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Comp MPG way off, Fuel Flow Sensor??

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by ggarman, Jun 6, 2006.

  1. ggarman

    ggarman Junior Member

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    I have a '03 and here recently the car is telling me I'm getting MPG that is nearly 10 MPG greater than what I calculate. My guess is that whatever sensor watches the amount of fuel consumed is 'slowing down' which would give me greater MPG.

    I know my calcs are correct, or at least as correct as you can get. I drive the car back and forth to work everyday on the same route and recently the MPG just started climbing, I expected a little bump from the warmer weather, but not this big of one, at least as far as the computer thinks.

    My fuel gauge seems to be acting correctly too, it's just the MPG's that are way off.

    I have the service manual for the car, but can't figure out where/how the fuel flow is derived, anybody know?

    Thanks,
    Greg
     
    #1 ggarman, Jun 6, 2006
  2. brandon

    brandon Member

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    How do you perform your calculations (equation)?
     
    #2 brandon, Jun 6, 2006
  3. keydiver

    keydiver New Member

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    I really don't think our car uses any kind of "flow sensor". I believe the ECU just adds up all the fuel injector pulsewidths, because it knows what size the injectors are, and can therefore calculate how much fuel it has injected per second/minute.
     
    #3 keydiver, Jun 6, 2006
  4. ggarman

    ggarman Junior Member

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    <div class='quotetop'>QUOTE(brandon @ Jun 6 2006, 10:03 AM) [snapback]266603[/snapback]</div>
    MPG = Miles Driven/Gallons Pumped

    I know there is some % error caused by not really knowing if you've completely filled the tank, but I always am on level ground and I 'click' the pump a few more times after the initial automatic shut-off to make sure it wasn't just bubbles causing it to shut-off.

    We just took a 500+ mile trip, 95% highway driving. I filled up as we were leaving, at our destination, and when we got back home. The first leg of the trip the computer said 45+ MPG and we calculated around 37 MPG and on the way home computer said 45+ and we calculated 40. I was very surprised with the first leg's MPG, the second legs discrepency is a little closer to what I'm used to seeing out of the car.

    On my drives to and from work, the computer is showing almost 49MPG and I calculate at around 40, it is really weird....I just wish I was really getting the mileage the computer is coming up with. :)

    I also have been resetting the Computer MPG at every fill-up.

    Thanks,
    Greg

    <div class='quotetop'>QUOTE(keydiver @ Jun 6 2006, 03:03 PM) [snapback]266795[/snapback]</div>
    If this is true then the computer having bad data probably isn't the culprit. If I'm thinking right, there is no way for anything to be wrong with the injectors, clogged or soemthing, because the car will detect this 'leaness' via the O2 sensor and richen the mixture back to normal.

    Something seems to definitly changed with the computer, it has never been this far off before and I caluclate at every fill-up. I usually calculate about 2-4MPG less than the computer.

    Hmmm.....

    Greg
     
    #4 ggarman, Jun 6, 2006
  5. Manksgloob

    Manksgloob Member

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    <div class='quotetop'>QUOTE(ggarman @ Jun 6 2006, 12:51 PM) [snapback]266818[/snapback]</div>
    Plus the fact there's an expandable "bladder" in the tank and so the capacity during one fillup might not be the same during another.
     
    #5 Manksgloob, Jun 6, 2006
  6. cdavid

    cdavid Member

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    I am very disappointed with the computed MPG figure. I've ran through about 5 tanks with my 06 now, and its always at least 2mpg below calculated. On my last tank, it showed 52.8 over the 475 miles, but when I filled up (had the flashing one bar) it was only 46.8, ouch! That's too far off. It cannot be relied on at all.
     
    #6 cdavid, Jun 6, 2006
  7. hdrygas

    hdrygas New Member

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    It does work better over time. The system learns. There are a few things that appear to me to throw it off a lot. They are early and late in the tank average. If you do really good during the first 1/4 of tank and thrash the rest you will (or at least I have) gotten a over estimation of MPG. As an Ob/Gyn I have occasion to have to do the Bonsai run into the hospital near the end of a great tank. There is no helping it, life is. That in my hands leads to a tank MPG lower than actual. I have wondered that if you drive the car in a "style" that is different than the "usual learned style" of the ECU that it causes a shift in the estimation of MPG reported on the MFD vs. the measured MPG. Just a thought.
     
    #7 hdrygas, Jun 6, 2006
  8. galaxee

    galaxee mostly benevolent

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    fuel flow is derived from injector pulse, injector volume and fuel pump flow.

    what's more precise? a coarsely calibrated fuel pump at the gas station or a finetuned fuel system? gas station pumps are calibrated probably to milliiters, but the fuel system measures in droplets, or probably microliters.

    the fuel system's calculations probably contain less error than the gas left in your tank plus whatever error is going on in the pump measurement plus the fuel bladder BS. it all contains error. the MFD does not report a precise measurement and neither do your calculated tank MPGs. it's a matter of what has a lower margin of error.

    in short, there's nothing that could be slowing down. and if there was, you'd probably be seeing some major performance issues as a result.
     
    #8 galaxee, Jun 7, 2006
  9. ggarman

    ggarman Junior Member

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    <div class='quotetop'>QUOTE(galaxee @ Jun 7 2006, 12:07 AM) [snapback]267083[/snapback]</div>
    I agree with you completely, my real issue is that fact that my margin of error has greatly increased over the past few months...something is different, therefore I'm little concerned.

    Probably as most of you do, I use my MPG data to give me an idea of the health of the system. So, with with the computer saying everything is great, but my calculated says not so great, this makes it a little difficult to gauge the health of the system.

    Thanks,
    Greg
     
    #9 ggarman, Jun 7, 2006
  10. McShemp

    McShemp New Member

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    <div class='quotetop'>QUOTE(cdavid @ Jun 6 2006, 08:19 PM) [snapback]266995[/snapback]</div>
    [sarcasm]OMG! Was that five whole tanks?[/sarcasm] :D

    You might want to reserve comments like "very disappointed" and "It cannot be relied on at all" regarding the FE until after you've put on some substantial miles (10 or 20k). This will give you more time to reflect (while upping your sample size). Doing otherwise may make you sound foolish.
     
    #10 McShemp, Jun 7, 2006
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