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How does fuel gauge on Gen 2 work?

Discussion in 'Gen 2 Prius Main Forum' started by VFerdman, Sep 1, 2018.

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  1. VFerdman

    VFerdman Senior Member

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    I wonder if someone has technical knowledge about how exactly does the fuel gauge work in Gen 2 Prius? We all know that the fuel gauge can be erratic sometimes and not consistent, but I would like to know how it actually works. Is there a sending unit in the tank that determines the approximate level? I imagine with a bladder that would not work out very well, so how does or North American bladder tank car determine how much fuel is in the tank to display those pips?

    The particular reason I am asking is this. My last fill up I had problems filling the tank. Usually those problems do not show up during summer when the bladder is pliable. In any case, last fill-up I had driven over 450 miles on the tank and was pretty sure I was down to fumes, although the last pip was not yet blinking. I knew my wife would be driving a good distance next day and decided to fill up before the pip blinking. I got about 2 gallons in and the pump stopped. I re-adjusted and got 2-3 more gallons in before the pump kicked out again. I knew I was close to empty before the fill-up, so I persisted and re-adjusted the nozzle again and tried again. This time it kicked out almost immediately. I only had less than gallons pumped at that point and I was not giving up. I re-adjusted again and this time the thing went all the way to... (gasp) 11 gallons total. I never pumped that much in one fill-up. low 10's is my max, but usually less than 10. But here is where I got REALLY surprised. As I pulled the nozzle out the gas started to gush out of the tank! that was new to me. Not a lot came out, just a few burps, and I was able to cap the tank and go on my merry way. Now I have almost 300 miles on this tank and the gas gauge is showing 9 out 10 pips. I am a bit worried that something went bad in the system and this is why I would like to know exactly how the system decides how many pips to display.

    If anyone knows, please enlighten me. I am a bit worried, but I think it will right itself out eventually. I just can not trust the pips on the gauge at the moment. I think I should be down 5-6 gallons from full. And given I was able to pump 11 gallons last time I should have about 5-6 gallons left and the gauge should be displaying 5 pips, maybe 6, maybe 4, but not 9!

    Thoughts?
     
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  2. DLC82SV

    DLC82SV Member

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    Just had the exact same thing happen to me. I went 545 miles on a 11 gal fill before it started blinking. Then I pumped 12.5 gallons. Just to have a good bit burp back out at me and down the side of the car only to have it start blinking this time at 435... Then I filled up this time and it barely held 10 gallons? Pretty bizarre...

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  3. bisco

    bisco cookie crumbler

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    better to trust average mpg than the fuel gauge.

    hopefully someone will have your answer, otherwise, it will be in the service manual.
     
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  4. padroo

    padroo Senior Member

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    I owned my 2008 Prius for 10 years and never liked the problem that the fuel bladder created being inconsistent taking fuel but I never had the problem you have. The sending unit in the tank is a float system like most cars but I think your problem is more likely the fuel bladder.
    If the bladder does not want to open back up the gauge will only read what the level was designed to read. Someone will read this and my guess they will say the bladders are starting to get stiffer with old age. It may be time for a new gas tank. This is just my opinion.
     
  5. VFerdman

    VFerdman Senior Member

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    If there is a float type sending unit, how can possibly work well in a bladder that's designed to always have a "full" level? I've been racking my brain on it for a while now. Also, this was the first time that this burping situation happened. Usually I just fill up and it stops the pump at something reasonable, if not always consistent. In the summer I have not had a problem before at all. Always takes a good amount of gas like 9-10 gallons. In the winter I contend with effectively a 9 gallon tank and usually I get about 8+ in it in the winter. But in the warm weather it's been fine. It was possible the pump was being silly, but I was using the pump I usually use at the gas station I usually use. It was not a new to me pump. This is why I wanted to know the technical details of how the fuel gauge worked. If it's a float, how can it determine the proper level if the bladder is constantly squeezing the air out and therefore making the level max? This is the whole purpose for the bladder, isn't it?
     
  6. Mendel Leisk

    Mendel Leisk Senior Member

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    The logic with the bladder tank was to reduce emissions; what a total design fail. :mad:
     
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  7. PriusCamper

    PriusCamper Senior Member

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    Fuel gauge doesn't work... It's a guess gauge... The equivalent of putting a float sensor inside a water balloon that's unaware that expansion of water balloon may change the readings.
     
  8. Harry Eaton

    Harry Eaton Junior Member

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    There is a small fixed, ridged "tank" adjacent to the bladder part of the tank. Fuel from the bottom of the bladder flows into this fixed tank which has a fluid level gauge. The top of the fluid level in this part of the tank sits at the level of the top of the bladder (which drops down as the bladder becomes less full). So the gauge works a whole lot like a standard fuel gauge. There is one slight difference in that there is a small pressure from the bladder which makes the level in the fixed tank a little different. This pressure changes somewhat with temperature so the MFD compensates the fuel gauge reading for temperature measured at the tank and also for vehicle inclination.

    The fluid surface area in the fixed tank is much smaller than the bladder or a normal gas tank, so the rate of evaporation is much lower which is the whole point of this system.

     
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