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?