Sudden drop in fuel economy

Discussion in 'Gen 2 Prius Main Forum' started by PeterPrius2007, Sep 4, 2017.

  1. PeterPrius2007

    PeterPrius2007 Junior Member

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    My car used to get a very good 52 average mpg. After a gas fill up it has suddenly gone down to 36 mpg even after driving about 150 miles on this tank. It seems the gas motor is coming on much more frequently than it should now.
    What could have happened? Does the traction battery fail suddenly like that?
    What else could be the cause?
    I had lent the car to a friend who drives it more aggressively than my very gentle driving, but that shouldn't matter.

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  2. dolj

    dolj Senior Member

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    It could, but they would have to have driven it a lot for it to make a difference. If the friend manage to change the tank average significantly while driving it, then maybe.

    The battery could fail in a way where it significantly drops the MPG without setting a code initially. If that is what happened, expect a battery code soon.

    Could it possibly be bad gas, or high ethanol gas?

    Have you checked you tire pressures lately? Other things to check is the oil level, is it significantly over filled? If it is drain off the excess oil and then check and clean the throttle body and the mass air flow (MAF) sensor.
     
  3. paprius4030

    paprius4030 My first Prius

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    I would check the 12v. battery especially if it's the original. One sign of the 12v. battery going bad is the gas engine running extra to keep it charged. Since your mileage drop happened so suddenly maybe your last start was the one to start the downhill slide of your 12v. battery.
     
  4. PeterPrius2007

    PeterPrius2007 Junior Member

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    Haven't checked tire pressure lately but honestly I can't see that causing more than 1 or 2 mpg change or something like that. I can check the MAF.

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  5. dolj

    dolj Senior Member

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    While it would be a good idea to check the 12 V battery, in any case, I doubt it is the cause of the ICE (internal combustion engine) running frequently or constantly. For this to happen the HV battery wouldl have to be discharged from 6 bars to 2 bars first then the ICE will kick on frequently. For the 12 V battery to even be able to do this, it would need to be seriously bad shape. It is more likely to be the A/C set at 74ºF on a 90ºF+ day. It draws heavily on the HV battery. Even more so if the HV battery's capacity is diminished, which is possible on a 2007.
     
  6. PeterPrius2007

    PeterPrius2007 Junior Member

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    Ok that's a good idea. The battery was totally dead and would not hold a charge when I bought the car. I filled it with distilled water and seemed to be totally fine after that. Maybe it's going bad again

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  7. PeterPrius2007

    PeterPrius2007 Junior Member

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    I don't use AC so that can be ruled out.
    The HV seems to always be around 6 bars or so, and the ICE just keeps kicking in at the slightest acceleration, when before it wouldn't.

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  8. paprius4030

    paprius4030 My first Prius

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    12v. Battery going bad makes the Prius do a lot of weird things LOL.
     
  9. dolj

    dolj Senior Member

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    It may seem that way to people who do not understand what is going on behind the scenes.

    But what you put forward is not even one of them. LOL
     
  10. dolj

    dolj Senior Member

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    In that case, it looks more increasingly like something is going on in the HV battery, if you are ruling out those other things I mentioned.
     
  11. StephenJ

    StephenJ Member

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    Did you reset the fuel mileage tracker in the display to see if it still gets that mileage with just you driving to take your friends driving out of the calculation?
     
  12. PeterPrius2007

    PeterPrius2007 Junior Member

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    Yes I did. I did notice highway mileage is still good, but city is where it has dropped do significantly. I think the other poster with checking 12V battery is on the right track.

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  13. PeterPrius2007

    PeterPrius2007 Junior Member

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    The 12V battery fails under load. I hope this is the problem. Looking to replace and see what happens.

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  14. bisco

    bisco cookie crumbler

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    someone just got a great deal on an optima at pep boys.
     
  15. dolj

    dolj Senior Member

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    Poly cos it's defective. LOL.
     
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  16. PeterPrius2007

    PeterPrius2007 Junior Member

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    Well it seems my fuel mileage is back to 53 mpg.
    I thought the 12V battery was failing under load, but I think I was using too high of a current sink during testing for that small of a battery.
    I bought an ODB II scanner + Torque Pro and really couldn't find anything obvious. The best and worst HV cell were within 0.2V pretty much all the time.
    The solution? The rear tires were pretty low on air and when I filled them to 40 psi my fuel mileage came back to 53 mpg.
    I had no idea this car is so sensitive to things being "just right" but I'm happy nothing had to be replaced.
     
  17. bisco

    bisco cookie crumbler

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    how low? i guess if they were 20 psi it would make sense.
     
  18. dolj

    dolj Senior Member

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    I have found anything less that 33 PSI makes a noticeable difference. Anything over 38 PSI make little significant difference, all other things being equal. I don't believe that over-inflating* for the sake of a couple of extra MPGs is worth the trade off with tires wearing quicker down the centre (which will occur if tires are over-inflated*) which means they wear out early verses the saving in fuel cost. To me it is a false economy as the cost of new tires outweighs the saving in fuel. The best option is to select the best LRR (bearing in mind the premium for LLR) tires balancing the fuel economy and eco premium.
    This is no surprise to people who have been around Prius for any significant time. We try and tell noobs all the time bit they don't believe it will much of a difference.

    Remember though there are many little thing that do not, in and of themselves, add up to much, but collectively, they can be the difference between good and bad MPGs.

    If it were any easy equation, we would have so many angst filled post of "which tire should I get for fuel economy".


    * Bear in mind the definition of under-inflation, correct inflation and over-inflation is governed by tire tread wear pattern and is the one to which I refer. Correct inflation is shown by even wear right across the whole width of the tire and not the use an arbitrary PSI figure. The actual PSI will be different tire to tire by +/- ~3-4 PSI.
     
    #18 dolj, Sep 21, 2017
    Last edited: Sep 21, 2017
  19. EricWagner

    EricWagner New Member

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    I have been dealing with mileage dropping over the last 4 months. At 100k, I decided to do some maintenance and with the help of NutzaboutBolts, I went ahead and performed:
    1) transmission oil change
    2)engine oil change
    3) spark plug replacement
    4) install oil catch can
    5) removeand clean egr valve amd tube
    6) remove and clean intake manifold
    7) remove and clean throttle body
    8) replaced O2 sensor
    9) replaced MAF sensor

    All of this was proactive maintenance. I had zero issues and no check engine lights.

    After all of this, I experienced a slow decrease in MPG- from normal of 50-54 to 44-47.

    No matter what I did, light on the accelerator, tires at 40psi, premium fuel, ECO Mode, and overall conscious driving g habits, I couldn't get this thing back to "normal."

    Today I decided to bite the bullet and buy a new 12v battery. I had no reason to suspect this was the issue because I purchased this Prius used 2 years ago and when I looked at the battery 2 months ago, the sticker on it said 8/17. I figured the battery is 2 years old- still good right?

    Last week, while sitting in my car watching my in-dash dvd player (which came with the vehicle) a message appeared that I had not seen in the past. I typically sit in my car for an hour or a little more, 3 days a week while my daughter has soccer practice. I'll usually pop a dvd in and watch it.

    The message said something to the effect- shutting down to conserve battery. I am always in Accessory Mode when watching the dvd.

    Today I swapped out the 12v battery with a new yellow top Optima at a cost of approximately $230 dollars at local AutoZone. Drove it right after on the highway for about 10 miles and I'm up over 54!!!

    Hopefully, problem solved
     
  20. dolj

    dolj Senior Member

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    Don't keep doing this, unless you want to be changing the 12 V battery regularly. If you must sit in the car using accessories, just leave it in READY and allow the HV battery to supply the 12 V bus via the DC/DC converter. The car will take care of keeping enough charge in the HV battery and the DC/DC converter will keep the 12 V battery charged up.
     
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