What Happens If You Overfill Your Car’s Gas Tank?

Can you overfill your gas tank? How do you even know if you have overfilled it and what should you do? Here's a super simple guide.

The average driver refuels their vehicle between 45 and 55 times a year.

During that time, you have probably overfilled your gas tank with diesel at one time or another and wondered about the consequences and dangers of overfilling the fuel tank.

When you are dealing with flammable liquid, safety is always a concern. And topping off your gas tank is no exception, and it can also potentially damage the vehicle.

The typical gas tank of an American automobile has a closed circuit system designed to incinerate the fumes of the gasoline.

Whether intentionally or by accident, overfilling the gas tank can overburden the system. This may eventually clog the system and damage its components.

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How Much Damage Does Overfilling The Gas Tank Really Do?

Filling up gas in a car

Will overfilling your tank once or twice cause significant damage? No. But you may want to know about the consequences of routinely or frequently topping off the tank when refueling.

We’ll go over how and why a vehicle owner might overfill their gas tank and discuss the potential problems, repair costs, and odds of an overfilled tank damaging the vehicle.

How Do You Accidentally Or Intentionally Overfill A Gas Tank?

Hand refilling a car with gas

Automobile gas pumps have a security measure to prevent the tank from being overfilled. It works by automatically shutting off the flow of gasoline when the fuel nears the tip of the pump.

However, it is possible to overfill a gas tank by accident if the handle of the gas pump nozzle is accidentally depressed again after being lifted from a full tank after the automatic shutoff.

Some drivers may intentionally overfill or “top off” their vehicle’s gas tank when pumping gas in an attempt to squeeze in a few more drops and, consequently a few more miles.

The excess gas in the tank suffocates the available air space and reduces the amount of air needed to help the fuel system process the fuel’s vapor.

Consequences my be a clogged fuel system, malfunctioning carbon filter, or even faulty circuitry due to the sensitivity of the high-tech onboard digital equipment.

Fuel systems have become more sophisticated over time to keep up with increasingly stricter emission laws.

John Ibbotson from Consumer Reports in a recent CR report.

To cut to the chase: Avoid topping off your gas tank—it can only lead to bad things for your vehicle, and it’s not worth any possible extra mileage by strangling a few extra drops out when refueling.

Most automotive experts, however, warn against overfilling your gas tank. The dangers of spilled gasoline pose problems for the public and the environment. It also has the potential to damage your car, SUV, or truck.

What Happens If I Overfill My Gas Tank?

Fire Risk or Fire Hazard

The consequence of an overfilled gas tank, of course, is spillage of the gasoline on the vehicle and ground. The fuel is flammable and, therefore, a fire risk.

But gasoline dries quickly, and a small amount of spilled gas on the ground should dry up in a few hours.

The real concern is the potential damage that regularly overfilling a gas tank can cause to your automobile.

One potential consequence is premature wear of the fuel pump due to the fuel system becoming clogged by overfilled gas tanks. The extra fuel prevents the system from evaporating the gasoline fast enough and causes clogs.

Here’s What To Do If You Overfill Your Gas Tank

Turning Ignition On Car
Turn off the car before filling up the gas tank.

If you find your gas tank overfilled, leave the vehicle off for a few moments to allow the overfilled gas to seep back down into the fuel tank and for any gasoline spillage outside of the tank to evaporate.

You should never pump gas with the car switched on, even though the statistical probability of it causing a fire is negligible.

Fortunately, gas dries quickly when it makes contact with air, usually taking a few hours for small spills to evaporate. Spilled gas is a fire risk for at least as long as you can still smell its vapors. But if there’s no ignition source, there is no immediate danger of fire.

However, overfilling your gas tank can lead to eventual damage to mechanical components within the vehicle’s fuel system.

Potential damage includes premature wear and tear on the evaporative emission system, which has to work harder to vaporize the gas because the air it needs to work more efficiently is being taken up by the excess gasoline.

Left unchecked, the fuel system issues could expand to create expensive problems with the engine. And anytime you’re dealing with engine problems, you can expect to pay between a couple hundred to a few thousand dollars in repairs.

How To Know If You Overfill Your Gas Tank

Hearing Click Sound

Is it possible to overfill your gas tank?

Yes, either by accident or intentionally, you can overfill your gas tank. You’ll know it because you’ll hear the safety lock click when the tank is full, letting you know to stop refueling.

If you squeeze the fuel hose grip again and add more fuel, it will likely splash back up and spill on the vehicle and ground.

You’ll also see the dashboard’s fuel level indicator at its highest possible level.

Dangers Of Overfilling The Gas Tank

Warning Danger

You’ve probably overfilled your gas tank at one time or another and wondered about the consequences and dangers of what happens when a gas tank overflows.

Overfilling your car or truck’s gas tank means you are now dealing with flammable liquid, and safety is always a concern.

Routinely topping off your gas tank can also lead to eventual damage to vehicle components that may include:

  • Engine
  • Emissions control system
  • Charcoal canister

Conclusion

Gas Station Worker

The typical gas tank of an American automobile has a closed circuit system designed to incinerate the fumes of the gasoline. Whether intentionally or by accident, overfilling the gas tank can overburden the system and eventually clog or damage the system and its components.

If early signs of damage are not dealt with, such as the engine light coming on, the fuel system issues could lead to engine damage and a high repair bill.

Avoid creating any potential future damage to your vehicle by listening to automotive professionals’ advice to avoid frequently topping off your vehicle’s gas tank.

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David M. Ciminelli

David M. Ciminelli is a Southern California-based journalist and longtime auto enthusiast who has contributed to a plethora of popular automotive sites.

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