Declining MPG (With charts)

Discussion in 'Gen 2 Prius Fuel Economy' started by jsinclair92, Feb 25, 2019.

  1. jsinclair92

    jsinclair92 Junior Member

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    Hi All,

    I have tracked my MPG since the day I bought my 2008 Toyota Prius. I have been extremely happy with the car and have had little to no problems with it. However, I have noticed that my MPG has been steadily and slightly declining. What are the common causes of this?

    A few things to note:
    • MIlage: 162,710
    • I take routine maintenance very seriously and use OE replacement parts
    • I have used Mobil 1 synthetic oil since day 1, and change every 5k miles
    • I rotate, balance and replace my tires regularly
    • My 12v battery is only a few months old
    • I have not changed driving habits, or routes since I have had the car
    • I hardly ever use AC, I like to ride with the windows down
    mpg.jpg

    service.jpg
     
    #1 jsinclair92, Feb 25, 2019
    Last edited: Feb 25, 2019
  2. mpg_numbers_guy

    mpg_numbers_guy Junior Member

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    Your hybrid battery may be dying. As the hybrid battery ages, it grows weaker and requires the engine to spend more time charging the battery, which reduces fuel economy.

    There may be other issues that more experienced Prius owners may know, but that seems to be the most probable cause.
     
  3. Robert Holt

    Robert Holt Senior Member

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    I think we experienced poorer mileage as our 12-volt battery was failing after 5 years—what seemed to happen is that the system voltage was staying in the “charging” range of 14.7+/-0.1 volt and that helped keep the ICE more often “on”. After replacing the 12-volt, the system reverted to cycling between the 14.7-volt and the lower 13.6+/-0.1 volt state that indicates the 12-volt battery is fully charged and the overall draw of the 12-volt accessories is low, and our mpg returned to normal levels.
    (I Should add that on older vehicles the simple wear and tear of increased mielage would result in worn rings, pistons, and cylinder walls, resulting in lower compression that could ultimately reduce the overall efficiency of the ICE, so lower mpg over a car’s lifespan was expected.)
     
    #3 Robert Holt, Feb 25, 2019
    Last edited: Feb 25, 2019
  4. bisco

    bisco cookie crumbler

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    Agree with post number two, could be a weakening hybrid battery.
    Have you done any engine maintenance besides plugs and fluids?
     
  5. sam spade 2

    sam spade 2 Senior Member

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    The two primary causes are:
    It is 10 years old and has travelled 162,000 miles.
    Do you REALLY expect it to still perform like it is new ??

    There is a LOT of information about this scattered all over this forum in hundreds of theads.
    A little searching would likely yield all that you need to know.
     
  6. jsinclair92

    jsinclair92 Junior Member

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    I should have also noted that I have done a compression test and the results were surprisingly good, though I do not recall the exact figures.

    I have not done any engine maintenance besides fluids and plugs, yet.
     
  7. fotomoto

    fotomoto Senior Member

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    FTFY

    It's been slowing dying since the day it was made but as experience has shown us once they get past the 10-12 year time frame death can come at any time and decline becomes more noticeable in several ways: lower mpg's, less power, more frequent ICE start ups, SOC fluctuations, etc.
     
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  8. fotomoto

    fotomoto Senior Member

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    MAF sensor cleaning and run a few bottles of fuel injector cleaner is always good too.
     
    #8 fotomoto, Feb 25, 2019
    Last edited: Feb 25, 2019
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  9. bisco

    bisco cookie crumbler

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    throttle body, injectors, test the 12v to be sure, tires pressure, oil level, maybe think about checking inside battery case for corrosion
     
  10. srellim234

    srellim234 Senior Member

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    The system is most likely charging more often as the weakest cell is at a lower level charge and reaching the recharge threshold quicker than the rest of the cells. That's what the computer is responding to. Conditioning and balancing cells should restore some (not 100%) of the efficiency and help the hybrid battery last longer. See if someone in your area has a Prolong system or is very comfortable with cell balancing using hobby chargers and can help you out.
     
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  11. mpg_numbers_guy

    mpg_numbers_guy Junior Member

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    Yes, as long as proper maintenance has been performed. ;)
     
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  12. sam spade 2

    sam spade 2 Senior Member

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    Good luck with that.
    It's not realistic......with any vehicle.
     
  13. mpg_numbers_guy

    mpg_numbers_guy Junior Member

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    Every vehicle I've driven has been able to easily exceed it's EPA rating regardless of age and mileage..
     
  14. jsinclair92

    jsinclair92 Junior Member

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    I fully expect the vehicle to perform just like new or at least as close to new as I can keep it.

    Thanks to everyone for the great tips! I'll be busy this weekend looking into hybrid battery cell balance, and ensuring tb and injectors are clean as well.
     
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  15. sam spade 2

    sam spade 2 Senior Member

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    Which is quite different than saying that they all run exactly like they did when new.

    Some hold up better than others.
     
  16. bisco

    bisco cookie crumbler

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    Many older Prius perform very close to new.
    Another suspect is brake drag
     
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  17. mpg_numbers_guy

    mpg_numbers_guy Junior Member

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    Fuel economy is only one point, yes, and if you remember, that was the symptom being falsely diagnosed as normal aging of a car.

    You own a Toyota. Not sure why you disagree considering both Toyota and Honda have remarkable track records for reliability..
     
  18. sam spade 2

    sam spade 2 Senior Member

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    I disagree with the impression that you seem to be saying that an internal combustion engine should never wear out.
    And that it should run exactly like new right up to the time it falls apart.
    Neither of those things is true, no matter what the make and model.

    Some things are (reasonably) fixable and some are not.
    Most things that are "diagnosed" in places like this are just wild guesses.
    Some turn out to be right and some not.
     
  19. mpg_numbers_guy

    mpg_numbers_guy Junior Member

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    I agree...left to its own devices no ICE will last forever, or run like you. But if you properly maintain it and replace things as they wear out, they will last for quite a long time, and run almost like new. Maintenance says more about an engine's health than it's age.
     
  20. Zeppo Shanski

    Zeppo Shanski Member

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    My father had a 1st generation Chrysler mini-van, a Dodge Caravan, that he put over 385,000 miles on. He got a letter from Lee Iacocca congratulating him for running it so well. I did most all the maintenance on that vehicle. It ran just as well on it's last days as it did on it's first.


    Maintenance is key.
     
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