Overfilling Gas Tank

I’ve been driving for 13 years and have gotten gas the same way since the beginning.

One day as I re-entered the truck, the wife looks at me and asks, “Do you think it’s safe to keep putting in gas when it’s already clicked.” I remarked, “The foam builds up and that’s what made it click off(BIG LIE?) and it still had room for about 5 or 6 gallons.” She, of course, scoffed, rolled her eyes realizing I wasn’t getting it. Of course an arguement ensued, but that’s not what I want you all to solve. I already did.

I am intrigued by that now. I never thought of it. Finally, out of boredom from waiting on a substantially larger gas tank than we’ve ever had to fill up, I read all the signs on the pump. In a whole list was, “Do not overfill tank.”

Here’s the two issues/questions:

I’ll admit when my wife’s right, and logic is telling me she is. (She’s never had a ticket or and accident, so I cannot win car arguements)

I think overfilling tank is “creating” too much pressure because the tank needs to be vented. That’s my idea, but I want to know the answer to this dumb question, “Why can’t I overfill my tank?”

The other issue is our 1999 Expedition. When it clicks off and I only put in about two more gallons I finish up and get back in. While I reset odometer I notice that the gas guage isn’t quite all the way up, not like my Taurus. So next time, I added about 6 to 8 more gallons and it finally read “full”. Why is this?

Thanks. You all will make her day by reiterating that she’s “right”. :slight_smile:

Take care.


i had a 99 4runner and i used to always fill it to the very to of the spout. then after a few years i stopped and noticed that when it clicked of my fuel indicator never hit the full mark. my logical guess was that the constant over filling had actually damaged the fuel sending unit in the unit in the tank. as far as causing damage structurally to the tank i am sure they designed the tank to handle it. i believe that the sign at the pumps was put out by the EPA. some dude that gets paid too much says that overfilling cause more polution. whether he or she is right is a mystery.

Overfilling the tank risks damaging the EVAP system in your truck. The EVAP (or evaporative emissions system) is designed to prevent raw fuel VAPORS from escaping the gas tank and fueling system to the atmosphere. The EVAP system captures these vapors, and holds them until the ECM determines it can purge the vapors through the engine to be burned. But, by overfilling the tank, LIQUID fuel can be sucked into the system and damage the valves, sensors, and charcoal filter media used by the system. The system can tolerate a certain amount of liquid fuel, but, if you regularly overfill the system, it can get overwhelmed and give you DTC codes. And, with today’s cars, most of these components are expensive.

Also, for the Expedition, the system may already have some damage. The EVAP system on your truck and as part of the gas nozzle for the station are suppose to work together to prevent fumes from escaping. But, if either system has problems, yours or the stations, the fuel nozzle can ‘click’ off before filling the tank all the way. That rubber cup or boot on the nozzle is designed to create a seal to prevent fumes from escaping. But, if the system is not venting correctly, it will cause the tank to pressurize. I have a similar problem with my Ford Explorer, and the only solution has been to pull the nozzle out an inch or two from the filler neck to allow the pressure inside the tank and nozzle to equalize. Then, I can fill it all the way, and it will still click once full.

Overfilling can damage the evaporative emissions control system, which is expensive to repair. I’m not willing to risk it. I don’t put any more gas in the tank after the filler clicks off, no matter what the gas gauge says.

I’ll bet the owner’s manuals for your vehicles have something to say about this.

You should not top off modern car's tanks.  It can do damge and can make filling the tank more difficult and may require a moderatly expensive repair. That is the reason the manufacturer tells you not to do it. 

Your explanation about how much fuel you can get in there after it stops, makes we wonder if you have not already damaged it.

In any case, it does not cost you more to follow the instructions and not top it off.

Busted did a very nice explanation.

I’ve found that how much more fuel can get in the tank after the automatic shutoff varies greatly from car to car. My Mustang is pretty much filled to brim once the pump clicks off for the first time. But my Bronco can typically hold another 2 or 3 gallons after the pump clicks off the first time. When the Bronco was my daily driver I got into a habit of topping off. With the Mustang that’s not really possible. As others have mentioned, topping off the tank can lead to problems with the Evap system. I know it does happen, but I’ve never had problems with it despite years of topping off the tank.

If it takes an additional 6 - 8 gallons after the pump clicks off the tank was not full, and the pump shut off early. There is no way the filler pipe, even on an Expedition, could hold 6 - 8 gallons. I have an 86 Blazer that will shut pumps off early, so it must have something to do with the filler neck and vent design that causes some vehicles to shut the pump off early.

Thank you so much for that wealth of information! I never heard of EVAP system. Does that differ from line a return fuel line?

My first two cars were old. One was a 1965 Chevelle and the other was an 81 Silverado. Granted they’re older and the chevelle liked to leak some of the excess out (the way I drove it didn’t help). I just never encountered that and no one has caught me overfilling my tank years ago.

I do not know fully how my wife’s Expedition was treated before, we’ve had to solve a couple of mysteries when we first got it. Can overfilling tank affect the IAC valve (idle air control)? I’ve had to replace it and DID NOT enjoy it as it was on the BACK of the motor!

I have noticed at begining of fueling, the pump keeps clicking off. I have not paid attention to different gas stations and how the nozzle runs. If I can hear it really gushing, something must be new and I really have to back it off. I also noticed my 1 Ton Ford Van does the same thing. My Taurus does not. Next time I get gas, I’ll pay attention to nozzle depth.

What is your take on the fact that my gas guage isn’t quite all the way up? First things first, I’ll try backing out the nozzle more (without washing the parking lot down with 89 Octane).

One more question and I’ll give your eyes a break. When we bought this truck, it was (is) outfitted with a BBK Cold Air Intake. I think he said something about a “chip” but I disregarded it. For the first six months our “check engine” light kept coming on and it was the same code over and over. ( I don’t have my book handy-everyone’s sleeping right now). I think it was along the lines of P-171 & 172 whatever… it meant “Bank 1 and Bank 2” running lean (or rich). I replaced several parts including a $150.00 MAF sensor. I also noticed that the hoses kept coming off of the cold air intake and everytime the engine would run at a sloghtly higher RPM almost pulling us through a red light. I fixed those hoses with clamps, they were just pressed onto intake port. After almost one year of this and finally, she started shuddering at highway acceleration. I gave up and took it to a shop. They just got a new system specifically for Ford/Lincoln/Mercury and he hooked his laptop up and we went driving. He looked at the screen and said, “Ohhh, your #1 coil is bad, that cylinder and dropping completely out, not firing.” All was fixed and she’s ran like a champ ever since, infact, I got a 2 mpg bump, taking mpg into the 20s.

I told you this story to ask you this question:

Does the BBK Cold Air Intake confuse my computer? Everynow and then I feel a little surge to the truck, not as bad as before, but sometimes she feels like she wants to go through the red light. Doesn’t happen all the time. I do notice that sometimes it’s a little warm(weather wise). Any notions?

THANKS AGAIN for all that knowledge you sent my way.

Take care friend.