Don't be a moron: Stop topping-off your gas tank.

"DMCVegas" (DMCVegas) 05/06/2015 at 17:41 • Filed to: Gasoline, Fuel Tank, Emissions, Bad Drivers, Ford, DMCVegas

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As noted in another Jalopnik thread about the irresponsibility of car owners, a very interesting subject came up: The smell of gasoline in car parking lots. Why is that? Drivers are ignorantly topping-off their gas tanks when they fill up, and are breaking their cars and wallets.

How is this possible? Well to understand the problem, you must first understand how something works in order to understand how it can fail.

Gasoline is volatile, and creates lots of fumes. It gets worse when there is a heat source near by, and for fuel tanks there usually is. Be it hot blacktop in the summer, but usually the exhaust pipe running near by on it’s way to the muffler at the back of the car. It’s not dangerous, but there is a whole lot of radiant heat around to warm the gas up, and cause to to atomize. The problem is that this builds up pressure in the fuel tank, and that’s not something you want. In the early days, the tank was just vented to the atmosphere. Problem solved, but more problems created for the environment. Gasoline doesn’t just magically disappear. Once it gets cold, it will condensate and get mixed in with water in dew, or rain.

The idea came about to capture the fuel vapors, and route them into the intake manifold where they could be safely burned away. Great idea! But we can’t just route the hose into the carburetor. It’s still going to vent vapor to the atmosphere when not running, and we don’t want to risk sucking liquid fuel into the engine. So what was the solution to all of this?

Behold! The mighty Carbon Canister!

The idea here is to let the charcoal inside of the canister collect and store the gasoline vapors. Then once the engine starts up, the manifold vacuum would extract them into the intake where they could be safely burned. The filter at the bottom of the canister would then allow fresh air from the outside into the canister to help purge the vapors, and to avoid placing suction onto the fuel tank.

The Carbon Canister’s individual function is clearly viewable in the handy-dandy diagram right there. Granted now, depending upon application they are available in different shapes, sizes, and port configurations. Sometimes the fresh air vent tube is on the bottom, and sometimes it’s on the top, where it then opens up at the bottom of the canister internally. Likewise the version depicted has a mechanical Purge Control Valve. It’s a vacuum-controled diaphragm that is activated by engine vacuum. Modern vehicles eschew the diaphragm vacuum line and will instead have an electronically controlled valve (like a solenoid) placed inline between the purge port and the manifold.

So what in the world does topping-off your gas tank have to do with this?

Well now that you know how a Carbon Canister works and know it’s role in the Evaporative Emissions Control systems of an vehicle, let’s take a look at it’s physical placement:

This is diagram is a great example that I love due to both it’s functional display of vacuum tubes, as well as it’s physical location in relation to everything else. (remember: this is an older mechanical example. On modern cars the Carb Bowl hose is deleted, as is the Control Line and the Purge Valve connected to it. Instead we just have the Vent Tube, Fresh Air Tube, and Purge Line that connects to the manifold. Other than deleting those lines and adding in the electronic in-line PCV, it’s the exact same.)

As with most vehicles, the filler neck runs a bit higher than is depicted here. That’s important to know, because when you top-off your gas tank, that’s where the excessive fuel goes: Up the Filler Neck. But that of course isn’t the only place that it goes.

Gasoline overflows into the vent tube and trickles into the Carbon Canister. Remember that once you start pumping fuel into the neck, you’re overflowing into the vent tube now that the fluid level is higher than it. Once the gasoline reaches the canister, it mucks up the entire thing so that it no longer is able to properly collect and then purge excess fuel vapors into the intake manifold when the car starts up. So the tank builds pressure, and starts to vent around the gas cap so that it doesn’t rupture. That’s where that smell comes from.

Ruining the Carbon Canister doesn’t come immediately. There are lots of ups and downs in the vent tube, and it takes some time to screw up that canister. But it WILL happen. But what are the effects:

Since gasoline vapors start leaking out of the gas cap, it becomes a danger inside of your garage if you have a furnace or gas water heater out there.

It’s bad for the environment.

You are literally pouring out gallons of wasted gasoline. A !!!error: Indecipherable SUB-paragraph formatting!!! from 2010 estimated that we loose over 31,700 gallons of gas a year just by those tiny droplets that fall from fuel nozzles when we refuel. So with continuously flowing vapor, it obviously can’t be far behind.

If you have a more modern system that triggers a failure of the EVAP system, it can absolutely be a costly repair.

And the repair thing is pretty interesting. Because with some vehicles, the canister is right there in the engine compartment where it’s easily accessible. But with some, manufacturers will hide the canister behind inner fenders and make it a bitch to get out. Or worse yet, you have repeat repairs. Allow me to start another round of DMCVegas’ Storytime Stories for that one...

When I worked for the local cable company in Vegas, we had a rash of Econolines going in for service due to this very issue. Excessive gasoline smells, which worried the field techs that they had leaking gas tanks. It was no problem to fix. At least in terms of finding a solution. But the cost was about $230 or so per truck, I believe, to replace the canisters and blow the lines out. They sent in something like 40 trucks in to be serviced, costing well over $10K in fleet repairs because techs were consistently topping-off their vans.

So pretty please, with sugar on top, stop doing this. I’m tired of smelling gasoline when I sit in traffic because you can’t work with anything but rounded-numbers when you balance a checkbook or need change back from the cashier at the gas station and hate coins.

lonestranger > DMCVegas 05/06/2015 at 17:56

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I’m a little confused.

What do you suggest doing as an alternative? Do you just guesstimate when your tank is nearly full, or do you just keep it at around half-full?

MarquetteLa > lonestranger 05/06/2015 at 18:00

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I think what DMCVegas is trying to say is to stop putting in fuel when the pump stops itself. Don’t “top off” by pulling the gas pump trigger more after it auto-stops.

DMCVegas > MarquetteLa 05/06/2015 at 18:15

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Exactly! The pump does all of the work for you, and let’s you know when it’s full by clicking off. The only exception that I’ve ever seen to this has been on my DeLorean where the inlet is completely horizontal @ 0°. Those vapor recovery pumps with the accordion over the nozzle doesn’t like that severe an angle, and work very well to where they shut off prematurely (usually a symptom of a bad Charcoal Canister). But all others are a-ok.

KylesPerGallon > lonestranger 05/06/2015 at 18:16

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Topping off is pulling the trigger again after it goes pop.

Trunk Impaired 318 > DMCVegas 05/06/2015 at 18:19

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Definitely not the case with my MR2, the auto stop from the pump leaves an additional 1.5 gallons of room and when your tank is only 10 gallons, that makes a big difference

norskracer98-ExploringTheOutback > MarquetteLa 05/06/2015 at 18:30

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I can’t do uneven numbers though. God that just bothers me.

Short-throw Granny Shifter is 2 #blessed 2b stressed > DMCVegas 05/06/2015 at 18:39

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Living in NJ, the knucklehead gas station attendants ALWAYS top off. It’s a pet peeve of mine. I hate it, so I always prepay an amount that is less than the it takes to fill the tank.

sebdel > DMCVegas 05/06/2015 at 18:51

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I totally get the point and see how this can be a problem, but putting too much gas in my tank would be a very nice problem to have..

dogisbadob > Short-throw Granny Shifter is 2 #blessed 2b stressed 05/06/2015 at 18:57

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They do this to round to the next dollar (yeah I know you know this, but the OP and others may not)

Tip: use a card instead of cash. They usually don’t top off when you pay by card, but they do top off when paying cash.

Also, Costco has a company policy prohibiting top-off

In Oregon, the other non-self-serve state, it’s actually illegal to top-off. NJ needs to do the same.

lonestranger > MarquetteLa 05/06/2015 at 19:22

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People actually do this? I thought “topping off” meant filling the tank until it’s full. When the pump automatically shuts itself off, it’s full. You’d have to intentionally pull the nozzle partially out in order to purposely overfill it up into the filler neck. If morons out there are actually doing this, of course it’s going to go in the vent tube, saturate the charcoal canister, and vent fumes to the atmosphere.

lonestranger > DMCVegas 05/06/2015 at 19:29

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So, by “topping off” you mean intentionally overfilling? I had no idea that people did this. To what end? If you’re trying to cram fuel into the filler neck up past the vent tube, of course there’s going to be issues. If this is what your former co-workers at the cable company were doing, they should’ve had to pay for the repairs out of their own pockets. That likely wouldn’t be feasible, but in a perfectly just world...

lonestranger > dogisbadob 05/06/2015 at 19:59

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Ahh, so the attendant doesn’t have to make change. I don’t think I’ve used full-serve nor used cash at all in the last decade, maybe more.

It still doesn’t explain why someone pumping their own gas at a self-serve would feel the need to overfill their neck.

R Saldana [|Oo|======|oO|] - BTC/ETH/LTC Prophet > DMCVegas 05/06/2015 at 22:01

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Guess I will no longer do this....

Whoops....

DMCVegas > Trunk Impaired 318 05/07/2015 at 01:24

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All shall be answered...

...in Part 2!

PNO TECH > DMCVegas 05/07/2015 at 06:41

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THIS! I used to fill to top, until the day when there was a pool of gas under my car eating the asphalt at work. Now, when it clicks, I’m done.

Took me two junkyard visits and close to a week to be able to drive without becoming dizzy-didn’t help that that week was quite rainy so I couldn’t open the sunroof.

This was an ‘82 Rabbit. It’s wierd, cause I had been topping-off my EA81&2 Subarus for years w/ out problem....or maybe the lines, like every other rubber line on my Roo’s, were so degraded they vented to atmosphere.

NJAnon > DMCVegas 05/27/2015 at 19:27

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Do I fill my gas tank all the way up with the “auto pump” thing? No, because what are you going to do if your fuel line breaks because your car is kind of old or if the auto latch thing doesn’t disengage? Are you going to attempt to drive with gas leaking onto the street to a auto place (assuming it isn’t 10pm and all the service places are closed) and watch your fuel gauge run down?

Yes I saw it happen and I felt bad for the guy but he should have known better.

Rykros the Disdainful - Supposed Petulant Capitulant Junkie > DMCVegas 06/02/2015 at 19:00

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Any time I notice someone topping-off, the grand majority of the time it’s an older person.

The only time I’ve ever “topped off” is when I paid cash, and I tried to round to the nearest 50 cents. When gas got really expensive last decade. and gas stations started enforcing pre-pay, I started just using my card. Which eliminates the issue.

Texican > NJAnon 06/02/2015 at 19:23

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Wait, what? You don't fill your tank for fear the fuel line will break? That.... Doesn't make sense. Do you mean the filler neck falling apart or something?

TheWrathofKamm > DMCVegas 06/03/2015 at 00:42

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Cox Communications, huh? Fleet?

nerd_racing > lonestranger 06/03/2015 at 08:55

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I personally witnessed an idiot topping off his newer escape (the kind without a fuel cap) and I saw him click click away at it as gas poured down the side of his car.

Drakkon- Most Glorious and Upright Person of Genius > DMCVegas 06/03/2015 at 08:57

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We have a Ford F-250 with a nearly horizontal filler neck and it takes about 4-6 onzes of gas at a time and then cuts off. It has literally taken 45 minutes to completely fill the tank. I’m not kidding. We called Ford, they were like.... yeah. That happens.

Thanks guys.

ivan256 > lonestranger 06/03/2015 at 10:37

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I know somebody who not only tops off, but will push the bumper down a couple times to “burp” the tank to get a couple more ounces of fuel in there....

El-Verde > DMCVegas 06/03/2015 at 11:43

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I always topped off my diesel Golf TDI with no issues. I usually filled up before a 40+ mile drive though...

I know older TDI owners would do a “ventectomy” to allow topping off with no issues. I was under the impression that newer TDIs didn’t need the modification. Am I right?

DeLM > lonestranger 06/03/2015 at 12:05

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you could grab an owners manual, see the capacity and fill accordingly. ex. 10 gallon tank, my needle is at 1/8th a tank. divide 10 by 8. that is 1.25 gallons per 1/8th a tank. therefore you should add just under 8.75 gallons.

Wrong Wheel Drive (41%) > DMCVegas 06/03/2015 at 13:26

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What if you live in NJ? It would be a chore to remember everytime to remind the attendant to not top off the tank. They also do not carry coins so if you are paying cash I am pretty sure they would ignore that request. Sorry, I live in a dumb state where its illegal to pump your own gas.

Ad_absurdum_per_aspera > lonestranger 06/03/2015 at 13:28

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Surreptitiously watch the people around you at gas stations and you’ll see that quite a number of them do several repetitions of vaguely Freudian maneuvers with the nozzle in order to get just a little more into the tank than the automatic shutoff mechanism thinks they should.

Sure, once in a while you encounter a pump whose out-of-adjustment mechanism tries to call a halt to the proceedings with multiple gallons of room left. However, for the most part, these people are just intentionally overfilling the tank (and, as described in this article, sending liquid where only vapor ought to be, which comes at a price for their car and the environment).

danonymous > DMCVegas 06/03/2015 at 13:45

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Screw the vapors. Topping off the tank is stupid because the amount of fuel it adds to the tank/tube isn’t worth the time to stupidly and repeatedly “bump” the pump handle a bunch of times.

I don’t have a lot of spare minutes in my day, and I sure don’t want to spend even one of them standing at a pump clicking the handle like a spaz having a seizure to add 0.1 gallons of fuel. I’ll still have to stop for gas the same day next time with or without adding that extra tiny bit of gas.

Plus it can lead to a spill, which basically pours money on the ground, ruins your shoes, and then you smell like a gas station for the rest of the day. No smoking near Joe! Whoosh! Oops! Sorry Joe...

Let the pump click off, just hang the hose, cap your tank, take your receipt, and go. Done.

ttyymmnn > DMCVegas 06/03/2015 at 15:00

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I was never a tank topper, but when I bought my ‘01 Golf the salesman made a point of telling me to stop pumping when the pump shuts itself off. Most likely to prevent damage to the EVAP system.

Ripper2660 > DMCVegas 06/03/2015 at 16:05

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I love to top off ! I'm the biggest topper offer you can find. Or was that tossing off?

NJAnon > Texican 06/03/2015 at 16:34

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I don’t know what specifically happened to the guys old SUV but I know he had the filler latch on the pump set to fill his tank up and when right as the tank went full something broke and made him leak gasoline all over the ground.

Prior to that, I may have filled my car to 80% or whatever but I know my car is old and I don’t want to be scrambling to fix fuel lines while fuel is spilling out everywhere and no one wants to help (or no one is around to help).

MonkeyMD > R Saldana [|Oo|======|oO|] - BTC/ETH/LTC Prophet 06/06/2015 at 17:50

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Same here. Topping off in my Honda Fit since fuel tank is under the front seats means from empty, I can get 12.5 gallons instead of 10. That is almost 100 extra miles of driving. Going to be hard to give up. Car now has 140k miles with no issues.

I like pooping. > DMCVegas 06/19/2015 at 22:11

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I have never been able to put an exact dollar or gallon amount of gas into a vehicle. say I have a rental car and I want to put 5 gallons in to get it back to the level I started at. I will end up at 4.983. or 5.012. or whatnot. then, at 3c per hundredth of a gallon, I’d be off by a few pennies from a target price no matter what.

CammedCTSV > DMCVegas 09/15/2015 at 22:13

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All the charcoal cans I have accumulated have met noble deaths, at the shooting range...loaded with gas fumes

BaconSandwich is tasty. > MonkeyMD 12/08/2015 at 22:19

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I know I’m a bit late to the party here as well, but I’m just as guilty. 240,000 kms and no issues.

BaconSandwich is tasty. > El-Verde 12/08/2015 at 22:32

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I’d be curious if other cars can have that same modification made.

Mike Miloserdoff > DMCVegas 12/15/2017 at 00:37

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Thanks for the great info. Just replaced my charcoal canister, today, hyundai accent, 2002, $125 for the part. took off the hoses, got a lot of charcoal beads. I can now pump gas once more, but p1403 obd code, looks like purge valve is bad, symptoms are consistent with stuck open diagnosis. Your diagrams and explanations help to understand the entire “fume flow”. Bought the car used, but recently, I had to have helped the canister to break down, putting gas in took about 10 minutes due to charcoal blockage.