Is topping off the fuel tank bad for your Prius?

That's never a good idea, as fuel expansion may force fuel into the evaporative control system.

As for Prius specifics, it depends on the type of Prius. Your Gen III Prius has a normal gas tank, and as such, is no more vulnerable to this sort of problem than any other normal car.

The NA Gen II Prius have bladders inside of the fuel tanks. Expanding fuel can be forced outside of the bladder, requiring a complete replacement of the sealed fuel tank assembly. This is an expensive repair, and not covered under any warranty.

As for the risk, I've only read about a couple of cases on this site.

Tom

 

Why is this considered to be environmentally Waco-unfriendly? Sorry I did not purchase my Prius to save the earth...Or to reduce my carbon footprint..

Thanks in advance

 

I to have heard the horror stories of overfilling the fuel tank. In my opinion it's not worth hurting anything in the fuel system...most of us live close enough to a gas station...besides overfilling makes the risk of running gas down the outside of your paint, why do that.

 

Those are mostly old wive's tales. On a Gen III with it's normal fuel tank, you can't overfill it by toppinig off the tank. The fuel system is designed to capture a couple of gallons worth of air in the top of the tank to assure room for fuel expansion.

If you slant you car steeply to the side to add extra fuel, then you could do some damage as could you if the vent system is defective or modified to allow overfilling.

For a Gen II with it's weird bladder system, I do not know.

The environmental hazard from topping a normal correctly functioning tank is when you spill gas on the ground, so if you can top the tank without spilling gas... ignore the alarmists.

 

There's a thread about what would you change, and of course
Toyota already changed this for 2010. I have a 2009, though, and
I have to say the bladder is a pain in the neck. First, I have
never, ever gotten a "full" fill of 12 gallons. The most I ever
was able to fill it was 9.5 gallons, and that was with the last
pip flashing. My average "good" fill is 8.5-9 gallons. Recently,
I've been only getting 7-7.5 gallons (at one pip). It has been
colder, but not that cold, so I wonder how the bladder can
be affected so much.

I try to top off the tank, but it takes hardly any more before
the gas pump repeatedly clicks off.

 

When you fill your Prius at the first auto click of the gas pump you
can still pump in another 2 to 2.1 gallons of gas right up to the neck.

I don't do it very often because in Oregon it is a violation of law to pump your own gas and attendants will not often top
the tank.

I do not think you will damage the fuel system, if there was
you would see big letters in the owners manual, "Do not fill to top of filler neck when pumping fuel".

I have never heard of anyone damaging their prius, at least on the solid tank models that all the 2010 Prius' have.

Its nice when you can go an extra 90-100 miles on a tank.

alfon

 

I've always "overfilled" my gas tank on every car I've owned and never had a problem, it makes it so I can go extra time before needing gas again. I've got a decent commute so I get gas once a week -- so every little bit helps time-wise...

If I'd have had an earlier Prius with the bladder, I might have done differently though.

 

I don't fill to the neck in this or any other car. GM's are somewhat notorious for having evap problems from people trying to stick an extra 1/4 gallon of fuel into the tank, and I'm sure that Toyota's evaporative containment system is pretty much designed for vapor instead of liquid as well. I get 510-520 miles out of a tank this way, which is good enough for me.
If you like to fill to the neck---good on you. I'm a live and let live kinda guy, but I have to agree with the owner's manual on this one.
It's just not worth the risk IMHO.

 

I don't get thank kind of mileage, only 460-470 miles... (Prius V 17" tires and 90% rural highways.)

I probably could get 500+ if I ran it down to less than a gallon, which is not a great idea given the type of fuel pump...

I've really never had a problem, but I'll post back here if I ever do!

 

Fuel Pump?
What kind do we have???

 

Electric. It is located inside of the fuel tank.

Tom

 

I was topping off to the very top (filler neck) every time for years. Then I got a P0441 code, which is "evap system" and I started reading about evap problems. In that reading there were people saying "never top off or gas gets into the evap system and wrecks your purge valve and/or your charcoal canister. Went to dealership and they diagnosed it was the purge valve. Replaced myself with OEM. No luck, kept getting code. Took it back to dealer....this time they said "bad charcoal canister", so I got one on eBay from a junk yard with "41k miles on it" (looked to be accurate from the appearance of the canister. Put that in. Don't recommend this....what a pain in the arse. Code off for a while....then back again. Took it back to the dealership. They said "Bad canister you bought" and were going to charge me close to $800 for a new canister (yes, after mis-diagnosing, and after causing me tons of hassle and grief). Anyway, I got a Lexus dealer to send them a brand-new OEM canister for half that price (or less, can't recall) and they put it in. All good for a while....then same code again.

This time, as they were responsible for all of it, I took it back and they happened to have a Lexus mechanic, who had run a large dealership maintenance department for many years, and happened to semi-retire in the small town in Colorado. This guy won't ever speak with customers...but he was willing to talk to me, once he saw how clean the engine bay was, how I had an oil catch can setup, and how I'd done a bunch of work myself. He found the problem...which nobody was going to find otherwise: Loose pin in the connector that has 20 pins and plugs into the computer that controls the purge valve. So the pin would sometimes activate the purge valve and sometimes not. He fixed it. No problems since. Super knowledgeable guy. Maintenance desk said, "He TALKED to you?! Was he ok to you?!?!" Apparently he has no patience for idiots but thought I wasn't one. (Got away with pretending to be an intelligent human for a few minutes).

Regardless, I'm now back to square one: My purge valve and charcoal canister were likely fine, after filling to the absolute brim, every time I'd fill up, for over seven years. So now I wonder all over again:

With the tank so annoyingly small, can I top it off without wrecking the evap system??? I absolutely hate how small the gas tank is. I fly airplanes and yes, I can go 5 hours without needing to pee. Large tanks allow for trips to take a lot less time by not having to stop. Currently, I'm commuting in my CT200h about 50 mins each way, at 78mph. (speed limit 75). This means that I get about two and a half round-trips before I have to fill up again. It's a pain to fill up every couple of days. Nuissance. Car gets about 35mpg (says 37.5, so I know it's right around 35) at 78mph. At 60 mph, I could get about 45mpg, but I'd be a friggin road hazard and drive everyone and myself crazy. Car just not made to go over about 65 efficiently.

Anyway, been following the guy trying to swap a corolla tank into a Gen3 Prius. Will have to read the rest of the forum, but it brings to mind:

1) With all the difficulties of swapping tanks (there are a lot of sensors, connectors, fitment issues, etc, etc), couldn't we just have a fuel cell with it's own pump mounted somewhere and have it pump fuel into the filler neck? That would seem to be an easy solution (and could be a completely sealed system, so could be mounted in spare tire well or something). I just would need to understand the evap system, where it pulls fumes from, where the vent is in the filler neck (seems to be right behind the small opening in the plate that blocks larger pump nozzles in the top of the filler neck).

2) Is it really a problem to top it off? Am I really going to damage my charcoal canister?

 

This is from the Car Care Nut:

 

Something about this doesn't make sense.
We have a Gen3 (2011).
For a decade now, my normal practice has been:
1) Do the normal fill

Based on the (incorrect?) "knowledge" that my car has an accordian / bladder at the top to minimize air space... and needs time to expand to hold an actual full tank...
2) I wait a few seconds
3) Then I begin filling again, but sloowly
4) When it clicks off a second time, I stop

AND: I always am able to add 1 1/4 to 1 1/2 gallons of extra fuel this way. That's well over 50 miles of additional range.

I have tried this with other cars, and do NOT get that extra space. Maybe 0.1 to 0.2 gallons max.

If my car doesn't actually have a bladder,
* why does this work?
* and, why does my fillup/range seem much closer to the stated capacity this way?

 

It’s bad to get gasoline in the charky canister becos if you calculate mpgs by how much gallons you put in or fill up, the math will be off because the gas that goes in the canister doesn’t get burn.

 

My drill:

Tap pump with phone pay
Select grade (regular)
Lift off nozzle and remove gas cap
Pump gas till it shuts off
Replace gas cap and stow nozzle
Take receipt and git

 

You don’t even have to select grade? I need pumps like yours!

 

Thanks for the edit, fixed.

 

Yep, when the auto-shutoff pops off...DONE...no "topping off"...not worth the risk of getting gas in the evap canister.