What gets damaged when fuel tank over-filled?

I’ve heard that over-filling the fuel tank in modern cars can damage something. I’m not talking about forcing fuel into the fuel tank under pressure. Just topping it off past where the pump shuts off. I can’t think of anything this would damage – permanently I mean. Where you’d have to fix something if you did it.

I understand doing this isn’t a good idea from a safety and environmental perspective. It could cause a fuel spill or could saturate the evap canister, which would allow gas fumes to leak out into the atmosphere. But I’d expect these issues would resolve themselves after a few miles, once the gas tank became less full.

Just curious, beyond the above, what could possibly be permanently damaged by over-filliing the fuel tank?

It does terrible things to the evap system and that includes the charcoal canister. The canister in newer cars is not vented to the atmosphere like the older cars, so once saturated with raw gas, it doesn’t clear after a few miles, it stays saturated for a while. This, of course, causes problems in the evap system and never ending ‘check engine’ codes. Other problems are raw gas mucking up evap sensors and valves, requiring them to be replaced. And some are not cheap. The evap system in newer cars is designed to sacrifice itself to prevent fuel spills from over-filling, so the constant punishment for over-topping the gas tank could lead to expensive repairs.

I have never seen the charcoal canister anywhere near the gas filler. I don’t think you would damage anything!!

The activated charcoal canister is connected to the fuel tank by a group of tank vent hoses…The canister itself is usually located tucked away under the hood…By over-filling your gas tank, you can force liquid gasoline into this vapor collecting device…This canister was not designed to hold liquid gasoline. It can not purge this liquid properly which results in the CEL illuminating caused by any number of EVAP Sys trouble codes which can be difficult to properly diagnose and repair and the parts involved can be difficult to find and when found, tend to be expensive…

Which leads to this question… Why would anybody want to force-feed every last drop possible into their fuel tank??

My wife utilizes one of the few remaining full service stations. She is under strict orders to tell the jockeys there to STOP AT THE FIRST CLICK!!! Since this is a co-op we belong to, they are well trained.

However, I’ve seen gas jockeys try to get an even number of dollars in so they don’t have to make change. I’ve even seen them spill the last bit of gas ON THE GROUND just to get the dollars even on the pump.

“However, I’ve seen gas jockeys try to get an even number of dollars in so they don’t have to make change.”

Make change? Who walks around with enough cash to be able to fill up a car these days?

Overfilling the tank can force fuel into the vapor canister–the device intended to store gasoline vapor until the car can purge those vapors and burn them rather than vent them into the atmoshere. Liquid fuel can deteriorate the charcoal in the canister, turning it into small granules that can in turn travel through the rest of the system and stick in valves and solenoids.

The fuel system may also use a fuel tank pressure sensor, which operates just like the MAP sensor on the engine does. This sensor my be damaged by repeated immersion in fuel.

Some cars use a “leak detection pump” which draws a slight vacuum on the fuel system and watches for a leak. This can also be damaged by liquid fuel.

I’m sure there are other things I’m not remembering right now.

Although the charcoal canister is several feet away from the tank and even several inches above the top of the tank if the car is overfilled and on an incline so that the fuel is entering the vent line when the system is in the vent mode vacuum will draw the liquid upward. Fuel can become trapped in a low section of the vent hoses where it will be eventually drawn into the canister. When you consider the shape of a tank, the location of the filler neck and the slope where the car is parked when filling up it is more easily recognized that some people scoff at the idea that stuffing fuel in causes a problem because they have been doing so for decades with no problem. They might be quite $urprised one day when they fill up at a different station where the slope positions their car to overfill. A nearby self serve can totally blow any effort to keep track of fuel mileage. If I enter from the street my mileage is 15 +/- but if I fill facing the street it is nearly 20. And the slope is not very noticeable.

Thanks everybody. I guess my cars are so old — early 70’s & 90’s – that they don’t have this new fancy fuel venting eqpt to worry about. My 70’s Ford truck just has a canister which captures the extra vapors, and vents with a hose to the air cleaner, where the vapors get sucked into the air intake when the engine is running. I had to replace the hose from the canister to the air cleaner last year, otherwise I’d not even know it was there. My 90’s Corolla, I think it is configured the same way, just a canister which vents somehow to the intake manifold. But I could be wrong about that. It might vent back into the gas tank on that car.

No worries. I don’t overfill the tank anyway. Not enough money! I’m lucky to get it to half full. lol …

But thanks for your illuminating comments, I can now see that indeed with newer cars at least there’s considerable damage that can be done by overfilling, and since there’s so little to gain by overfilling, why risk having to take the time and pay the dollars for repairs when the diagnosis is often time consuming and the repairs expensive, and it can all easily be prevented by stopping fueling at the first click. Good advice!

Most carbon canisters for the EVAP system on modern vehicles are mounted very near the gas tank and not under the hood.

The EPA realized that overfilling the gas tank can damage the EVAP system thereby producing more emissions so they demanded the auto makers devise a method to prevent that from happening. So the auto manufacturers came up with the RO/OF valve. ROLL OVER/OVER FILLING valve. This is a ball in a cage with a weight at the bottom at the end of the fill tube. When filling the gas to the full level in the tank the ball floats up and blocks the fill neck thereby shutting off the gas nozzle. And no matter how much you try you can’t overfill the gas tank because the ball prevents gas from being introduced the nozzle keeps shutting off. In the event of a roll-over accident, the weight under the ball forces the ball against the end of the filler neck thereby preventing gas from running out of the gas tank. The EPA doesn’t want unburned gas in the air, in the water, or on the ground.


Recognize that the charcoal canister that everyone is alluding to is the only method your gas tank has of breathing in to fill the void left as the gas is pumped out. Should it become saturated, the tank will be unable to breath in, the pump will struggle, the car may stall, and you might experience premature pump failure.

have there been any reported cases where you removed the pump nozzle when it stopped but are still told that you overflowed the tank. This is happening to us and we don’t ever top off the tank.

Is there supposed to be a stop valve or control mechanism such as a float in the tank that lets the gas pump know the car is full?

I think the gas pump itself detects pressure building up in the fill tube. That’s how it knows to shut off automatically.

And the gas tank itself can collapse from the suction.

Sort of. There is a valve in the EVAP system that shuts off the tank air’s path to the charcoal canister when the tank reaches a predetermined “full” level, stopping the flow of air to the vent port next to the fill hole and causing the pressure change sensed by the pump handle.

I’ve attached images of a typical system along with the description of the components. I hope they’re readable.

Au contraire! Our 1976 Ford Granada had such a vapor canister. I remember the Ford dealer quoting $500 to properly purge the system and replace the canister. It did not have a warning light and all the other electronics.

To who’s comment?
I should point out that EVAP systems have evolved greatly since '76. :grin:

I was referring to George San Jose’s comment on his old vehicles (70s to 90s) not having the evap equipment. California installed this many years ago in the 60s and it was followed by the 49 states later.

Are you certain? The Clean Air Act wasn’t ratified until 1970.