I Want to Determine How Much Fuel is Left With One Flashing Blin

Discussion in 'Gen 2 Prius Technical Discussion' started by ydpplqbd, Nov 4, 2019.

  1. ydpplqbd

    ydpplqbd Junior Member

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    I really want to find out how much fuel I have left when I have one ("1") flashing blin left on the fuel gauge (AKA low fuel indicator). I have read here that running your Prius out of fuel should be avoided. As a result, I am considering doing the following:

    1. Drive my Prius to 30 miles after the "blin" begins to flash (the point where I have been refueling);

    2. Shut the Prius down in my driveway on a level spot and install my electric fuel transfer device (essentially an electric fuel pump mounted to a piece of plywood with input hose, output hose and independent fuel filter on input hose) which involves putting one hose down the fuel tank opening, attach pump to 12v from Prius and ground from Prius.*

    3. Pump remaining fuel into an empty fuel can which is on a scale.

    4. Weigh the remaining fuel and then put fuel back into the Prius tank.

    Question to all: am I going to damage something using this course of action?

    Please provide your input and thoughts.

    Many thanks.

    *- I will likely ground the metal safety can to the Prius also.
     
  2. SFO

    SFO Senior Member

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    The amount remaining will vary as the gauge isn't accurate, many have been stranded on one or more bars before.

    Just fill the gas tank when it reaches 2 bars and you won't have to worry about it :whistle:
     
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  3. ydpplqbd

    ydpplqbd Junior Member

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    Hey thanks for the response. I am scrupulous about keeping track of gas mileage and miles drive between fill-ups. I am planning on taking my Prius on a couple thousand mile journey in the near future. I often drive late at night and early morning on secondary roads. As a result, fuel stations are not always open when I need to refuel. As a result, I really need to know how much fuel remains in my tank with one flashing blin.

    Any comments on possible damage by pumping remaining fuel out with one flashing blin remaining?
     
    #3 ydpplqbd, Nov 4, 2019
    Last edited: Nov 4, 2019
  4. VFerdman

    VFerdman Senior Member

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    I think you have a solid plan of action. I do not see any issues. One thing I would suggest is to have a 5 gallon can of fuel filled up on hand during this experiment. There is some logic in the Prius that will not allow the car to start after running out of fuel until it has at least 3-5 gallons added. If you trigger this logic with your experiment you may need that amount of fuel to get it to start again. I have no idea if your experiment will trigger this condition, but to be safe I would either have 5 gallons of fuel on hand or be ready to go get some (like another running vehicle and a 5 gallon gas can).

    More notes. North American versions of Gen 2 Prius have a flexible fuel bladder in the gas tank. This does not always expand and contract as designed and consequently has different capacity in cold vs. warm weather. This is really one of the most annoying quirks of this car. Your range will differ in cold weather vs. warm weather. Just something to keep in mind. I am not sure how this will effect your particular experiment. It may not effect it at all, but I do not know how the gas gauge works as far as logic. You may want to conduct this experiment in different weather conditions and compare the results.

    Good luck and keep us posted. I am very interested in your results.
     
  5. ydpplqbd

    ydpplqbd Junior Member

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    VFerdman, many thanks for the heads-up on the no start after running out of fuel unless about 5 gallons is put into the tank. And, I will definitely update the group on my progress with this experiment.
     
    #5 ydpplqbd, Nov 4, 2019
    Last edited: Nov 4, 2019
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  6. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    Uh oh...

    Yes, running out of gas should be avoided. The 2nd gen has a couple of very good reasons: the bladder tank, which means you're playing Russian Roulette with low fuel, and the car's propensity to switch to electric-only mode if you run right dry, and then run the battery down dangerously low.

    Good luck with that: most gas tank fill pipes have convolutions expressly to defeat what you're trying to do.

    Maybe ask yourself what you're trying to achieve? Stretching tanks? What does that achieve? And what are the risks?
     
  7. ydpplqbd

    ydpplqbd Junior Member

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    Thanks for heads-up on the filler neck convolutions. I found a pic of this item from ebay. See attached pic. It appears that the diameter of tube reduces significantly and also looks like a 90 degree bend.

    If I cant pump the fuel out directly from the tank because of the filler neck obstructions, then is there a better access point for me to pump out the remaining fuel. As an alternative, what comes to mind is unhooking a fuel connection under the hood and then powering the fuel pump (with a motorcycle battery) with car completely off (or perhaps disconnecting low and high voltage batteries) and fuel pump connector disconnected.
     

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  8. VFerdman

    VFerdman Senior Member

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    Knowledge is power. I would want to know the answer to OP's question myself. I usually let the tank go to either one pip or a blinking pip before filling up. Not sure what I am trying to achieve besides visiting the gas station less often (not a fan, think it's a necessary waste of time and reducing it is a good thing). I never ran my Prius out of gas, nor do I want to, but knowing the safety zone of range once the blinking pip appears is a very useful piece of information.
     
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  9. ydpplqbd

    ydpplqbd Junior Member

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    Agreed. +10
     
  10. spdracrm3

    spdracrm3 New Member

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    Yea we all want to know but i think like mentioned above it's not going to be a consistant volume due to bladder tank .
    I just got my $400 02 prius in July so it's my first winter driving it ,parked in a garage at night and sits outside at work but not even down to freezing in portland but I've lost 90mi range already (410 per fillup at one pip summer to about 320 at one pip now). And it seems to drop faster from two to one to flashing pip much faster in the cold. Came home from work couple weeks ago and it had just dropped to two pip , next day went to go to a local hiking trail 15miles from my house and just pulled into parking lot and it starts flashing one pip in 15miles ,this was my first time being down to it flashing needless to say nervously returned to town while engine was warm I made it 18 miles and it only took 7.5 gal ,what.....

    I just want to replace this silly tank with the European non bladder tank so I know what's in the tank for sure, no more guessometer

    SM-G960U ?
     
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  11. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    If you've got an idea of your mpg, do the math as to what your range should be, with say 8 or 9 gallons, reset trip meter when you fill up, keep an eye on that trip meter, and the other eye on the gas gauge.
     
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  12. ydpplqbd

    ydpplqbd Junior Member

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    Last two fill-ups were at 7.75 and 7.5 gallons. I had traveled exactly 352 miles before each fill-up. It is hard for me to believe that the bladder has shrunk from 11.9 gallons to 7.5 gallons (back of the envelope math says that this would be more than 33% shrinkage).

    PS Even if the shrinkage varies by temps, I can measure and analyze so that I can see effect of temperature.
     
  13. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    If it had shrunk that amount you would have run out of gas twice. You want a bit of a buffer. Too: if you fill up sooner, but while you happen to be passing by a gas station, maybe that's better than a last minute desperation trip for gas only?

    What it comes down too is how much you want to risk being stranded, vs a few extra gas station stops.
     
    #13 Mendel Leisk, Nov 4, 2019
    Last edited: Nov 4, 2019
  14. frodoz737

    frodoz737 Top Wrench

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    Considering all variables, the fact still remains a 13 YO bladder tank does not provide a constant. The experience provided here suggests your "experiment" unwise. More power to ya, but you are over thinking this ydpplqbd.
     
  15. ydpplqbd

    ydpplqbd Junior Member

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    IMHO, the relevant questions are:

    1. How does tank capacity correlate with average temperatures; and
    2. Currently (and over the next 6 or 9 months), what are minimum and maximum tank capacity for my Prius.
     
  16. frodoz737

    frodoz737 Top Wrench

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    There are many, many threads about this. IF you are truly determined to repeat earlier experiments first hand...just make sure you have that 5 gallon can full and in vehicle.
     
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  17. spdracrm3

    spdracrm3 New Member

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    I've never had more than 8.0gal in put in (we dont pump our own gas here) ,there is a post about having to put the last two or so gallons in very very slowly after it has clicked off allowing bladder to stretch inside, but who has time for that.

    Agree you will have to know what cold remainder and warm weather remainder is but even that wint be very acurate

    SM-G960U ?
     
  18. VFerdman

    VFerdman Senior Member

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    I don't think the experiment is pointless. No one here has mentioned first hand knowledge of how the low fuel warning is generated logically in the car's "brain". I don't know this information and as far as I know no other participants in this thread do either. Yes, the bladder shrinks and expands differently in different seasonal conditions and therefore provides various range values for the "full" tank. That much we all agree on. BUT! who knows how and when the car generates the "low fuel" (blinking pip) warning? Is it based on an estimate based on the whole tank? Is there an actual low fuel level sensor (float/sending unit), is there another way the car keeps track of the fuel level in the tank? There could be a very well designed method (using a sensor/float/sending unit) to trigger the warning correctly regardless of the bladder's feelings about the weather. We do not know this and this experiment can shed a bit of light on the behavior of the system, if not the actual design of same. The experiment just needs to be designed with a certain methodology. I would perform it the way the OP suggests and do it in the winter, spring, summer and fall. Yes, a long-term enterprise, but that's the way of science sometimes. We could also try and find someone whith actually has first hand knowledge of this system and have them enlighten us, but I think that is a long shot (they may be a Japanese speaker).

    So, I think it's worth the trouble to try and figure this out empirically. Do not confuse this experiment with the one attempting to figure out the bladder's capacity. There is a subtle difference as there may actually be a special and separate system in place in this car to help prevent running out of fuel.
     
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  19. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Each and every tank, the bladder will have contracted a different amount based on:
    1) air temperature;
    2) how the bladder folded, so how much fuel is trapped in folds; and
    3) how much air is blocking fast fill ups

    The North American Gen 2 Prius is one of the few cars where you can get more fuel in in the heat of the day. (Normally fuel is denser the colder it is)

    No amount of measurement will improve the accuracy of the 'guess gauge'.

    The tank may take as few as 7 gallons and as much as 10.5, it really is changing size through the year. (MA may get smaller and stay smaller than MS.)

    If you run out of gas:

    Stop immediately, you can add gas, recharging the HV Battery is a Toyota Distributor level problem (lots of towing and waiting)

    You must add at least 3 gallons of gas before you power back up the car, the computer will insist it is empty,even if full, if you 'check' after each gallon.

    If you try and fail to start 3 times, it will set a code the dealer needs to clear. (less towing and less time than running the HV Battery down but still pricey)

    This is the best advice I have:

    If your mother in law is in the car, get gas with three pips, running out does not bear thinking of.
    If your spouse is in the car consider 2 pips Empty, you will never hear the end of it.
    Solo or with good friends, one pip is time to get gas.
    If the pip starts blinking, get gas RIGHT NOW. (I have run out within 8 miles of it starting blinking)

    I hope this helps
     
  20. sam spade 2

    sam spade 2 Senior Member

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    There is an old saying "There are no stupid questions." but this one comes REAL close.
    And it has been asked in other forms many times too.

    Just PUT GAS IN IT at the 1/4 mark and there won't be a problem with running out.
    (That would be one or two "pips" showing on the gauge.)

    If you are in an area where stations are scarce.......do it sooner.

    Your time will be MUCH better spent on something else.
     
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