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Unexplained major drop in city fuel economy

Discussion in 'Gen 3 Prius Fuel Economy' started by web1b, May 15, 2013.

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  1. web1b

    web1b Active Member

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    I have had the car for 3 1/2 years and 35,000 miles and I generally have averaged about 42-45 MPG in my normal mixed driving of about 90% suburban surface street driving and some short highway drives.
    I never bothered to "try" to get high mpg other than than not driving "hard." No special hyper-miling techniques used.
    All of a sudden with no change in driving habits or weather compared to prior years, I'm now seeming the lowest mpg I have ever seen since the car was new. I have started to get about 35mpg in the city and it is a struggle to get any higher. If I turn off the a/c and drive extra lightly, I still cannot crack 40mpg according to the display. If I get on the highway, it shows 50+mpg at a steady 65mph.
    Of course I checked the tire pressure and it is the same as it always has been. I set it at the levels shown on the door jamb and that has not be altered in 3 years.
    I have also had the issue through the last two tanks of gas purchased at different gas stations and I am not carrying any more passengers or ant more weight than before.
    Since it is not the tire pressure nor a change in driving habits or location, what else would suddenly cause a drop in city mpg?
     
    #1 web1b, May 15, 2013
  2. skwcrj

    skwcrj Member

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    #2 skwcrj, May 15, 2013
  3. web1b

    web1b Active Member

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    That is a link back to this message.
     
    #3 web1b, May 15, 2013
  4. JMD

    JMD 2012 Prius 4 Solar Roof

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    Check your tire pressure.
     
    #4 JMD, May 15, 2013
  5. skwcrj

    skwcrj Member

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    Sorry. I pasted the wrong link.
     
    #5 skwcrj, May 15, 2013
  6. skwcrj

    skwcrj Member

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    I think he already tried that.

    Recap: OP said
     
    #6 skwcrj, May 15, 2013
  7. web1b

    web1b Active Member

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    I mentioned in the post that I already checked and it is not tire pressure just so that no one would make that suggestion.
     
    #7 web1b, May 15, 2013
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  8. JMD

    JMD 2012 Prius 4 Solar Roof

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    Sorry about that chief. You are correct. It could be a number of things. Sometimes MPG drop when the 12v battery start to decline. If you car is 3 1/2 years old chances are that is not your issue. I can't say, dirty air filter, fuel mix change, poor calculation standards.

    Perhaps one of the master mechanics will pick up your thread and respond
     
    #8 JMD, May 15, 2013
  9. spiderman

    spiderman wretched

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    Given the obvious things, might be a bad cell(s) in the HV battery.

    Might try a hard reboot. Disconnect the 12v for a few minutes. That will reset your FE stats however.
     
    #9 spiderman, May 16, 2013
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  10. F8L

    F8L Protecting Habitat & AG Lands

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    Actually the 12v battery would be on my suspect list if it were mainly city fuel economy that is being affected since good city mpg relies on the engine turning off. On the highway the engine stays on so it shouldn't be affected as bad.
     
    #10 F8L, May 16, 2013
  11. skwcrj

    skwcrj Member

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    Hey Justin,

    He did say that this is happening in "my normal mixed driving of about 90% suburban surface street driving and some short highway drives". Maybe the short highway driving is not enough to offset the horrible city mpg? Just thinking aloud....
     
    #11 skwcrj, May 16, 2013
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  12. JMD

    JMD 2012 Prius 4 Solar Roof

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    #12 JMD, May 16, 2013
  13. skwcrj

    skwcrj Member

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    Hey wait a minute!! Why involve professionals? This is the internet afterall. We have a huge collection of "experts" here. :D
     
    #13 skwcrj, May 16, 2013
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  14. F8L

    F8L Protecting Habitat & AG Lands

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    That's what I was thinking. He said that on the freeway it will hold 50+mpg so that sounds pretty normal. He also said that turning off the A/C didn't help. The main problem with the A/C in the city is the engine doesn't like to turn off as often so we have a similar condition to a 12v drain.

    I'm not a big pusher of the 12v failure idea usually but this one seems weird that the loss is city only.
     
    #14 F8L, May 16, 2013
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  15. web1b

    web1b Active Member

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    The reason why this is unusual is that this style of driving is not new. Nothing about the driving style has changed. I have been doing mostly the same driving in the same area for the last 3 years and the mileage just changed on the last couple tankfuls of gas.
    I have ordered a new air filter that I will install this weekend and I will have the 12v battery checked at the dealer. I don't remember ever getting the battery replaced since the car was new. I would have to look through all the service records to check, but I doubt it.
    Does the 12v battery generally need replacement at around 3 or 4 years?
     
    #15 web1b, May 16, 2013
  16. JMD

    JMD 2012 Prius 4 Solar Roof

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    12V battery failure at 4 years old is rare but possible. Worth it to test and eliminate that as a potential issue.

    Air filter sounds like it especially if it has never been replaced.

    I have heard of a dealer adjusting the parking break to tight which impacted the MPG.

    I also heard overfilling the oil can be a Culprit
     
    #16 JMD, May 16, 2013
  17. web1b

    web1b Active Member

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    Air filter has definitely been replaced at least once since the car was new.
    I have had a previous car where the 12v battery started going bad before the car was 3 years old and was replaced under warranty. This car is now out of basic warranty by age, so I thought that maybe 3 1/2 years might not be that unusual for battery to start degrading. I don't see any other sign of the battery failing since it starts up normally and I don't see anything else such as dim lights.
    Not sure how many years is normal for a 12v battery on a Gen III Prius.
     
    #17 web1b, May 16, 2013
  18. JMD

    JMD 2012 Prius 4 Solar Roof

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    It is hard to say how long they last. It depends how you cared for it.

    I put this from another post that may shed some light.



    image.jpg
     
    #18 JMD, May 16, 2013
  19. spiderman

    spiderman wretched

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    Has your SOC gauge been "normal"? Although my FE is normal, one thing I have noticed is my SOC has been a bar or two lower than "normal" for the last three months. We will see if that changes with summer coming.
     
    #19 spiderman, May 16, 2013
  20. skwcrj

    skwcrj Member

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    Just to be sure... Have you run the 12v battery completely dead recently? Either way, doing the test in the link above could help eliminate 12v battery issues.

    One more question... have you noticed any odd behavior (rapid charging/discharging) behavior of the HV battery as you drive around the city?

    Keep us posted. We are all curious....

    Edit: Sorry spiderman. You beat me to it. Great minds must think alike...
     
    #20 skwcrj, May 16, 2013
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