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Fuel economy decrease after replacing fuel pump

Discussion in 'Gen 3 Prius Fuel Economy' started by newguywithpups, Nov 23, 2021.

  1. newguywithpups

    newguywithpups Junior Member

    Joined:
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    Location:
    Seattle(ish), WA
    Vehicle:
    2011 Prius
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    N/A
    When I bought my Prius used a couple years ago it had a P0171 "system too lean" error.

    Over the past couple years I tried different fixes that had no effect - replacing MAF sensor, changing spark plugs and fuel injectors. They also didn't seem to affect the fuel economy, which hovered around 46mpg as measured by the amount driven (odometer) / amount of fuel refilled.

    A couple months ago I tried exchanging the fuel pump with a used one from eBay - immediately my LTFT dropped from ~+30% to around 0, and since then the check engine light hasn't come back on, which all seems to indicate the original P0171 error is fixed.

    However, exactly after the fuel pump replacement, my fuel economy has sunk to around 42mpg.

    From other threads where people fixed their P0171 by replacing the fuel pump, they reported significant increases in fuel economy, so I was expecting the same.

    Any ideas what the issue might be? Maybe I should've just kept the error. I don't have smog checks in my state anyway. :( It's like I paid all this money to troubleshoot and make the error go away, only to effectively decrease my fuel economy and nothing more...
     
    #1 newguywithpups, Nov 23, 2021
  2. ASRDogman

    ASRDogman Senior Member

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    Perhaps the "used" is weak and not able to get the pressure up where it should be.
     
    #2 ASRDogman, Nov 23, 2021
  3. douglasjre

    douglasjre Senior Member

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    You probably have a small evap leak that will show up after a couple more drive cycles. If it does you'll have to put some silicone grease on the seal
     
    #3 douglasjre, Nov 24, 2021
  4. Melthias

    Melthias Junior Member

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    Vehicle:
    2012 Prius
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    Three
    This may have happened to me too. I replaced the fuel pump in my 2012 with a cheap one when I was trying to track down issues with misfires. It turned out to be counterfeit plugs, and putting the old ones in fixed that. However, fuel economy is down from about 52mpg to 47mpg. Did you ever resolve this?
     
    #4 Melthias, Sep 19, 2023
  5. nicoj36

    nicoj36 Active Member

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    Put a bottle of fuel-treatment and do this reset
     
    #5 nicoj36, Sep 20, 2023
  6. Melthias

    Melthias Junior Member

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    I have done a seafoam fuel-treatment and yes, I have done that reset.
     
    #6 Melthias, Sep 20, 2023
  7. douglasjre

    douglasjre Senior Member

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    Where'd u get the plugs from
     
    #7 douglasjre, Sep 21, 2023
  8. Melthias

    Melthias Junior Member

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    Location:
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    Vehicle:
    2012 Prius
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    Three
    I got new plugs from Ebay, but on close inspection they appear to be counterfeit. I figured out something was up after uncontrollable misfires in cylinder #4. I was just getting back from a 500mi trip when things started going bad, and I managed to get the last few miles home (mostly downhill. The engine was a JDM low mileage swap that I had just completed. On the long drive I was getting a bit over 50mpg and going about 70mph much of the way. With the newer fuel pump in and the old plugs I can't seem to get above 46mpg under the same conditions. I do not have a baseline for the older plugs, but getting new ones did not seem like a bad idea. That is, until the misfires. I have some Denso twin tips on order from rock auto now, and hopefully that will help out.
     
    #8 Melthias, Sep 22, 2023
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