Here’s How a Gas Pump Knows When to Shut Off

Learn all about the mechanics behind the humble gas pump

Co-authored by Duston Maynes and Devin McSween

Last Updated: January 25, 2023 References

How Gas Pumps Know When to Stop

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Why Gas Pumps Stop When Your Tank Isn’t Full

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Can I keep filling my tank after the pump stops?

You put gas into your car so often that it’s automatic: the nozzle goes in the tank and then the pump does all the hard work. But as you rest against your car, it hits you: how does a gas pump know when to stop pumping gas? You might be surprised to learn that the answer is much simpler than fancy cameras and sensors! In this article, we’ll go over the mechanics working inside the gas pump that tell it when to stop. Then, we’ll answer why gas pumps sometimes stop pumping before your tank is full and whether you should top off your tank.

Things You Should Know

A gas pump nozzle contains a venturi tube that sucks in air as gas enters your tank. When gas covers the tube, it triggers a valve to stop gas from pumping.
A gas pump might stop pumping prematurely if the gas flow is too fast, there’s a mechanical issue with the pump, or your car’s vent lines are damaged.
Do not top off your gas tank once it’s full. This can damage your onboard refueling vapor recovery (ORVR) or cause gas spills.

Section 1 of 3:

How Gas Pumps Know When to Stop

Gas pumps have something called a "venturi tube" that tells them when to stop. The venturi tube is a small tube that runs through the pump nozzle and handle. When you pump gas, fuel goes through the nozzle while the venturi tube sucks in air from your tank, creating a vacuum inside the pump. When the gas in your tank covers the venturi, it can no longer take in air. This suddenly decreases the pressure flow inside the pump, which triggers a valve in the handle to shut off the gas.^{[1]
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- Think about sucking air through a straw. When the end is open, it’s easy to suck in air. But if you put your finger over the end, you can’t suck in any more air. If you continue to suck, the pressure inside the straw decreases and it crumples in on itself.^{[2]
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Section 2 of 3:

Why Gas Pumps Stop When Your Tank Isn’t Full

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The splash back from fast flowing gas can trigger the pump to stop. Some gas pumps have a faster flow than others. A higher flow can cause the gas to splash back and hit the nozzle head. This covers the venturi tube, telling the pump to stop.^{[3]
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- When this happens, slow the pump down. Either press lightly on the pump handle yourself, or move the pump handle clip to a lower position.^{[4]
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- Flipping the pump nozzle upside down as you fuel up can also help the splash back issue.
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The pump is clogged or facing some other mechanical issue. If the gas pump isn’t pumping gas quickly, but it’s still stopping prematurely, it might be an issue with the pump itself. Wipe the end of the nozzle with a clean cloth to remove any potential debris blocking it. Then start pumping again to see if the issue is fixed.^{[5]
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- If the pump continues to malfunction, tell your gas station attendant. There might be an issue inside the pump, which they can get fixed.
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Your car’s gas tank vent line is clogged or damaged. If every pump that you use stops pumping prematurely, your car’s vent line might be damaged. The vent line releases air from your gas tank as it fills up with fuel. When air isn’t released from your gas tank, the fuel has nowhere to go. The pump nozzle senses this, so it shuts off the pump even though your gas tank is still low.^{[6]
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- If you suspect the gas pump shuts off because your vent line is clogged or damaged, take your car to a mechanic. They can check if that’s the issue and fix or replace the line.

Section 3 of 3:

Can I keep filling my tank after the pump stops?

It’s dangerous to top off your gas tank when it's full. You might be tempted to fill your gas tank to the brim, but this can cause more trouble than it’s worth. Overfilling your tank can damage your car’s onboard refueling vapor recovery (ORVR), which absorbs harmful gasses when you refuel your car. If your ORVR doesn’t work like it’s supposed to, it needs to be replaced, which can cost several hundreds of dollars.^{[7]
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- Overfilling your gas tank can also cause fuel to spill out of your car. This is dangerous to you, your car, and to the environment.