Accurate fuel fill without topping off?

[sbergman27]

Traditionally, on my cars I've always had a standard procedure for ensuring an accurate fuel fill. Run the pump till it auto-stops. Top it off. Bounce 15 times. And then top off again. However:

1. Topping off results in increased pollution from vapor.

2. I've read on this forum that topping off the Yaris can cause problems with the canister/vent system. (Is this true, I wonder?)

3. Topping off the Yaris involves tediously putting in over a gallon of extra fuel, which takes a while and is a real pain in the cold and wind.

4. The way the filler is designed, it's hard to top off without getting fuel on the paint every time.

So... is there a better way to do this while preserving accuracy? I travel a lot, and so always using the same pump is not possible.

-Steve

[YarisOwnersDad]

Steve:

The error in fuel mileage calculation due to the degree to which the tank is filled should be minimal over several tanks of gas. The error should cancel itself out over several tanks.

I have read the same thing that you have about how over-filling can be harmful to the fuel evap system, so I stopped doing that. When the pump kicks off the first time, I remove the nozzle.

Tom

[YarisSedan]

You should never top off gas. Your going to destroy the charcoal canister.

1. you're paying for gasoline you aren't using, as it is sucked back into the pump's vapor recovery system

2. gasoline can drip from the nozzle onto your car or on the next customer. not only will this damage the paint and leave you or your car smelling like gasoline, these released vapors aren't recovered go into the atmosphere. (besides a slight fire risk...)

3. gasoline vapors are unhealthy to breathe. (known to cause cancer)

4. topping off your car's fuel tank may cause fuel to enter your car's vapor recovery system (charcoal canister), poisoning it and causing it to be unable to function properly, which may get you a check engine light on your car besides putting out more emissions and costing you more money to repair it.

4. As the temperature changes, your fuel needs somewhere to expand to. If you top off your tank, this expanding fuel/vapor is forced into your vapor recovery system (see above).

So in the end its not at all worth it to top off your gas. Thats why at the gas stations they have signs everywhere saying not to do so.

I would just run the pump till it clicks and stops. Hang the nozzle back up and put your gas cap back on. Any discrepency will be so minute. If you want to get the most true acurate readings i would suggest purchasing a ecometer. Then you can graph your monthly mpg adverage there will be less likely error over longer period of time. Maybe 2 percent.

[sbergman27]

Steve:

The error in fuel mileage calculation due to the degree to which the tank is filled should be minimal over several tanks of gas.
True enough. I do like to have accurate readings over the course of a tankful because I can more easily correlate conditions with mpg. But as I gain more trust in my Scangauge II, that may become less of an issue.

But it is nice to have that extra gallon of fuel, and the extra range it represents.

-Steve

[Hershey]

1 click of nozzle at full speed of fuel . Try to use same pump at each fill . The ECO-METER has been off by about .5 since less need for idling in morning to defrost windows . More the idling the wider the margin from calculated m.p.g. .

[Uber_Otter]

I stop at the first click, I fill at the first click on the pump handles so its slow. The wide open click on the gas pumps tend to vary a lot. The slowest doesn't vary much from station to station.

[Yaris Hilton]

That's been my assumption, that the slow fill is more reliable. So I fill it wide open till it clicks off, wait a few seconds for the bubbles to clear out, start it back as slow as it will go and let it click off once more. Never adds more than about 1/10 gallon, and seems to be consistent.

[BailOut]

I always just fill on the slowest setting and stop at the first click, then turn the nozzle upside down to get out anything left in it.

[127.0.0.1]

You should never top off gas. Your going to destroy the charcoal canister.

1. you're paying for gasoline you aren't using, as it is sucked back into the pump's vapor recovery system



ummmm where do you think the vapors go ? they don't vent to the outside

they vent......into........the intake.


so, no one is paying for any gas they do not use.

[YarisSedan]

ummmm where do you think the vapors go ? they don't vent to the outside

they vent......into........the intake.


so, no one is paying for any gas they do not use.

Maybe you misunderstood im talking about the vapor recovery portion of the gas pump when you pump the gas.

http://www.sbcapcd.org/edu/dtofactsheet.pdf

[sbergman27]

Hope you don't feel I'm picking on you, YarisSedan. I appreciate you response. But I can't help commenting upon this lovely public service publication aimed at the general public.

There's more than a bit of propaganda in that link, mixed with some valid points. But I'll just put some numbers on one of the points mentioned.

After the first click-off, the Yaris tank can still hold another gallon of fuel. (Slightly more, but let's keep the numbers round.)

One mole of a gas occupies 22.4 litres at standard temperature and pressure. The average molecular weight of (the components of) gasoline is 108. So 22.4 litres of gasoline vapor weighs 108 grams. That's 4.8 grams per litre, or about 18 grams per gallon. Liquid gasoline weighs about 3000 grams per gallon. So the lost vapor from the top off works out to about 1/170th of a gallon. Hardly enough to break the bank. It's worth about 1.5 cents. So if you're the sort who bends down to pick up pennies and takes them home to put in a jar then this might be significant to you. Note, however, that likely about half the vapor is captured by the Yaris' capture system. So you're probably only saving about 3/4 of a cent, in reality.

Another amusing highlight from the link is this:

====
I picked up the nozzle and gas spilled out. What should I do?

It’s likely that someone before you topped off a tank and caused this problem for you, and the damage is done. The nozzle may still be able to function properly, but if
you notice any more problems, tell an attendant at the station, and use a different pump.
====

Because after such abuse as the thoughtless sociopath in front of you at the pump has inflicted upon it, the nozzle may not ever be able to function again... :-0

-Steve

[Uber_Otter]

How can I reduce my risk of breathing gas vapors at
the station?

Use the nozzle’s hold-open latch to pump the gas
continuously, and stand upwind (but stay close enough
to monitor it). Always remember to remove the nozzle
before driving away – and be sure to tighten your gas cap.


Ha ha


I started to fill my tank and there’s a strong smell of
gasoline. What should I do?

Tell an attendant at the station, and to report a problem,
call the APCD at 961-8800.

[Revsson]

Is that Pam after the breast reduction?

[Reddog99]

You should never top off gas. Your going to destroy the charcoal canister.

1. you're paying for gasoline you aren't using, as it is sucked back into the pump's vapor recovery system.... This is true in California, but the OP lives in OK, where they don't have the gas pump vapor recovery systems like in Cal. gas stations (I think).

By the way, the extra room in the tank is meant to leave room for expansion and sloshing, and to prevent the flooding of the charcoal canister (part of the car's vapor recovery system). So it's not a good idea to overfill the tank.

[127.0.0.1]

Maybe you misunderstood im talking about the vapor recovery portion of the gas pump when you pump the gas.

http://www.sbcapcd.org/edu/dtofactsheet.pdf

yeah I misunderstood. I used a weak pump once and gas poured out
of the coaxial line onto my paint from the dub who gassed up before me
and it got sucked up the hose, but not all the way

[127.0.0.1]

heh

[docB]

Filling to the very top is nice if you have all the time in the world. Plus you are hauling the weight of that extra fuel. Keep track of all your gallons and divide it into a total mileage from your odometer and the refill amount won't mater. Base it all on the mileage from your next fill up. Reset the B trip odometer and keep all your gas receipts. Then add up all the gallonage and divide that into the trip B reading. Changes in driving habits, traffic patterns, weather, etc... will all blend in.
doc

[Yaris Hilton]

It'll still clog up your charcoal canister.

[sbergman27]

Filling to the very top is nice if you have all the time in the world. Plus you are hauling the weight of that extra fuel.
Well... That extra 1.1 gallons of fuel gets me an extra hour (or so) down the road before the next refill. So on a "time economy" basis that doesn't make sense. However, on a time spent out in the cold and whistling wind" basis it might, since a fuel stop involves substantially more time than just the pumping time. (Exit, find the station, find a pump... etc. etc. ... update the scangauge, get back to the highway, get back up to speed...) But the time spent out in the cold is significantly increased. And I really, really don't like the cold.

Gasoline weighs about 6.5lbs/gal. So 1.1 gallons weighs about 7lbs. Or about 0.25% of the weight of the car (with me and my suitcase in it). Of course, the *average* added weight over the course of the tank is half that, or about 0.125% of the weight of the car. And on the Interstate, wind resistance dominates. The difference made by the extra weight works out to about 0.02 mpg.

However, despite the humorous flights of fancy in the public service announcement, there are enough rational reasons not to top off that I've switch to filling at the pump's slowest setting and then pumping as much as I can by pulling the trigger *ever so slightly*, just once to make sure that the automatic mechanism got it right the first time. It doesn't work out to more than 0.1 gallons more. And the method seems to give me a consistent adjustment factor on the scangauge.

-Steve

[STC]

I fill up at the slowest pump speed. Once the pump clicks and shuts off, I wait about 10-20 seconds and fill around .100 of a gallon more.

By filling up at the slowest speed reduces the gasoline vapors, therefore not wasting the gas that has evaporated.

This is less volume in a fill-up... saves a tiny bit! :smile:

[bkrownd]

I've always done one shot past the first shut-off just to make sure it wasn't cutting off prematurely - occasionally pumps will shut off a bit prematurely. It's absolutely restrained compared to how my relatives pump gas. ;)

We don't have the vapor sucking pump nozzle in Hawai'i. Mmmmm...those lovely heady gas vapors. :D

[Ilovemyyaris]

It'll still clog up your charcoal canister.

How many times till that happens? Is that just a long term effect? :frown:

I've been topping off all the way to the top of the tube to make sure my mpg calculations were as accurate as possible. I haven't noticed any bad side effects. I just thought it wasn't a good idea because of the risks of spilling gas when it gets close to the top. I wont be doing that anymore.

[Idahotom]

I just use the same pump at the same station every time (locally of course) and just quit when it clicks off. I'll even wait for that pump to be available, and I park the exact same way and spot, that should be consistant enough. I use the fast pump mode, I just like getting in and out of the station quickly, while other drivers are still pumping (and paying).!

[TLyttle]

Yeah, why is that so hard to understand? If it was a good idea to clog that cannister, they really wouldn't install it in the first place! It is a SYSTEM, dammit!

[The Limo]

If you are really interested in your mileage then join a site such as fuelly.com or spritmonitor.de (European site). These sites keep track of your MPG plus other expenses. I find after about 10 fillups that your MPG stays about the same.
Roy

[Em_]

topping off is very bad on the charcoal canister

i topped off my my 2005 echo a couple of times and i had to endure the check engine light 5 months p0446 i think

i erased it regularly and finally after 5 months it didn't come back it' like some gas went somewhere where it wasn't supposed to and it took a while to evaporate