How Do Gas Pumps Know Your Tank Is Full?

If you drive a gas-powered car, you know the drill when it’s time to refuel: You pull up to the pump, scan your card or go inside to pay with cash, put the nozzle in your tank and pull the trigger to let the gas flow. If you’re like me, you lock the trigger in the on position so you can do other stuff while the tank is filling. When the tank is full, the pump shuts off automatically, and you’re on your way.

While pumping gas, you have plenty of time to wonder how the gas pump knows when the tank is full. “You’d think there’d be an electronic sensor,” says auto insurance specialist Melanie Musson, “but that’s not how it works.” It’s a mechanical system that relies on vapor pressure. The mechanism is somewhat fragile, and you can damage it by ignoring the signs warning you not to top off your tank. As this post explains, you can also damage your car.

How Does the Gas Pump Know When the Tank Is Full?

The standard gas pump nozzle owes its operation to a fluid dynamics principle discovered by 18th-century physicist Ace Ventura. Just kidding…that name would actually be Giovanni Venturi, and the phenomenon is known as the Venturi effect. Venturi discovered that air flowing through a tube speeds up when the tube narrows into a constriction, and this creates pressure. Engineers used this principle to design a nozzle that employs air pressure to keep the valve open and allow gas to flow.

The design involves a narrow tube that runs the length of the nozzle and terminates with a small opening near the end. Air flows through this tube while gas is being dispensed, keeping the valve open, but when the tank is full, and the tip of the nozzle is immersed, gas blocks the opening and cuts the flow of air. The gas pump knows when the tank is full when the pressure changes and the valve closes and cuts the flow of gas.

Did you get that? If not, maybe Musson’s explanation will help clarify: “There’s a hole at the end of the nozzle that pulls in air, which offsets the pressure of the gasoline going through the nozzle. When the gasoline in the tank reaches the hole in the nozzle, the pressure changes because gasoline is different from air. That increase in pressure causes the membrane that allows gas to flow freely to move, which closes the valve, stopping the gasoline.”

FAQ

Is it bad to top off your tank after it’s full?

Full disclosure…I do this all the time. But I shouldn’t, and automotive expert Evaldas Zabitis cites two reasons why not:

“Gas stations often warn you not to top off your gas tank, not only because of accidental fuel spills but also because it can damage the fuel nozzle. These nozzles depend on a delicate vacuum system to shut off, and then the tank is full. When you top off your gas tank, that vacuum system may become flooded and malfunction.”

Not only that, says Alex Black — another car care expert — but topping off is also bad for your car: “Newer cars possess an evaporative emissions (EVAP) system for capturing the vapors from the gas and avoiding pollution. Flooding the charcoal canister through overfilling can cause costly repairs.”

Why is my gas tank overflowing when it’s not full?

Normally, the gas pump knows when the tank is full, but sometimes it makes a mistake. This could be because the nozzle is faulty, says Zabitis, but there’s another, more likely, possibility: “One of the most common reasons is a clogged EVAP system. If the tank’s air can’t escape fast enough as fuel flows in, pressure builds up, causing fuel to back up near the nozzle and trigger an early shutoff.”

A third possible cause of blowback, says Zabitis, is the size of the tank opening: “In some cases, a filler neck is narrower than normal. This is quite common in cars from overseas. The nozzle blocks the entire filler neck, restricting the airflow and activating a shutoff valve long before the tank is full.”

About the Experts

• Melanie Musson is an auto industry expert with AutoInsurance.org.
• Evaldas Zabitis is an automotive expert who writes for carVertical, a global and decentralized car history registry
• Alex Black has been a car expert for more than 15 years. He is the chief marketing officer for EpicVIN.

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