It doesn’t happen too often, but this is one of those times where I’m writing an article because of something my mother said. My mom—a woman of a certain age who drives around in an adorable mint green Fiat 500—was complaining to me about how the gas pump kept shutting off while she pumps gas, and expressed the usual panic that this meant something was wrong with her car. After asking around, I realized there’s a lot of misinformation about this common occurrence, so we may as well tackle it head-on. Ready, mom?
First, and most importantly to the people I’ve spoken with about this, is this bit of information: a gas pump that keeps annoyingly cutting off does not mean there is something wrong with your car. Now, that doesn’t mean that components of the car and how they’re designed aren’t playing a part in all this, just that it does not necessarily mean your car has some sort of mechanical problem.
In my informal polling about this minor but pervasive annoyance, you’d be surprised by the number of people I encountered who felt that the car itself had some say in when a fuel pump would cut off, as though the car had some sort of mechanism to disrupt fuel flow on the pump when it determined it was full.
The truth is that no car I’m aware of has any such device or capability. The refueling process on cars really hasn’t changed appreciably in around a century, at least on the car’s end. It’s still pretty much just a tube that leads to a tank. There is a cap on the end of the tube, and that’s basically it.
Now, on the pump side, things are much different. Gas pumps have changed a lot, and modern pumps are much different than they used to be, including many safety-related changes, the sorts of things you’d expect from a publicly used machine that’s basically one lighter away from becoming a stationary flame thrower.
We’ve actually covered how fascinating gas pumps are before, and even shown you this wonderfully geeky video about them:
This time, though, I just want to focus on the key part of the pump that affects the fuel cutoff, though, because that’s the root of all these fuel cutoff issues: the Shut-Off Sensor. You can see it in this diagram I highlighted from Methodology for Evaluating Fuel Nozzle Dispenser Characteristics, which is a real article:
So, here’s how the shut-off sensor works—it’s all basic mechanical stuff, no fancy electronics: there’s a small hole near the end of the pump nozzle, and this hole connects to a little pipe inset in the main fuel nozzle, like a little hose shoved up an elephant’s trunk.
The pipe is sucking in air via the Venturi effect, and when that little hole is able to just breathe in air, everything flows nice and easy. As soon as the hole gets covered—say, by the rising level of gasoline in your tank—then all of a sudden the hole can’t suck in air as easily (or at all) and where that flowing air was once traveling, there’s now a vacuum, and that vacuum sucks down the cutoff valve, stopping the flow of fuel.
Here’s a little video of a version of this device, set to some smoooooth free production music:
What this all means is that if you’re filling your tank and the pump keeps cutting off, something is blocking the little hole, preventing a flow of air through the pipe. Most commonly, the culprit is just some gasoline splashing back enough to block that hole momentarily, triggering the cutoff.
Now, the question is why is it splashing back? In the case of my mom’s little Fiat 500 (and my small cars like my Nissan Pao, Yugo, and Beetle) the issue seems to be a relatively short pipe for the fuel to flow before it hits the tank.
On cars with shorter intake pipes, a fuel pump with a relatively fast flow could easily flood that little column and trigger the cutoff; that’s why the first solution given to pumps that do this a lot is to lower the flow rate of fuel. Here’s a troubleshooting guide that says so:
So, if you’re constantly plagued by fuel pumps that keep, annoyingly, cutting off on you, over and over, it could be because the flow rate is simply too high for your fill pipe length. In that case, you can try adjusting the trigger setting to a lower notch, or squeezing it less with your hand, or possibly angling the fill nozzle to give the little breather hole a better chance of not getting splashed or submerged by gasoline.
If none of that works, try another gas station that may have slower-flowing pumps. I guess if you were really, really desperate and had a real contempt for safety, you could maybe rig up a little snorkel device that would fit over the end of the nozzle and give the cut-off sensor an uninterrupted flow of air, no matter what.
I bet that’s illegal, though.
With that in mind, the takeaways here are that it’s not a problem with your car, and the best thing to do is to pump gas a little bit slower.
Got it, mom?
UPDATE: As some have pointed out, I was remiss in not mentioning something on the car’s end of things that could be a contributor as well: the fuel tank vent line.
Fuel tanks have venting lines that can get clogged or blocked; in cases like this, the situation caused by fuel back-up in the fill pipe can happen, since the clogged vent line will prevent rapid refilling, and that can trigger the shut-off valve.
So, there could be something wrong with the car, specifically, the vent line. If this happens a lot to you and you have a car with a long filler tube, that could be worth looking into.
An even better UPDATE: We reached out to a fuel systems engineer at a major OEM (he’d prefer anonymity because you know how weird OEMs can be), and got a very interesting response that gives even more—and better—information:
“So, Torch did a decent job on the article, and honestly this one constantly plagues me trying to figure out exactly what is causing a premature shut off (PSO) on a vehicle. The most common culprit that we see for a PSO is a damaged nozzle from the gas station. Most fuel nozzles have a shelf life around 3 years, next time you are filling up, look at the nozzle, especially under the rubber boot over the front and you will likely see a replace by date that was a few years ago. When the gas station wants to save some money and not replace the nozzles, you will see the end of the nozzle start to peen over from many many insertions into vehicle filler pipes. This causes the spray from the nozzle to no longer be uniform (which is how we design our fuel fillers), causing turbulence in the fuel filler pipe, bouncing back, and momentarily blocking the venturi on the fuel nozzle.
Also, one correction to Torch’s article is how a gas nozzle actually shuts off when the fuel tank is full. We don’t design our systems to have fuel back up the pipe and block the venturi. Instead there is a float valve in the fuel tank, once the correct volume of fuel is dispensed into the tank, the float lifts, preventing the tank from venting. This causes a spike in the pressure of the filler pipe, and shuts off the nozzle by having a large enough pressure differential across the venturi.
Now if a customer is having issues with PSO’s, the first thing to try is slowing down the flow rate. The second thing we try is to actually flip the fuel nozzle upside down. This lifts the venturi of the fuel nozzle to the top of the filler pipe, and usually gives you a little more tolerance for fuel splashing due to a bad nozzle. If neither of those things fix the issue, it’s likely an issue with the vehicle; the most common issues we check is that all vent lines are hooked up and clear and then check that the float in the fuel tank is not stuck shut.”
That’s interesting about the float valve spiking the pressure to trigger the cutoff—I believe that’s a relatively new innovation, and this page has some nice details about how that works and an excellent diagram. Also, the upside-down nozzle trick is a great thing to try if this issue is plaguing you.
Yet another UPDATE: The engineer told us a bit more about how that valve goes about shutting off the pump, saying:
“[Fuel] can rise up the tube, however in my experience it is more likely that the tank pressure alone shuts off the nozzle. A majority of fuel systems have two primary fuel tank vents, one going to the evap canister (which is how the system displaces the air in the tank per the article you are referencing) and another going to the fuel filler pipe. The line going to the filler pipe ... is actually used to reduce the amount of vapor generated during refueling.
[This] line is also connected to the float valve that shuts off the canister vent line, so when the float shuts, the air path to the filler pipe also is blocked off, and the fuel going into the filler pipe causes a spike in pressure. The venturi on the filler nozzle will actually shut off from this spike in pressure before the fuel reaches the nozzle. This is more common on vehicles with longer fuel filler pipes...”
He also mentioned that a bad float valve or a plugged evap canister can cause issues with premature shutoffs, thusly:
“And a bad float valve in the fuel tank will absolutely cause this PSO’s. The evap canister has a fresh air filter that vents the air out of the fuel tank, if this filter is plugged from dirt or spiders nest, this will also cause PSO’s.”