I am a relatively new Prius owner (2006) and I have just over 7,000 miles. I love the car, but I am having an issue that I'm sure members of this forum can address.

I understand the fuel tank has a bladder system thus providing varying "fuel capacities." My concern, however, is recently I have been experiencing a smaller and smaller fuel tank range over the past 3-4 fill ups. Initially, I waited until the fuel gauge would show one or two bars and then fill up. This would generally result in my putting in somewhere between 7.75 and 9.75 gallons and my range would vary from 400 to 510 miles.

So, I thought I would see what my maximum range is on a tank and have waited until the last bar begins flashing. in the last three fill ups, I can't get more than 9.4 gallons into the tank. Overall my mpg haven't changed (51-54) but I'm wondering where the other 2.5 - 3.0 gallons of my fuel tank went (100 - 150 additional range miles).

Is this something I need to take in to the dealership to address or are there things I can do to "re-inflate" the bladder? Is it even a bladder issue?

Your thoughts are appreciated.

UTpiper

 

It is maddening, the variable size fuel tank. If you check my top-off records, you'll see considerable variability. I even tried a double fillup: (1) fill-up, and (2) drive 5 miles for second top-off. Nothing seems to work.

I suspect air or gas entrapment between the outside of the bladder and the tank walls. But to test this hypothsis, I have to put a vent in the gas tube and I'm not ready to go that far. So instead, I just 'live with it.'

Since you are under warranty, go for it. You have nothing to lose.

Bob Wilson

ps. I've only tried to 'drive dry' two tanks and got 566 and 562 miles. The 566 mile tank ended not with the blinking pip but because the next 25 miles were at 1:00 AM and it was raining, 11.2 gallons to refil. The second 'drive dry' had the empty tank indicators and took 10.6.

 

Same issue here. My Prius just had its 1 year anniversary.(38k miles) and the very first tank of gas was 10.4 gallons. since then i have never put more than 9.45 gallons in it. And yes even when i ran it out of gas and had to drive almost 12 miles on ev, i only put in 9.3 in it. Weird...the way i look at it, as long as im still getting the milage, so what, ill just have to fill up a day earlier is all. Dean

 

I'm still in the dark as to why it's a "bladder" - and by the way the title of this thread is pretty funny. UTpiper, I hope you've taken care of everything.

I've read the bladder "reduces vaporization." But vaporization where and to what benefit? The fuel vaporizes.. in the tank? Is that bad? Somewhere further down the line? Why would a bladder affect anything after the tank?

I also guessed, but perhaps this isn't true at all, that if the fuel tank is a bladder, then filling it from the pump causes it to stretch a little to hold all the gas, possibly reducing the work needed by the fuel pump at the beginning of the tank because the pressure coming from the bladder (which you really got for free from the pump) is pushing the fuel into the car. Or is that just overestimating the Prius designers?

 

Hi,

The USA Prius has a fuel bladder within the metal tank and an arrangements of tubes and valves that controls the flow of trapped gas vapors in the space between the bladder, carbon cannister, a 'Purge line' that goes to the throttle assembly and also operates a "hydrocarbon absorbtion control" for a valve on one of the catalytic converters. I don't know of any good description of how the assembly works although the maintenance manual is good about how to find and fix problems.

We know that in the NHW11 Prius, 01-03, there is a valve on the catalytic converter that can go bad and result in an expensive repair action that is part of the system. There have been suggestions to lubricate it with a high temperature lubricant. My understanding is the 04-current Prius uses the coolant canister to avoid this hydrocarbon absorbtion assembly but I don't have any details.

There are a lot of unknows about the fuel system and speculations that the bladder volume is temperature sensitive. One of these days, I plan to put a low-pressure range gage to measure the pressure in the space between the bladder and tank. But the effect, a variable size fuel tank, has been pretty well confirmed.

This means individual, calculated fuel tank mileage is subject to substantial variations (see my vehicle log.) But the overall average across multiple tanks is accurate. It is one of those 'nagging problems.'

Bob Wilson

 

That's really interesting. So are you saying the vaporized gas is actually diverted and burned separately to power separate systems not driven by the rest of the vehicle's power system/drivetrain? And the volume of vaporized gas surrounding the bladder actually dictates the pressure pushing on the bladder?

 

Hi,

From what I can tell, that seems to be part and partial of how the system works. Unfortunately the maintenance manuals tell us "what" not "how."

Traditional vehicle tanks have a hose that goes to a carbon canister. As the tank fills up, the carbon canister absorbs the fumes and later, they are purged when low pressure in the intake manifold opens a 'fresh air' valve. A modification of that system still exists in the Prius but the valves appear to be computer controlled.

The maintenance manual has a schematic of the fuel system pipes and valves but lacks the 'principles of operation' that would clarify what is going on. My thinking is a low-pressure gage, 0-10 psi, should let us find out if the pressure between the bladder and tank increases during fill-up . . . sometimes.

Working on the hypothsis that engine operation might 'normalize' the pressure, I tried topping off the tank, driving 5 miles and topping it off again. I treated these two as one ''tank." But the same variability remained.

The next test, when I get around to it, will use a pressure gage on one of the hoses to the space between the bladder and tank. This will hopefully tell us if vapor pressure here has anything to do with it.

Bob Wilson

 

I just thought I would revisit this old thread now that I am two - three months into the winter driving season here in the Salt Lake area. Based on my own anecdotal evidence, it certainly appears the "flexible" bladder is impacted by the outside ambient temperature. Since Thanksgiving the highest capacity fill up I was able to acheive was just under 8.5 gallons and that was with the last fuel indicator light blinking for about 20 miles before I filled up and with my inadvertant overfill of the gas tank (the auto shut off on the pump didn't).

The only frustration with the increasingly limited fuel tank capacity is the need to fill up after just 300 - 350 miles (generally when there are two bars left on the gage). Since I can routinely put 200-275 miles on per day, I am back to filling up every couple of days, just like when I drove my Subaru Outback. Of course, my calculated mileage is much improved (2.5X) but the psychological impact of frequent fillups is frustrating. Hopefully as spring and summer approach, the bladder will loosen(?) and I can get back to achieving 550 - 625 miles per tank.

FWIW

UTpiper

 

OWCH! This is wrong.

I live in N. Alabama so we only have cold spells of a couple of days and then we're back in the 50s-60s. I can't replicate the problem but it sounds terribly wrong.

Bob Wilson