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Topping off at the pump?

(All Years) 
12K views 34 replies 20 participants last post by  whobodym 
#1 ·
Can someone explain this to me? Some people are dead set against this; others think it doesn't matter. Can someone tell me whether or not this is a good idea or a bad idea, and which is right or not?
 
#4 ·
I do it everytime I fill up, but I have been told it is better if you drive 50 or so km's afterwards.
I have not noticed any untoward behaviour in my last 3 cars - 1986 Holden VL Commodore (14yrs) - 2001 RX Liberty Sedan (2.5yrs) - 2003 Forester (5yrs in 01 Jul) and it was the same procedure for all 3.
 
#6 ·
I always end up grabbing the trigger a couple more times.

Ive been to some pumps where they shut off very early, grab the trigger again and itll add another 1/4 gallon. Other times, it trips almost imediatly. When it trips the first time, you are dang near full, not much need unless you are dead set on having a nearly over flowing tank.

It depends on the vehicle a lot of the times, too. My work van trips the pump very early, where as the Subaru pretty much fills up before it trips.
 
#14 ·
At the top of the filler neck, under the screw cap, there is a small hole with a hose to the evaporative emission control charcoal cannister. Overfilling past the first click of the pump risks getting gas into the hole and into the charcoal cannister. That is why the Owners Manual says:
3. Stop filling the tank after the fuel filler
pump automatically stops. Do not add any
more fuel.
Maybe it takes a few times, a certain amount of liquid gas. But when the charcoal gets enough gas, it will not work properly, the system will sense it and it will cause a trouble code. As to whether it affects the way the engine runs and must be fixed, I don't know.
 
#8 ·
Used to but since it says not to why do it? Less chance of spilling over and smelling gas for a week...

On that note I read what I believed to be an urban myth recently about filling your car on the slowest pump speed to avoid excess aeration. It didn't take much longer than usual but I got 3-4 MPG more than normal! The next few tanks will be the proof...
 
#13 · (Edited)
... I read what I believed to be an urban myth recently about filling your car on the slowest pump speed to avoid excess aeration...
It was part of a"tips" chain letter that Snopes began investigating last month:
http://www.snopes.com/inboxer/household/gastips.asp

The “tips” start with a fact, and then go astray.

There is a Kinder Morgan but they mainly pipe natural gas and crude oil, and would not alternate refined products daily in the same pipes -- http://www.kindermorgan.com/.
The gas station tanks are deep, underground temperature is constant and does not fluctuate between day and night, and does not affect the density of gas.
Gas pumps do have vapor recovery, but pumping fast does not make more vapors than slow. To cause more evaporation into vapor, the surface area of the gas has to be increased as by spilling or spraying.
A half filled tank of gas does not evaporate much, and fumes are confined to the tank by the cap seal, charcoal cannister, etc. My gas cans and motorcycles don’t lose fuel over the winter.
A big refinery storage tank has a internal floating cap, but it is so that as the level drops, the tank is not full of fumes that would be an explosion hazard.
Gas is filtered at the delivery truck and at the pump and by the car, and “dirty” gas has not been a problem for decades.
None of these “tips” will save gas like avoiding unnecessary speed, accelerations, and braking.
 
#10 ·
Gasoline is not supposed to flood into any part of the evap system. Subaru will replace the damaged goods. Any other shop.... well, I'm sure there are ways around actually replacing, and as far as that would go I don't know. The manager at my dealership laughs when people come in complaining of bad fuel smell and poor engine performance. He asks them (like a salesman) when the last time they filled up and about how much gallons, leading them to admit they topped off. Then he'll say have a good day, it's not warrantied! Of course if the tech actually find fuel in the canister.

Well not like that to the customer of course but he'll tell me that.

Truly this all makes sense. It's not the ole' Big American Iron where you can just top it off and let carburetion do the work.
 
#12 ·
Well, every sensor at the pump trigger's differently. Who's to say (I don't advise..) that if you were to damage something you could say the gas flow never stopped! FWIW, I used to fill until there was fuel in the top of the fill tube. Or to the nearest dollar. Honestly, I've built more change up in my pocket that I empty in my house and it adds up a lot faster if I don't round to the nearest dollar.

Also one time I was pumping gas and I was at the driver side windshield washing it, and these hotties in the drive-thru at Wendys across the road were yelling and waving at me and I was like "Heeyyy" and waving back. They kept doing it and I thought they were drunk or something, but about 5 seconds later I found out they were not hitting on me but trying to tell me the gas at the pump was flowing out of the car! The sensor never shut the fuel offeven with fuel all over itself and it was everywhere. The first time I paid for 21 gallons of fuel in a Subaru.
 
#16 ·
Our subys have that special valve in the filler neck so that someone can't siphon out your gas and the vapors don't get out, either. I am tired and don't know the exact specifics or terminology right now but I will get to my point...

...It really depends on the gas station and their pump. Sometimes I can just put in the pump and it will pump until full and click off. I can then go on my merry way knowing I have a full tank. BUT sometimes a gas station will have a nozzle that doesn't sit in the filler spout good and/or the pump is older technology that doesn't quite like the suby special valve so the pump will stop on me after 1-3 gallons (even though I still need 8-14 more gallons).

At this point I do two things:
  • Check my current gas level to determine how much is needed
  • Push pump nozzel farther in filler opening and/or hold it in place
If the pushing it in farther doesn't work, I just keep filling in slowly keeping the pump trigger held on the low notch catch. If it clicks off and it hasn't reached my estimated gallons needed. I wait a second, then put it on the low notch catch again until I get there. Kind of annoying but sometimes needed.

Maybe it is just me? This has happened on both my old '04 and my new '08.
 
#20 ·
This happens to me all the time it drives me insane, especially when its like -15 below and I have to sit there making sure the darn thing doesn't stop. Because the gas stations around me apparently suck, my car never gets a full tank of gas so I never have the ability to "top it off"
 
#18 ·
Judge for yourself:

1. There is a mechanism in the nozzle that shuts off the pump when it senses splashback in the filler neck. Splashback occurs when the tank is full and the gas is starting to rise in the filler neck and encounters the incoming stream.
2. After the pump shuts off, you can start it again and again, several times, until the tip of the nozzle is immersed in gas in the filler neck, which makes the shutoffs instant.
3. You can increase the amount of gas in the car by filling up the filler neck in this way. Over-riding the first click might add a quart, over-riding successive clicks adds smaller and smaller amounts.
4. There is a hose that comes off the top of the filler neck that takes gas fumes to the charcoal cannister that is part of the evaporative emissions system.
5. When gas reaches the top of the filler neck, it might go into the hose to the charcoal cannister.

The Owners Manual anticipates this and says to stop pumping after the first click. Some people feel it is important get a little more gas by pumping until the second click, and some like to keep pumping until the click-off is instantaneous.
 
#26 ·
I have always topped off mine until the filler neck is full. I can see a difference in range and never had a problem with any emission codes other than the typical P0420 cat codes when the cat died. I have always topped off all my car, never had a problem and one thing that helps is pretty much every drive in the cars is 40+ miles, which helps to fully purge the charcoal canister.
 
#28 ·
I have always topped off mine until the filler neck is full. I can see a difference in range... 40+ miles, which helps to fully purge the charcoal canister.
I did not realize that the neck held 1.5 more gallons of driving range, though I can see that topping it up full of gas could purge the charcoal cannister pretty good. Many will be attracted to the benefits of doing this :-(
 
#27 ·
I usually fill up at the same (local) station and never top-up; the reason for this is not technical, but it enables me to do a quick'n easy fuel consumption check because I reset the tripcounter with every fill-up. So I compare the actual mileage (or kilometers) with the amount of fuel. Since it is the same gas station I use, this gives a fairly accurate check.

Just my 0.02...
 
#29 ·
In SoCal we have vapor recovery systems both in the vehicle and in the pump. Topping-off can mess these up and is strongly discouraged. I let the pump run on automatic (medium speed) until it shuts off. I check the GAL guage on the pump and if it's around 10 I assume that the fill-up went OK. I never top off or round up.
 
#35 ·
My general feeling is, "don't top off" has 2 main valid motivations: 1) outright safety for the most extreme one-in-a-million case of filling the car with extremely cold gasoline in the winter and then immediately driving to an indoor parking space that happens to be much warmer. The thermal expansion of the gasoline definitely can make a spill happen - years ago my father-in-law, who ran a wholesale nursery greenhouse back east that was well heated by a gas boiler, drove the old Dodge nursery van back into the greenhouse straight from the gas station one winter evening. He avoided catastrophe, but he did have a huge pool of spilled gasoline to mop up, a few hours later. 2) environmental benefits of avoiding hydrocarbon air pollution. That said, I usually do a few extra clicks in all of my own cars, especially if I am in the middle of a long trip where I'm immediately going to burn up lots of fuel. One time I got over 17 gallons into a Mazda5. And one time I got 563 miles out of one tank in another Mazda, a 3 with 2.5L engine, Butte MT to Othello WA. I've never noticed any car problems from 50 years of this "bad habit". Another observation is, big metro area gas pumps seem to always have the more complicated modern vapor-recovery mechanisms, and are harder to overfill cars with. That Mazda5 overfill escapade was in a rural small town, as was the Mazda3's fill in Butte for its record run.
 
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