Best practives to record fuel economy

My college buddy had owned his 1st gen Prius since brand new. He stated that his Prius has a bladder gas tank. One day he overfilled his tank and it caused problems with the 'fuel sensor'. Luckily at that time his car was still under waranty, so Toyota was able to fix it with no charge.

As far as recording miles and fuel consumption...I keep it simple. All you need is the total mileage and the amount of fuel in gallons you put in the tank.

Drive safe.

 

Generally the term is used to describe a practice of overfilling the tank by continuing to fill after the nozzle has clicked off the first time by continuing to squeeze the trigger despite how many time it clicks. In old vehicles some would continue this till you could see fuel in the filler neck. lol

 

I went to a new owner clinic at the Toyota dealership last week and they mentioned this issue of "topping off". They basically said "don't do it". When the pump shuts off, consider it done. By overfilling, you will ruin a "carbon filter", by getting gas in it. This is not a warranty issue, unless someone is feeling generous, and it cost $600 to replace.

 

+1.

 

It isn't recommended to top off the tank on any car because it can cause the fuel vapor recovery system in the pump to clog or worse, spill gasoline over the side of your car.

Heck, there are signs on every single gas pump telling you not to do it, but I'd guess that well more than half the people I see filling up their tank top it off anyway, even the pump-men in Oregon.

I don't really see the point in topping off, anyway, you get to sit there clicking the pump to squeeze in another half-gallon which extends your next fill-up 25 miles in a Prius or half that in your average other car? The same people could probably just wait until the gas light goes on before filling up the tank.

 

For me it has always been a way to ensure consistent fills at different stations and even different pumps at the same station. So, using the old adage of "just drive it", I didn't change my process for filling the tank when I bought the Prius

I fill with the "hold open" device at it's lowest speed. When that clicks off, I do two more clicks manually. I've never caused an FVRS problem that I have detected and I get fairly consistent tank fills judging by the deviation between the MFD and hand calculations.

But, I've only had my Prius for 3K miles so far and I may well live to regret this choice. As always, YMMV!

Brian

 

CleanMPG.com and other sites have a spreadsheet where you can record your data, and will display it on a graph. At CleanMPG, there is room for other stuff, such as temperature, type of gas, price, driving terrain and techniques, if you want to include that also.

 

Excuse me while I put on my flame suit but, isn't there some way of overfilling the tank that must not cause any damage to the vapor canister? The reason I ask is that I have been topping off the tank on my '07 since I bought it and there hasn't been a problem.(according to my logs at CleanMPG and GH I have filled 21 times since buying the car on 5/19/07) I fill until it clicks off then slowly dribble fuel in until I see it just below the top of the hole that goes into the tank. I feel that this is the only way to ensure consistent results when filling up since, even at the same station, the pump can click off at different times due to different conditions such as number of people pumping, temperature etc. Perhaps it has something to do with the fact that the closest station to my house is over 6 miles away and I use enough of the fuel out of it by the time I get home that it doesn't get into the canister? I have used this practice on every car I have ever owned and have had no ill effects mechanical or otherwise. Just my $.02 worth.

 

I suppose such a statement could be taken to an extreme:

"Once a week my entire family celebrates life by gathering in my back yard and shooting a rifle straight up into the air. We've been doing this since I was a child and nobody has ever gotten hurt, therefore it must be safe."

The fact that you haven't ever had a problem doesn't necessarily indicate that the practice isn't risky nor does it guarantee that you won't have a problem in the future. If you choose to continue topping off, I sincerely hope that you never have a problem, but if you ever do damage the charcoal filter or other components of the vapor recovery system, I really hope that Toyota charges for the replacement of the gas tank, pump, gauge float assembly, bladder, etc. I'd hate to think that all the rest of us would have to pay more for Toyota products because they are wasting money repairing damage that you caused to your own car.

That said, I'm curious as to what you mean by "ensure consistent results" as well as why you feel it is necessary to know that your tank is all the way full rather than full enough.

What exactly isn't consistent enough, and why is that important?

 

When pumping gas, there seems to be some foaming/frothing that seems to be dependent on the speed of the flow. As I mentioned before, if there are several people pumping the fuel flows slower so the fill could vary from one time to another depending on how many are pumping. Fewer people means more foaming and the pump shuts off faster(less fuel put in) and more people pumping and there is less foaming and more fuel gets pumped before the pump shuts off. By filling to a level that I can see it seems that that the fill would be more accurate. I'll admit, it probably wouldn't make too much difference, but I like being exact.

As for paying for damages, you are right. If it happens to me, then I should be charged because I have been warned and I have no excuse. What I'm getting at is that people have reported damage after topping of one or two times and I have done this 20+ times with no problems, does it possibly mean that there is something about the way I'm doing it that will never cause the problem? Have I played russian roulette this many times and just gotten lucky or is the problem not as certain to happen as some think? I mean, if one car blows up because of a rare malfunction or an odd set of circumstances, should we stop driving all cars because it could happen to another car? I want to keep an open mind and will change my habits if needed.

 

I disagree. I think Toyota should design properly a fuel system that cannot be damaged by "topping off" the fuel filler tube. A significant number of people routinely top off their tanks on nearly all makes and model vehicles with no adverse problems. It would be very easy to incorporate a design that accomplished just that. Sure, a defective fuel delivery nozzle that forced fuel under pressure into a vehicles tank may perhaps cause damage, but such failures in fuel nozzles are rare.

I was not aware this was a significant design flaw with the Prius as many posters have stated on this thread. I also did not know that you could blow a bladder, damage the fuel sensor, pump, and other various components of the Prius by topping off the tank as you stated above. I wonder how many blown bladders, damaged fuel sensors, fuel pumps, and such have been replaced by Toyota from topping off the tank.

I too have topped off the tank on my 2006 for the last two years (60,000 miles) with no ill effects. Perhaps my Prius is somewhat immune to blown bladders, flooded canisters and sensors???

Rick
#4 2006

 

This is the kind of thing that points to the possibility that topping off will not cause certain damage. It suggests that the technique in doing so plays a part.

 

No mater which car I am filling up, I just put the nozzle on slow fill and when it clicks off, that is good enough for me.

 

See now you are just making things up, and when that happens I tend to disregard most of what you say.

I never said anything about "blowing a bladder", "damaging a fuel sensor", or "damaging the fuel pump". I just said that when fuel damaged the vapor recover system, all these other parts would be replaced.

If you took the time to understand what you are doing and why it is not reccomended, you would understand that those are all components of the fuel tank assembly. When you "top off" the fuel to the point where you can see it at the top of the filler neck, you run the risk of overwhelming the vapor recovery system and ending up with fuel in the tank outside the bladder, and a flooded charcol canister. This is repaired by replacing the entire tank assembly.

You can see the locations of some of these components in the photographs here:

http://priuschat.com/forums/showthread.php?t=30593

And there is a good explanation of the fuel delivery and vapor recovery system here:

http://www.autoshop101.com/forms/Hybrid13.pdf

A significant part of the reason that the Prius is rated as AT-PZEV is because of this system of preventing the venting of fuel vapors to the atmosphere.

 

I'm still not sure what results you want to ensure are consistent. Also, it's not clear to me why you want to know that the height of the fuel in your filler neck is always the same whenever you fill up. You are aware that this in no way ensures that you have the same amount of fuel in the tank, right? The fuel bladder may expand or contract to hold more or less fuel regardless of whether the filler neck is filled to the same height. I don't recall which post I saw it in, but a source that seemed reliable at the time stated that the bladder capacity could fluctuate by as much as 5 liters. As such, even when you fill to the same height in the filler neck, you can still have a variation in fuel volume of as much as 5 liters.

The pump tells you how much fuel you have put into the tank. Filling the neck higher doesn't make this number any more exact. The car odometer tells you how far the car has traveled, filling the neck higher doesn't make this number any more exact. I'm trying to understand here, but it seems like you are taking a $1,000+ risk here and not getting much out of it other than the ability to say "I haven't had the problem yet that others have had." As you and viking31 have pointed out, you may never have a problem, but it seems like an expensive risk with very little reward.

 

I would have to disagree with this. If as I said, the fuel was flowing faster or slower the amount that could be put into the tank before the pump clicked off would vary from time to time.

As for the bladder expanding and contracting, I understand this. This will seem odd(and it probably actually is) but, I try to time my fillups from one time to the next to be as close to the same temps as possible to try and reduce this fluctuation.

The problem I'm having in seeing this as risky is that there are at least two of us(and I suspect there may be more who are keeping quiet for fear of being chastised) who are doing this for extended periods of time and nothing has happened. I have done this for 11K+ miles and viking31 for nearly 6 times that amount. It seems as though if it hasn't been a problem for us it most likely never will. As I've said before, this points to the very likely possibility that different circumstances/techniques could mean that it may not certainly happen to everybody with everything being equal.

 

I'm a new Prius owner for two weeks and I haven't filled my tank since the dealer filled my tank. I looked through the Prius Owners manual and did not see any mention of topping off. I would think this is an obvious scenario the Toyota engineers would've thought of in case the fuel nozzle doesn't cut off or the owner intentionally tops off. My dealer didn't mention this during the short orientation or test drive. I'll post this question to them. In the meantime I won't take any chances and will not top off. Usually on my Honda Element I'll top off to the nearest 50 cent increment since at almost $3.00/gallon this comes pretty fast and doesn't put much fuel in

Kelvin