Maybe you notice that your car has needed to be refueled more often. Maybe the pump at the gas station has just gotten quirky all of a sudden. Or maybe you catch a whiff of gasoline as you get in your car one day. Whatever the clue, you’ve come here to answer an alarming question:
Why is my gas tank holding less gas?
A clogged EVAP system can result in false gauge readings, and cause the gas station pump to stop repeatedly. Line leaks and faulty fuel gauges can also create the impression that your tank is holding less gas than usual. In other cases, the issue may be with the gas station pump and your fuel tank could be completely unchanged.
Below, we do a deeper dive into all of the reasons your car could appear to hold less gas than it used to.
1. A Clogged EVAP System
The EVAP system (Evaporative Emissions Control System) is there to capture and control fuel vapors. As your car is in motion gasoline creates volatile organic compounds (often referred to as “VOCs”) that can have adverse health and environmental impacts if they are not managed.
Your EVAP system is designed to capture and control harmful vapors, but over time, components like valves and sensors may fail or develop leaks, which could lead to issues such as a false fuel gauge reading or premature pump shut-off at the gas station.
These clogs can interfere with the fuel system pressures, which may affect the vehicle’s on-board diagnostics leading to a false fuel gauge reading.
A malfunctioning EVAP system can lead to other performance issues in your vehicle and can even lead to poorer fuel economy, and potentially increased wear and tear.
When your EVAP system is compromised, it should cause the check engine light to go on. Repairing or replacing parts on your EVAP system is usually straightforward and (relatively) inexpensive. You can learn more about what this looks like as a mechanic walks through the process here:
Is There Anything I Can Do to Avoid a Clogged EVAP?
Keeping the area around your gas input clean can help avoid debris accumulation. Opting in for scheduled car maintenance can also help.
You know when you go to the mechanic for an oil change and they say, “It says you’re due for a new charcoal filter.”
Then you say, “Is mine broken?” And they so no. And you say, “How much will it cost?” And then they tell you, and you hightail it out of there as quickly as you can. Well, those scheduled maintenance expenses can help reduce the need for larger repairs down the line.
A new charcoal filter for your EVAP system can go a long way toward reducing your risk of clogs. Filters should be changed every two years or so (approximately 24K miles).
Cool. Is There Anything Else I Can Do?
Stop topping off your car. The pump shuts off for a reason when your tank is full. Overfilling can cause liquid fuel to enter the charcoal canister, which is designed to handle only fuel vapors, leading to potential damage over time.
There is no fixing a broken EVAP canister. You’ll need to have it replaced, which will set you back a few hundred dollars.
2. Fuel Line Leakage
Fuel lines may develop leaks through corrosion or general wear and tear. Your car vibrates and fluctuates in temperature constantly while you are driving. Over time, this can cause your lines to become brittle and develop leaks. Physical trauma—for example, a car accident—can also create leaks in your fuel lines.
Sometimes the signs are very clear. Puddles of gasoline beneath your car when you park. A strong smell of fuel anytime you get in your vehicle.
Other times, the symptoms are more subtle. Minor delays when you start your car, which can closely resemble the symptoms of an aging spark plug.
Most of the culprits listed in this article are (more or less) harmless. Items to be fixed, yes, but not significant safety issues. Fuel line leakages are an exception. Dripping gasoline can create an increased risk of vehicular fires or explosions.
It is never considered safe to drive a car with a leaking fuel line. Set up a tow, and get your car to a trusted mechanic right away.
Fuel Gauge Malfunction
Another possible explanation is that your fuel gauge is malfunctioning independently from any other error. In this situation, your tank is holding the same amount of fuel that it always did. However, the readings on your gauge are off, making it impossible to know how much fuel you have.
Inside your gas tank, there is a float attached to a sender unit, which transmits the fuel level information to the fuel gauge. That float is connected to a wire that measures its position and creates resistance. If any of those components malfunction, or if there’s an issue with the electrical circuit connecting the sender to the gauge, it can cause an inaccurate read on your fuel gauge.
In some vehicles, a higher resistance indicates a higher level of fuel, while a lower resistance indicates a lower level of fuel, although the design can vary.
If any of those components malfunction it can cause an inaccurate read on your fuel gauge.
From a strictly functional perspective, nothing is wrong with the way your car is using or storing fuel. However, you obviously can’t set off for a road trip with no idea how close you are to running out of gas on the highway.
Usually, the repair will involve replacing the sender wire. Sometimes, a new gauge is required. But this is just one of the many problems faulty sensors can cause.
3. It Could Be The Gas Station Pump (Not Your Car)
There could be times when it’s not your car, but the gas station pump that’s throwing a wrench in your refueling routine. These pumps can malfunction, like anything else.
Here’s a brief on how these pumps work: when you insert the nozzle into your car’s gas tank and squeeze the handle, fuel flows until either you let go or the pump automatically stops when your tank is full. You already know that, but it’s still worth pointing out that the automatic shut-off is a “smart” feature to prevent overfills and is triggered by a mechanism in the nozzle that senses when the fuel level hits a certain point.
However, if the pump isn’t working right, this shut-off can engage too early, making you think your tank is full when it isn’t. Even if you continue to squeeze the handle, it still might push more fuel even if your tank has the space.
You may see this more often in pumps that have inconsistent flows, push fuel at a snail’s pace, or otherwise just seem off. So before you decide that your tank is holding less fuel, try fueling up at a different gas station or at least switch pumps.
Why Does The Pump Shut Off When My Tank Isn’t Full?
That is a tricky conundrum that can confuse some drivers—particularly after we mentioned that you should always stop fuelling when the pump tells you to. But, if you’re sure you’ve only filled it up a quarter of the way, something has to be wrong…right?
To answer that question, it’s important to understand how the shut-off feature on the pump works. Modern pumps have a mechanical shut-off mechanism triggered when liquid reaches a certain level, preventing overfilling. If your EVAP system is having issues or if the fuel is being pumped too quickly, it could trigger this shut-off mechanism prematurely, making it seem like your tank is full when it isn’t.
When your tank is full, gas splashes on that sensor, telling the pump it is time to stop. But certain things can trigger that shut off prematurely.
We mentioned earlier that EVAP issues can cause the pump to stop even though the tank isn’t full. Sometimes, however, the explanation is more innocent. For example, how fast is the flow on the pump?
Really quick pumps are nice when you are in a hurry, but they can cause a backsplash effect that triggers the pump sensor.
If you find that you are getting a wonky shut-off response at one gas station only, it’s probably the pump. If it happens everywhere, there may be something going on with your EVAP. Either way, don’t let it fool you: your tank is still holding the same volume of liquid. In fact:
Your Tank Capacity Doesn’t Really Change
Unless you fill the tank up with cement, it’s going to hold pretty much the same amount of gas for the entirety of its useful life. There may be minor capacity changes as your car ages. Dirt and sediment may accumulate in the tank, but usually not to the point that you will notice a change in your vehicle performance.
Your fuel readings can change. Your fuel economy certainly can change. But your car’s ability to hold gas is one thing that should remain consistent for as long as you own it. Comforting in a way, right?
Other Factors To Consider
It’s also worth keeping in mind that some circumstances may make it feel like your car is holding less gas than it really is because they impact your fuel economy. For example, running your AC on high uses up fuel faster than keeping it on a lower setting. Even your car’s defroster can change your fuel efficiency if left on for long enough.
Driving through cities uses more fuel than driving the same amount of distance on the highway. Even low tire pressure can harm your fuel economy.
If it seems your car isn’t holding as much gas as it used to, it may be worth your time to give these possibilities some thought. After all, many mechanics will charge you $80 just for parking in their lot (only kind of a joke). You’d hate to bring your wheels in only to find out that a small change in your driving habits is to blame.
Identify the variable that might be impacting your fuel economy. Address it and see if the problem subsides. If not, it may be time to take your car in.
While many issues can be resolved relatively quickly and at a reasonable cost, some more serious problems like fuel line leaks or tank deformation may require more extensive repairs, both in terms of time and cost.
It’s worth keeping in mind that many of the issues described in this article can be avoided through routine maintenance and making sure you don’t put anything funky in your fuel tank and that includes old fuel. Get your EVAP filters changed. Stay on top of your oil changes.
Cars, like people, thrive under attentive preventative care. Show your ride a little TLC.