Average fuel efficiency steadily decreasing

Discussion in 'Gen 3 Prius Fuel Economy' started by Charlie Monk, Jan 1, 2018.

  1. Charlie Monk

    Charlie Monk New Member

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    We have noticed the average fuel efficiency of our Prius slowly decreasing over the last 6 months. We have had this car for over 5 years (and a gen 2 Prius before that) and have become used to a range of 55 to 65 mpg over varying types of journey. This started to drop during the summer and the car now struggles to get anywhere near 55 mpg and currently averaging about 53 mpg over varied driving. We have not changed our location since 3 years ago. Driving style and type of journey has not changed. No reason to suspect tyres. Weather has varied from hot/dry to cold/wet over the period so nothing to do with weather. Toyota were not able to suggest anything helpful. Any other ideas? Battery?
     
  2. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    Could be the season change, that imposes a penalty every year. Even if you're hypermiling your guts out, the car's reading the (dropping) ambient temperature, and varying the warm-up regimen accordingly. So both the weather and the car's programming will reduce mpg.

    Try dropping the cabin temp down, or shutting the system right off at red lights? That'll help trigger engine shut-down when the car's partially warmed.

    Also: how many miles on it?
     
  3. grid

    grid Member

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    When was the last time you changed engine air filter? All cars that I've own, when mpg dips, 9 out of 10 times the engine air filter was the culprit. Check that first before hittting the panic button.
     
  4. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    Just a note: op's mpg numbers are more-n-likely miles per imperial gallon. For example 53 mpg (imperial) equates to 44 mpg (US).

    Still, that sounds a lot like our summer vs winter numbers, and we're similar (west coast) clime.

    upload_2018-1-1_10-29-7.png
     
    #4 Mendel Leisk, Jan 1, 2018
    Last edited: Jan 1, 2018
  5. Charlie Monk

    Charlie Monk New Member

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    Thanks for the engine air filter tip. I can see it should be replaced at a service every 40,000 miles so should have been done at 80,000. We are on 100,000 now. I will check when it was last replaced. Are there any other long term items that might be causing this?

    Please note this has been going on since 6 months ago so it is not seasonal as suggested above. We are very aware of all the normal fuel saving techniques. This is a changed pattern after many years of regular results.
     
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  6. Charlie Monk

    Charlie Monk New Member

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    Yes - Imperial Gallons when I say 'mpg'
     
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  7. grid

    grid Member

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    Measure the 12v battery voltage ; after your next commute power car down, wait for 30 min and measure the voltage, if it's 12.6vdc or greater, then battery appears fine for now. Also wait over night and measure voltage again next morning.

    Could be 12v depleting slowly. When I left parking lights on for 4 hours by accident, I turned the engine over and hv battery went from 8 bars to 2 bars within 5 minutes in park, the 12v battery sucked hv battery to be recharged. This was the only time I ever saw hv battery depleted quickly.
     
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  8. JimboPalmer

    JimboPalmer Friend to those who want no friends

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    Some (by no means all) of the failure modes of the 12 volt battery have the inverter trying to recharge it continuously when in Ready. This electrical draw pulls power, so reduces mileage. You may wish to check the battery voltage when sitting over 8 hours but before putting it in Ready. (a zero cost rough estimate is to operate the power windows in ACC, then in Ready. If the speed is WAY different, you need a 12 volt battery) I do not know British brands of batteries.

    http://www.amazon.co.uk/Optima-Japanese-Terminals-Reversed-8072-176/dp/B005CU65SO
     
  9. bisco

    bisco cookie crumbler

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    welcome!
    there are so many items on prius that can have a small effect.
    it's difficult to know for sure how your mpg's are doing if you haven't kept long term tank records.
    from tires, brakes and wheel bearings, to fuel, oil, throttle body, maf, egr and everything between. it's a slow process of elimination.
    and with such a small drop, it could be anything or everything.
    all the best!(y)
     
  10. Sam Spade

    Sam Spade Senior Member

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    A good detective does NOT automatically rule ANYTHING out without good evidence.
    At this point, you have NO good evidence of anything.

    Cold weather uses more fuel. Not much more but the change you noted isn't much either.

    At 5 years old, it could be a LOT of things. Many of them just due to tiny changes because of age that add up over time.

    If you must obsess over this ~2 MPG change, then test/check EVERYTHING.
    Tires, brakes, pcv system, throttle body, spark plugs, exhaust blockage, plugged air filter, battery cooling and read codes to see if any "hidden" ones are set.

    Or let a shop give it a good going over.
    Or trade it in on a new one.

    Or don't worry about it until it gets worse........if it does.
    It might recover next spring.
     
  11. Charlie Monk

    Charlie Monk New Member

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    I tried measuring the 12v battery. I have not driven it for 4 days (easily the best way to save fuel :) It measures 12.2v. Does not sound like a problem(?)

    I would not call this a 'small drop' and it seems to be a steady decline this year. Previous winters we would have been disappointed if it dropped to 55mpg. Usually nearer 60. Now 53 is best case. Car gets all the full service treatment at the Toyota dealer, which should rule out many of the wear and tear items. Perhaps maf, egr etc are worth checking.

    Thanks
     
  12. Charlie Monk

    Charlie Monk New Member

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    To save people reading back to the start of this and missing my point, I must stress - This is NOT a winter problem, it started in the middle of the Summer. It is NOT a change from 55mpg to 53. It is from 60 or more to 53. (55 was only mentioned as the previous worst case).
     
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  13. JimboPalmer

    JimboPalmer Friend to those who want no friends

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    While it is a bad result, it does not seem like the symptom of gratuitous charging.
     
  14. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    12.2 volts is a battery in significant trouble. If multiple days idle is a regular occurrence, I'd invest in a smart charger. That's precisely our situation btw, and I hook it up any day the car's not going to see use, and leave it on (which you can do with no ill effects) till the next use.
     
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  15. grid

    grid Member

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    In your shoes, Id go and buy chevron complete fuel system cleaner concentrated and dump it in the fuel tank before filling up all the way. It's $4.97 at Walmart. Sounds like proper maintenance been done and a dosage of that bottle might help.

    Where do you fill up on gas?
     
  16. bisco

    bisco cookie crumbler

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    agree, your 12v needs charging and possible replacement. after that, you can eliminate it from the list of potential problems.
     
  17. Charlie Monk

    Charlie Monk New Member

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    Sounds like I should investigate the 12v battery further. Maybe fuel system. (I am in UK so where I get gas probably won't help).
    Thanks for the suggestions
     
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  18. Sam Spade

    Sam Spade Senior Member

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    No 12.2 likely indicates that there IS a problem but it might not affect your gas mileage.
    A healthy, fully charged AGM battery should maintain a resting voltage up around 12.7 volts for WEEKS.
    12.2 likely indicates that it is about 50% discharged and that is not normal.

    AND.....the second sentence quoted above tends to show that you don't "get" what I said about not automatically ruling out ANYTHING.

    Good luck.
     
  19. James D Smith

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    Mileage on my 2010 Prius was fairly steady 43-44 mpg through Sept 2015, then the average mileage took a noticeable drop, almost 2 mpg, after replacing Bridgestone tires with Michelin Defenders. Replaced the air filter at 46661 and 80126 miles.
     
  20. bisco

    bisco cookie crumbler

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    that sounds right. why would you put on defenders if you don't want to lose mpg's?
     
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