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MPG is completely 'in the tank,' no apparent cause

Discussion in 'Gen 2 Prius Main Forum' started by pmelc1, Jan 17, 2024.

  1. pmelc1

    pmelc1 Junior Member

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    I looked over the sticky Weak/Max EV state post and I may be dealing with that.

    My combimeter was out for 7 years so I wasn't tracking mileage, though I suspect it was in the high 30s / low 40s. I only reached 50ish for brief periods, maybe 10% of the time prior to combimeter failing.

    Recently installed a new battery and got my combimeter repaired, and replaced fuel pump/tank (pretty high mileage). Mileage on the drive home was great [for me], averaging 45-46mpg, which I was happy with.

    The next day it was much worse (mid/high 30s), and today it's atrocious, averaging around 22-25mpg. The car feels slightly 'heavier' (like it does when hybrid battery is low / relying on ICE more) and ICE seems to kick on sooner. Hybrid battery bars are mid-range / in the blue. When I don't hit the gas/coast, mileage goes up to 99mpg.

    I ran Dr. Prius app before and after the recent repair the and there don't seem to be any issues with the hybrid battery, no engine codes other than catalytic converter below efficiency (original stolen, have aftermarket cat), and ABS pump / module (been driving without it for a decade, may finally get it replaced since I have the part on hand from a junked Prius).

    I was doing 12V resets almost weekly due to worn-down battery posts and a finicky positive terminal / some other electrical issue that was causing me to lose 12V connection constantly, and I know 12V resets are thought to be linked to Weak EV State.

    Only other variable is that it's 10-12 degrees colder today (mid/upper 20s Fahrenheit) since the day I got it repaired. I had the heat running both days. Planning a road trip way south soon so that will allow me to compare.

    Anyone deal with this / did it get better or do you have a solution?
     
    #1 pmelc1, Jan 17, 2024
    Last edited: Jan 17, 2024
  2. PriusCamper

    PriusCamper Senior Member

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    Usually I quickly resolve people's low MPG complaints by adding air to all the tires and there's usually one tire that's way lower on air than all the other ones, so the MPG plummets in big ways. So that's the first step, to inflate them as much as you're comfortable with but more is way better MPG.

    Of course in your case, this is just the freezing weather. Your battery ECU protects your hybrid battery in sub-freezing weather by using it way less often. Your best MPG happens at about 75-80 degrees F. at about 45mph.
     
    #2 PriusCamper, Jan 17, 2024
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  3. Frontporch

    Frontporch Member

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    I wonder if there is some learning that happens when you have battery issues. I had a dead battery recently twice while I was working on my car and it seemed to get worse fuel economy and slowly improve over time. Maybe it’s just me but my driving doesn’t change much
     
    #3 Frontporch, Jan 17, 2024
  4. PriusCamper

    PriusCamper Senior Member

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    Disconnecting the 12v resets all the engine settings back to default, so yes... But when you're way below freezing temps there's no such thing as getting good MPG.
     
    #4 PriusCamper, Jan 17, 2024
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  5. Frontporch

    Frontporch Member

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    Yes. I do expect at least 5 mpg less even if I don’t idle for 10 minutes to melt ice like I did this morning.
     
    #5 Frontporch, Jan 17, 2024
  6. PriusCamper

    PriusCamper Senior Member

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    I'm talking about temps cold enough that ice doesn't melt... Once you get into single digits or below zero your battery ECU is going to cut back use of your hybrid battery to less than 10% in some case and you'll be down to 20mpg no problem.
     
    #6 PriusCamper, Jan 17, 2024
  7. mr_guy_mann

    mr_guy_mann Senior Member

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    If you want to do anything with MPG you need data - keep a fuel log with miles, gallons, and temperatures. Do that for a couple months and then see what you're really getting.

    Having a number of repairs at once and then have 12V resets here and there while relying on the MFD reading makes it tough to know if the data is any good.

    Posted via the PriusChat mobile app.
     
    #7 mr_guy_mann, Jan 19, 2024
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