After a weekend with a bunch of MIT friends in Las Vegas, and then a Sunday morning trip to see the Hoover Dam, I found myself back in Vegas filling up for the ~300-mile trip home.  However, the car did not want to cooperate, and the gas pump would shut off within 1-2 seconds of starting, sometimes leaving a small puddle above the flap in the filler tube.  I moved to a second pump at the same gas station and had the same issue.  No matter how slowly I tried to add gasoline, it would stop within a few seconds.  After about 15 minutes, I had only added about 2 gallons of gas -- that's almost as bad as charging an electric car!

What's the big idea here?  The gas tank is a fixed volume and is not under much pressure, so when I push a gallon of gasoline into it (which takes < 10 seconds at a typical gas pump), a gallon of air and gasoline vapors has to be pushed out to make space.  If the vapors don't move, then the pressure inside the tank builds, which forces the one-way fuel fill valve between the tank and filler pipe to close.  The gasoline then builds up in the filler pipe and touches the nozzle, which forces it to shut off.  (Click here to see more details and illustration of the automatic shutoff gas nozzle.)  Similarly, if no fresh air can enter the fuel tank as the car is driven, then the pressure would drop and would create a vaccum relative to atmospheric pressure.  So to summarize, air needs to be able to enter the tank slowly while driving (at a rate of at most a few gallons per hour), and air needs to be able to exit the tank rapidly while refilling (at a rate of a many gallons per minute).

Nowadays, the air/vapors exiting the tank are cleaned in order to prevent hydrocarbon vapors from simply being emitted to the atmosphere -- "Onboard vapor recovery" has been mandated by the EPA since 2000.  Now, the path looks something like this (from the Hyundai service manuals website, linked from this helpful forum post):

In fact, this shows less than half of the "plumbing" involved around the fuel tank, so this diagram was a bit confusing as we tried to attack the problem.

On this car, the gas tank filler hose is on the driver's side.  Below is the view from under the left rear tire, which shows three connections.  The left (small) tube goes to the small ports around the main filler pipe.  The middle is the main filler pipe which carries gas to the tank.  On the right is a odd-looking unit which happens to be the "liquid-vapor separator".  I can't be 100% sure, but the hypothesis is that it's just a physically-higher unit which ensures that if one were to really over-fill the tank, liquid fuel would still have a hard time crossing to the canister.

And below is the view of the same thing from underneath the car.  The large pipe on the left is the main filler pipe, where gasoline goes when refueling.  The liquid-vapor separator is the long rounded object parallel to that pipe, and in this photo, the bottom connection to it is coming from the fuel tank's vent valve (at a point high up on the fuel tank), and the top connection is going into the box on the right of the photo.  The box on the right of the photo is the charcoal canister which collects fuel vapors.  This serves two purposes: one is that during vehicle operation, those vapors can be purged into the engine intake and combusted, which prevents the release of hydrocarbons.  Second is that this is the path for air intake/exhaust for the fuel tank.  The upper (thicker) hose in this photo going to the charcoal canister from the liquid-vapor separator.  The lower (skinnier) hose in this photo coming out of the canister is for the purge system, which sends those vapors back into the engine.

After a little bit of work, we removed the canister assembly, as shown below.  This involved removing two bolts for the canister holder, disconnecting the two vapor hoses, removing an electrical connector, and loosening the zip-ties.

The canister assembly is composed of three primary elements: the canister itself, an air filter which is open to the atmosphere, and the canister close valve, which connects or disconnects the air filter from the canister.  The last two photos in this gallery show the canister separated from the air filter+CCV.

Well, after all of that, we didn't find anything wrong.  No kinks or obstructions in the hoses, no significant back-pressure or other issues.  Since most people on the forums described finding hoses filled with little carbon pellets, we knew that that sort of thing should be obvious enough, and it wasn't.  So we put everything back together, drove to the gas station, and guess what -- it filled up just fine!

An unsatisfying conclusion!  Still, that fill-up in Vegas was the hottest the car had probably seen in its lifetime, which makes me wonder whether it may have been related.  Or it could have been the two pumps I tried, or any number of other things.  A learning opportunity, and an opportunity to get more comfortable working on cars, and a working vehicle at the end of the day, so I think I'll call it a victory.

I recently made this bottle opener on the milling machine from some aluminum bar stock. As you can see from the photos, it has changed a bit since my original CAD sketch, but I've decided to include the drawing anyway.

It's very strong and light. If I were to do it again, I'd probably make it thinner than the current 0.200" thickness, and would reduce the length of the thin extended arm and increase the length of the part before the "hook" to give the user a slightly longer lever arm.

Humberto had the idea of having the step in the thin extended arm to allow it to better deform the metal bottle cap (contacting roughly at its center) instead of simply prying it off (contacting at the far edge of the bottle cap as per the CAD sketch). Any opinions on which is better?

Two photos and a sketch: