention of this assembly ANYWHERE in the Clymer manual. I cleaned it, applied a little grease and bolted it back in, tightening in a star pattern until the seal was tight and looked a little "squeezed" in place.

No problems thus far!

[Billy Ricks]

If the tank is vented properly there is no way this should be happening. In the 21 years I've had my 700 it has never happened even in triple digit temps and the heat of the engine.

[Jethro]

I have never had this problem and I always fill the tank past critical mass.

[DimitriT]

I've had it happen and I wish I could figure out a solution. Right now the only way to keep it from happening on my bike is to leave extra space under the neck at the tank inlet when filling up.

[JHawkins]

The only time I had gas come out the gas cap was when the kickstand bolts had loosened up a bit and the bike was leaning over more than usual, gas started pouring out the top when I filled up and then parked to go into the store to pay.

[Billy Ricks]

I've had it happen and I wish I could figure out a solution. Right now the only way to keep it from happening on my bike is to leave extra space under the neck at the tank inlet when filling up.

Check the gas cap vent.

[Sandy]

The tube in the filler hole is to prevent over filling the tank but if the fuel is up to the bottom of the tube it has no where to go but out the top the of the tank, whether the breather in your cap is working or not. The problem is more the air trapped in the top 1 inch of the tank expanding due to heat rather than the the fuel itself expanding. Air expands much faster with heat than liquid. When the air expands and the fuel is covering the the bottom of the tube the fuel simply gets push out the opening either through the cap gasket if the vent is not working or through the vent itself if it is working, it has to go somewhere. One trick I've used in the past is to drill a 1/16" hole in the top right side of the tube to allow the expanding air out and vent through the cap. This seems to fix the problem most of the time. Just put a gob of grease on the bit and where you're drilling and hopefully you won't have too many sparks. If this freaks you out you'll have to pull the tank, drain it and properly vent it or fill it with water before drilling.