A miracle happened today. I used up 8 gallons and at the pump, it filled up to a complete 8 gallons before shutting off. This had never happened before. I then held my breath and brought it up to 8.5. Breakthrough?

 

Jason - can you attribute the change in fill level to anything in particular? Has it been warmer than usual? How many fills have you completed so far, and do you think it's possible that the bladder is becoming more flexible?

jw - Tideland AM, Colorado

 

Jason (and others) - this really is a VERY bad practice and could end up damaging the fuel recovery system - eventually leading to lower mileage.

There exists in the car lots more in the fuel system than just the gas tank and delivery system. I don't have the gory details, but suffice it to say that once the pump quits (unless you are CERTAIN that the normal fuel limit has NOT been reached - ie: sensitive pump) then STOP!

Forcing more fuel in to the system is feeding fuel into the fuel recovery system - capable possibly (depending on tube lengths and equipment placement) of accepting another gallon or two - but it is made for vapour recovery, not fluids! If you pump your charcoal recovery canister full of gas, well that sort of quashes it's ability to do what it is there to do!

As I mentioned in a seperate tirade (see the Guess Guage thread), it doesn't matter how much gas you put IN the tank - it matters what you get OUT of it. If you want more gas in your car, carry a spare fuel tank!

Jack

 

Jack,

I think this is one of those moot points. 90% of people will disagree with you. I, personally, have no idea. I can tell you, though, that it is widely believed that the Prius' shutoff is overly early and topping off would cure the problem.

 

Jason

I have a few issues with this. First, it cannot be a "moot" point - as it is not inconsequential. It is, however, entirely up to you if you wish to continue the practice. I hope it is not causing a fuel system problem, but my knowledge of fuel systems leads me to state otehrwise.

Second, it is not a matter of how many people disagree with me - as the only VALID opinion on the matter is from someone who knows how the fuel system operates in all its detail - not someone who forces fuel in at the pump. Can you put more fuel in? Surely - is it fine to do so? Not if you understand how the entire system works.

Third - the Prius has no direct control over the fuel shut off. The pump does that. Are there sensitive pumps? Yes. I have one locally that with an empty tank, I can barely get it to keep pumping. But it is not common - easily 90% of the pumps I fill up at have pump shutoff systems well capable of recognising an adequate shutoff pressure.

What APPEARS (to me) to be happening though is that the fueling inlet on the Prius is MUCH more tightly sealed than any other I am familiar with. This may cause a slightly early shutoff sense on the pump - but emphasis on "slightly". If it is an early shutoff on a properly functioning pump, I would estimate perhaps UP TO half a gallon is a safe addition, so your last post of adding a half gallon is probably (IMO) in the safe margin.

However, earlier posts of adding up to two additional gallons indicates no understanding of how the fuel system works. We are not dealing with a simple can into which a hose is placed for the purpose of extracting fuel! This is a complicated system and it is WELL known that forcing in additional fuel in any relatively recent (last 10+ years) fuel system is strongly ill advised.

You can choose to believe it or not - but the mechanics remain the same regardless of belief.

Jack

 

Why would you buy a car with such environmental benefits and then seek to undermine them?

Topping off adds evaporative emissions to the atmosphere. Period. That's not moot. Certainly not to me.

My only goal in getting all the gas in the car I can is to my range so that I do not have to fill as often ... reduce the number of times I have to fill up in a year and you reduce refuelling evaporative emissions.

I hope that everyone will reconsider if they are topping off. It is risking wasted gas. When the engine is on in the car and you aren't going anywhere, at least you're regenerating. Evaporative emissions and fuel spills are truly wasted. You get nothing for them.

 

Jack,

I can always count on you for lengthy posts
It's not that I believe you're wrong, because I don't. But, to me, it's the same as the practice of filling up when the tank is half full because it's "best for the car." Or sleeping on yoru side curled around your pillow because it's "good for your back." The fact of the matter is that barely anyone will ever notice the difference.

 

cybele,

No argument there. Admittedly, I've been driving for less than 2 years. I still have a bit to learn.

 

I am working on it!! At least, the long post part!

I noted earlier that the "fill up when half full" issue is also poorly formed, based on my level of understanding of "How Things Work" in the tank. You are quite right about that part - with one exception that does NOT apply in the Prius: keeping a "normal" gas tank full removes a degree of tendency for the tank to accumulate water from condensate from the air that is drawn into the tank. But most folks don't pay any attention to that in a car (including me!). It is VERY important in an airplane, though!

However, filling a tank BEYOND the shut-off point at the pump (again, always presuming the pump shutoff is working properly!) CAN and DOES cause things to happen that you cannot see and may not even recognise until later - maybe much later. It is like saying it is fine to eat a stick of butter a day. It doesn't hurt and can really make popcorn and bread taste great - until one day, 25 years hence when you end up under a knife because your blood no longer has any way to get out of your heart.

By squeezing in more at the pump than the pump tells you is enough, you can and will (particularly with large quantities such as the two gallons mentioned) cause damage that will not be apparent! But hey - it's your car. I have seen guys take all the oil out and race an engine until it killed the beast - just because they thought it was a good idea at the time!

I am just saying - this iis not a good practice. It can and will damage things that are not apparent. Take it as advice or ignore it as you will. Just be advised: I have been into more engines in more cars than many people have owned - even though I am not a "mechanic" in the professional sense of the word.

And as cybelle pointed out, it may have other environmental consequences as well!

Jack

 

Hi I'm not a Prius owner, but have been trained on Prius since 2001 at our dealership.
Over filling your tank can cause the fuel to leak in between the bladder and the tank. When this happens you will turn on the check engine for an EVAP code. The only way to get the fuel out is to replace the tank.