Today’s cars use a vapor trapping system aka, carbon canister. That lets air into the fuel tank to make up for the liquid fuel that you remove. The carbon is a large filter medium that filters the raw fuel from evaporating into the air when the car is at rest and the vapors try to flow the other way. If you overfill too much on some cars you can force fill the carbon, wetting it and then the air doesn’t pass. When that happens the fuel doesn’t flow “so good” to the engine and things are unhappy in engine land.
OK to stop customers from doing that many car manufacturers do two things. First they extend the end of the fill pipe into the tank so that even when fuel is pumped back up the filler there is usually about a 5% expansion area to prevent true overfilling and also to allow room for expansion when you fill and park in the sun. So when you “overfilled” you really didn’t – except for the note below (*).
Second, to allow the system to be closed and not give Al Gore a runny nose from gas vapors the cap seals the system and when it is screwed back in it opens the line to the canister. If the cap is left loose then the system is open to the air, the air isn’t flowing through the carbon trap and another sensor detects it and the warning lights go off. Some of you that took physics can see that this has to be this way because if the cap closed tight air couldn’t come in then the system wouldn’t pump either. So to prevent owners from pumping fuel back into the canister these manufacturers use that valve and this is the case on the smart. Look inside toward the front of the filler neck when you have the cap off (DON’T USE A LIGHTER) it is hard to see but there is a small, black (+/- ¼”) plastic gismo that is sticking out the neck.
Here is where the (*) note comes in. If you fill where I do the Wal-Mart has a ridge on the neck of the gas nozzle that can trip my trigger…..so to speak. When it happens I can hear the sound of the gasoline bubbles bursting in the top of the tank because the vapor hose is now open, but not to the carbon canister, but where I am pumping. This overfilling can flood the canister. I even use this knowledge to my advantage. I HATE TO TAKE THE TIME TO FILL, so I usually buy cars that go a long way between fill-ups. My daily driver goes about 800 miles which means I stop once a week, I hated the last car because it would only go about 450 and that wasted at least 20 minutes a week from my life doing something that is boring. I don’t drive the smart to commute, so not an issue. HOWEVER, when I fill my smart, if I know that I am going to drive another 15-20 miles before I shut it off for the day, I use the end of the nozzle to trip the valve and then I do get more in, but not up the neck. Yes, I risk saturating the canister, but a risk I choose, on my own.
