overfilled gas tank

With gas prices being what they are today, it’s only natural to want to squeeze every last drop into your tank, especially if you’re a member of a perks program that gives you one-time massive discounts on the price of a gallon of gas.

Most of the time, when you try to fill your tank to the brim, the pump’s automatic shut-off switch will detect the rising fuel level and trips the handle to turn off. Of course, you can keep squeezing beyond that point to get as many drops of fuel into the tank as possible. Perhaps even to the point that fuel threatens to spill out of the fuel-filler neck.

Though this begs the question, what happens if you push it too far beyond the pump’s automatic shut-off? Can you damage your fuel system by overfilling your gas tank?

Unfortunately, even a single severe over-filling event can possibly damage your fuel tank’s emissions control system. However, to understand what trouble this can pose to your car and the environment, we’re going to have to take a little bit of a deep dive into how your car’s fuel tank works.

Can I Overfill My Gas Tank?

Most gas pumps will let you manually squeeze the handle even after the automatic safety switch trips the hand. The pump will keep tripping itself off, but it is possible to keep squeezing repeatedly to fill the tank straight into the fuel filler neck.

The pump itself has a little diaphragm in it that senses the available vacuum of air pressure inside your car’s gas tank; when it reaches the bottom of the fuel filler neck, the pressure changes, which trips the diaphragm in the pump, causing it to kick out the lock on the handle.

This feature was put in decades to prevent people from locking the handle and spilling fuel on the ground while they were distracted doing other things like washing the windows. Though you can easily squeeze the handle to force out more gas before the diaphragm trips a second or two later.

How Do You Know When You’ve Put Too Much Fuel in Your Car?

Overfilling a gas tank shows different signs as the fuel level rises. First off, you’ll notice the automatic shutoff switch on the handle popping it out of the locked position, which cuts off the fuel supply.

If you continue to force it, you’ll likely start to smell gas fumes coming up the fuel filler neck. This might be followed by the sound of rushing fuel getting louder.

Some cars with an anti-siphoning valve in the fuel filler will block off the flow of fuel coming back up the neck. This is usually because you have to hold the handle slightly ajar to let the fuel come up beyond the pump’s automatic shut-off threshold.

11 Things Happens When You Overfill Your Gas Tank

If you’re lucky, overfilling your gas tank will just cause an expensive mess on the ground. However, it could also cause problems with your car’s emissions control system, possibly damaging the fuel system if you push it too far.

1. A Mess on the Ground with Legal Ramifications

A Mess on the Ground with Legal Ramifications

If you have an older car without any sort of safety device in the fuel filler neck and you try to max out your gas tank to the point that it spills fuel on the ground, you could be facing safety and EPA violations. A few drops aren’t going to be a big deal, but in some states, a major spill of more than a gallon can come with a fine. It’s best to tell the attendant so they can put down some absorbent granules.

2. Damage to the Paint Job

Damage to the Paint Job

If gas is overfilled, gas spills out of your tank and gets on your car’s exterior, and if it isn’t cleaned off, it causes the paint to blister. This can eventually lead to rust around the fuel cap, affecting how the cap seals. Gasoline acts like a solvent, but if you can wash it off with some warm soap and water, the paint will likely be okay.

3. A Contaminated Emissions Control System

Understanding Emissions-Control Systems

If excess fuel gets into your car’s evaporative emissions control system, it can clog it up, potentially affecting other components. The EVAP is designed to capture unburned vaporized hydrocarbons and deliver them to the intake system to be re-burned in the engine without venting them into the atmosphere.

4. Problems with the Air Intake Manifold

Problems with the Air Intake Manifold

If overfilling causes too much fuel to invade the EVAP system, it can start to affect the air intake system’s performance. This makes it increasingly hard for the ECU to correct the fuel/air mixture. This system also uses a charcoal canister with microscopic nucleation sites on the surface of the charcoal. It’s only meant to interact with vapors and will fail if a significant volume of liquid gasoline floods in.

Evaporator system charcoal canister replacement, Subaru Outback - VOTD

Compounding this problem, the charcoal canister might need to be replaced, which could come with a mechanic’s repair bill of $300 or more.

If you’re particularly handy, you might be able to replace it yourself for about half that cost. Just bear in mind that you’ll be dealing with active fuel vapors with fire safety risks and then some.

5. The Check Engine Light Comes On

The Check Engine Light Comes On

When gas overfills to the point where it invades the charcoal filter in the EVAP system, the check engine light might come on. When it does, it will throw a code that you can read to determine the severity of the problem.

  • Code P0456 means the ECU has detected a very small leak in the evaporative system during the vehicle’s self-testing.
  • Code P0441 indicates that the car’s EVP system is experiencing an incorrect purge flow.

If the overfilled gasoline is affecting other engine systems like the air intake, you might get codes that are specific for those faults. While the

6. Deformed & Leaky Fuel Tank

temporary fix to a leak in the fuel tank

Gasoline expands and contracts with changes in temperature and pressure, and the tank can deform when you severely overfill it. This could be a bulge near the fuel filler neck, a swelling near one of the welded seams, or the port that connects the fuel sending until to the gas gauge.

If the tank deforms but doesn’t fail under the pressure of excess fuel, you might not notice it. If the tank deforms near the fuel-sending unit or one of the seams, you might eventually end up with a nasty leak in the fuel tank.

In a scenario like this, you might overfill your tank in the morning when the temperature is low. Then, you seal the gas cap, trapping the pressure dynamic in the tank. If the day gets hot, and you’ve filled the tank beyond the tolerance for expansion, it could deform and possibly cause a leak even though it’s hours after you filled up.

7. Strain on the Fuel Pump

Strain on the Fuel Pump

In some models, the pressure strain of an overfilled gas tank can affect the fuel pump. This could essentially start pushing gas through it when it isn’t running, or during the day, the pressure increase with the outside temperature puts excess stress on the fuel pump that can’t keep up.

8. Fuel Filter Debris & Tank Varnish in the Fuel Rail

Fuel Filter Debris & Tank Varnish in the Fuel Rail

The same pressure dynamic affecting a fuel pump can also send tank varnish and other residues previously trapped in the fuel filter into the fuel system. While it’s relatively rare in modern vehicles, this could wreak havoc. If loosened fuel filter debris makes it past the fuel pump, the contaminants can move on to the fuel rail, where they can affect the solenoids in the fuel injectors and foul other engine components.

9. Gas Residue in the Cylinders

Gas Residue in the Cylinders

In a severe case where the overfilled gas tank pressure drives excess fuel through the fuel rail, it can flood the cylinders. This will affect performance when you first start the engine, causing it to run very rough. You might even get a few misfires, which turn the check engine light on.

10. Fouled Spark Plugs

Fouled Spark Plugs

Fuel that makes it into the cylinders from over-pressurized gas tanks can also foul your spark plugs. This can cause blistering on the plugs themselves and lead to excess carbon deposits. Either of which can grossly affect the engine performance.

It must be replaced, even if it’s just one spark plug that’s fouled. However, when you replace one spark plug in a gasoline engine, you need to replace them all to keep the engine from running off balance. So, your attempt to save a little money overfilling your gas tank might lead to a major loss in the cost of replacing all your spark plugs.

11. Misfires

Engine Misfires

Unburned fuel that’s forced into the fuel rail by an overfilled gas tank can easily cause a misfire on one or more cylinders. This can be a double whammy that goes beyond the potential damage to a cylinder. You can easily end up with multiple spark plugs, leaving unburned fuel in the engine causing misfires. This fuel can also migrate down into the exhaust system, causing expensive damage to the catalytic converter.

Possible Scenarios That Could Cause an Overfilled Gas Tank

Why Not to Overfill (Top Up) Your Car's Gas Tank

A few possible scenarios could cause you to overfill your gas tank, which goes beyond a failure in the gas pump’s safety systems.

1. A Bad Pump Diaphragm

Gas pumps have a diaphragm that monitors the ambient air/vapor pressure inside a gas tank and shuts off the flow of fuel as that pressure decreases. A pump diaphragm can be prone to freezing up, especially in cold weather, allowing the gas to keep pouring out even as the level climbs the neck of the tank. This is just one of the reasons why service stations have signs saying you should never leave a gas pump unattended.

2. Capitalizing on Pump Discounts

Credit card programs, travel group memberships, and even grocery store clubs will often offer discounts on the price of gas. These are one-time discounts that you can only use for a single fill-up. So, people try to pump every last possible cent of savings out, which can drive them to go beyond the automatic shutoff and manually force the fuel pump to deliver more.

3. Loose Insert of the Nozzle

For the diaphragm of a gas pump to work properly it needs to be completely inserted into the hole. If you only have the tip in, and there’s enough void to let outside air pressure affect the diaphragm, it might not detect the pressure change as the gasoline starts to fill into the neck of the tank.

Frequently Asked Questions

If you overfill your gas tank will it leak?

Overfilling a gas tank when the temperature is cold outside and sealing the cap with fuel in the neck of the tank can potentially lead to a leak later on. If the day gets hot, the gasoline and vapors will try to expand in the tank, which could exceed the design tolerance, causing a tank leak.

Will putting too much fuel in my car cause damage?

Most of the time, the biggest risk to your car from an overfilled gas tank is just getting fuel into the EVAP system, with the outside chance of deforming the tank. If some gasoline spills on your car’s exterior and it isn’t cleaned away, it can cause the clear coat and the paint to blister, leading to rust near the gas cap.

Is it safe to overfill my gas tank?

Overfilling your gas tank to the point where the fuel level is coming up in the neck of the tank is never safe. It can lead to a variety of issues, big and small. Not to mention the real risk of spilling gasoline out into the world.

Is it a waste of money to overfill my tank?

If you’re trying to capitalize on a one-time gas discount deal, the consequences of forcefully overfilling your gas tank can ultimately lead to a net loss. If you really want to make the most out of a one-time gas fill-up, the wiser move would be to bring a gas can with you. At the very least, you can pass on the savings to your snowblower and lawnmower. If you have a boat or a snow machine, bring it with you on the trailer and fill it up at the same time.

What should I do if I’ve overfilled my car with gas?

If you overfill your gas tank and spill more than a few drops on the ground, you need to inform the attendant. If you drive off, leaving an unreported mess on the ground, the security cameras can backtrack to figure out who did it.

If it’s just a minor case of overfilling, the anti-siphon features won’t let you draw any fuel out. The best move is to start the engine and get on the highway. Driving at high RPMs will help burn through the fuel. Usually, the fuel filler neck won’t hold more than a gallon of gas. So, if your car is rated for 25 MPH highway, put in 25 miles at highway speed, and it should be back to a reasonable level.


The temptation to pump every last drop into your gas tank when you’ve got an active discount deal can be powerful. Just know that it comes with risks and potential consequences that can erase those savings than then.

If you do overfill your tank and spill only a few drops, your best chance of avoiding pitfalls is to get on the highway and drive enough miles to consume at least one full gallon of fuel. If you spill more than, say, a cup of gas when overfilling, make sure to tell the attendant so they can safely clean it up.

Signs of a problem due to overfilling a gas tank typically manifest as a check engine light with a Code P0456 or Code P0441. This might come with a hard idle and some minor engine stuttering. If the engine performance issues don’t improve in time and the check engine light starts flashing, then you likely need the EVAP system’s charcoal container replaced.

If the engine is misfiring or you suddenly notice it is sluggish, the excess pressure inside the tank may have somehow affected the fuel system. While this is relatively rare, you usually need a mechanic to diagnose it.

Jason Farrell

Written By

Jason Farrell

Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.

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