1. Attachments are working again! Check out this thread for more details and to report any other bugs.

2008 Prius MPG dropping like a rock after new 12v battery and coolant pump change

Discussion in 'Gen 2 Prius Fuel Economy' started by Jon Good, Feb 28, 2023.

Tags:
  1. Jon Good

    Jon Good New Member

    Joined:
    Oct 27, 2022
    20
    3
    0
    Location:
    Seattle
    Vehicle:
    2008 Prius
    Model:
    Two
    Hey guys- I have a frustrating mystery. I recently changed my inverter coolant pump and my MPG dropped slightly from 40 to 38.. and then two weeks ago I had a bad battery and bought a new one from O’Reillys and now the mileage is going down every day and it is currently around 28 mpg. I hooked up the ODB scanner and looked at the hybrid battery with the free version of Dr. Prius, seems to be working fine.. I also put in a new air filter.
    What do you think might be going on??
    Much thanks for any responses.
     
    #1 Jon Good, Feb 28, 2023
  2. ColoradoCrow

    ColoradoCrow Active Member

    Joined:
    Sep 2, 2019
    923
    377
    1
    Location:
    Leawood, KS
    Vehicle:
    2008 Prius
    Model:
    I
    hmm. check your tire pressure. Also check and make sure you seated the new air filter correctly. Have you cleaned your throttle body or intake manifold? How many miles on her? If you are 125K or more and original..it is time.
     
    #2 ColoradoCrow, Mar 1, 2023
  3. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    107,686
    48,942
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    I think when you put in a new battery, the computers have to relearn everything.
    Track a couple tanks and let us know the actual mpg’s
     
    #3 bisco, Mar 1, 2023
    AzWxGuy and ColoradoCrow like this.
  4. Jon Good

    Jon Good New Member

    Joined:
    Oct 27, 2022
    20
    3
    0
    Location:
    Seattle
    Vehicle:
    2008 Prius
    Model:
    Two
    I think that’s what’s happening but it has taken MULTIPLE tanks of gas, like you said, as it’s returning to 40mpg ish. I ran it on the freeway a few times at 60, resetting the dash counter and then it started creeping upwards again. thanks for the many responses I appreciate it!
     
    #4 Jon Good, Mar 1, 2023
    bisco likes this.
  5. edthefox5

    edthefox5 Senior Member

    Joined:
    Jul 25, 2007
    10,096
    4,795
    0
    Location:
    Clearwater, Florida
    Vehicle:
    2007 Prius
    Model:
    Two
    lol...the 12 volt battery has little to do with mpgs.
     
    #5 edthefox5, Mar 7, 2023
  6. Jon Good

    Jon Good New Member

    Joined:
    Oct 27, 2022
    20
    3
    0
    Location:
    Seattle
    Vehicle:
    2008 Prius
    Model:
    Two
    Yeah I’m not sure what’s going on now because the mileage creeped up a little bit when I took it on the freeway it goes all the way up to like 47 mpg and then Intown drops like a rock down till like 30.. ugh.
    But all this started when I changed the inverter coolant pump and battery. Before I was getting a rock solid 40 mpg all the time.
     
    #6 Jon Good, Mar 7, 2023
  7. dolj

    dolj Senior Member

    Joined:
    May 14, 2012
    7,483
    3,763
    0
    Location:
    Wellington, New Zealand
    Vehicle:
    2007 Prius
    Model:
    N/A
    So which MPG meter are you specifically using?

    If it is the Average dispalyed on the Consumption screen (which is a lifetime since last reset), then the is a high chance that this was reset when you changed the battery.

    Depending on if you have ever pressed the "Reset" button on that screen then the average you saw before the battery change could be the average over many thousands of miles.

    With the average now reset, the average will be since the reset.

    The best way to know what is the true MPG is to calculate the true MPG by dividing the mile run on the cuurent tank by the numser of gallons you added the tank. Record the numbers in a biook (or better still create an account on fuelly.com) so you have a running record. When you fill up reset the Trip A and the average on the consumption screen. do the recording all over again at the next fill up. For this to work, you need to fill up completely, not partial fill ups.
     
    #7 dolj, Mar 8, 2023
    srellim234 and Jon Good like this.
  8. Jon Good

    Jon Good New Member

    Joined:
    Oct 27, 2022
    20
    3
    0
    Location:
    Seattle
    Vehicle:
    2008 Prius
    Model:
    Two
    Thanks for the answer I’ll try that and report back
     
    #8 Jon Good, Mar 8, 2023
  9. edthefox5

    edthefox5 Senior Member

    Joined:
    Jul 25, 2007
    10,096
    4,795
    0
    Location:
    Clearwater, Florida
    Vehicle:
    2007 Prius
    Model:
    Two
    You never stated the miles on the car or how long you have owned it?

    You need a new hybrid battery based on what your saying. if you just bought it you def need a new hybrid battery.

    Needs new hybrid battery is probably #1 reason people sell there G2.
     
    #9 edthefox5, Mar 10, 2023
  10. Jon Good

    Jon Good New Member

    Joined:
    Oct 27, 2022
    20
    3
    0
    Location:
    Seattle
    Vehicle:
    2008 Prius
    Model:
    Two
    .. well here’s the conclusion to the story. Consistent 37 mpg after I did all that work dropping from around 40. Stay that way for a few months and then I went on a big road trip 300+ miles and it went back up to 40 and now it’s at 41.7 and has stayed there for months. Go figure.
     
    #10 Jon Good, May 20, 2023
  11. edthefox5

    edthefox5 Senior Member

    Joined:
    Jul 25, 2007
    10,096
    4,795
    0
    Location:
    Clearwater, Florida
    Vehicle:
    2007 Prius
    Model:
    Two
    Yup your in Stage 1 hybrid battery failure. A prolong charger can prop it up and get you another year and forced charging can help but its going to need to come out and be investigated before it leaves you stranded.

    A failed hybrid battery on a G2 is the number #1 post on this site. very common its just the age of that car. 15 years is a awesome run on any battery.

    Forced battery charging or power braking is very easy.

    When your in gear and stopped put your left foot on the brake and then gently press the gas till you see the charging bars on the mfd start to route power to the battery from the engine. Not necessary to floor it just press the gas pedal down enough to start the engine and get the bars moving.

    Practice with no one in front of you for a while to get confident at that. I did that for months at every traffic light before buying a Prolong..

    It works it will add blue bars to the battery. Or at least get you out of purple bars.

    Here's a link to the Search Forums list with the search word "Hybrid Battery Failing". Se how other people dealt with this:

    Good Luck!

    https://priuschat.com/search/429441022/?q=hybrid+battery+failing&t=post&o=relevance
     
    #11 edthefox5, May 21, 2023
  12. Jon Good

    Jon Good New Member

    Joined:
    Oct 27, 2022
    20
    3
    0
    Location:
    Seattle
    Vehicle:
    2008 Prius
    Model:
    Two
    That’s not what’s going on.
     
    #12 Jon Good, May 21, 2023
  13. edthefox5

    edthefox5 Senior Member

    Joined:
    Jul 25, 2007
    10,096
    4,795
    0
    Location:
    Clearwater, Florida
    Vehicle:
    2007 Prius
    Model:
    Two
    OK
     
    #13 edthefox5, May 21, 2023
  14. Jon Good

    Jon Good New Member

    Joined:
    Oct 27, 2022
    20
    3
    0
    Location:
    Seattle
    Vehicle:
    2008 Prius
    Model:
    Two
    The point of this for forum is to help people and when you’re purposely peddling misinformation to sell stuff to working class people struggling with car repairs -that’s a real crummy thing to do.
     
    #14 Jon Good, May 22, 2023
  15. edthefox5

    edthefox5 Senior Member

    Joined:
    Jul 25, 2007
    10,096
    4,795
    0
    Location:
    Clearwater, Florida
    Vehicle:
    2007 Prius
    Model:
    Two
    wow...
     
    #15 edthefox5, May 22, 2023
  16. SFO

    SFO Senior Member

    Joined:
    Feb 7, 2017
    5,286
    4,225
    0
    Location:
    Northern California
    Vehicle:
    2007 Prius
    Model:
    N/A
    In town driving and short distance trips often result in lower MPGs, and disconnecting or replacing the 12v will cause the computers/ECUs to reset the learned fuel mapping/curves, which then reset to the factory default settings. Generally a good idea to clean the MAF sensor after any 12v work, unless already clean. Then use a toothbrush dipped in vodka or rubbing alcohol to clean the butterfly valve and throttle bore.
     
    #16 SFO, May 22, 2023
  17. Jon Good

    Jon Good New Member

    Joined:
    Oct 27, 2022
    20
    3
    0
    Location:
    Seattle
    Vehicle:
    2008 Prius
    Model:
    Two
    I see, thanks
     
    #17 Jon Good, May 22, 2023
  18. Ozzbo

    Ozzbo New Member

    Joined:
    Jun 22, 2023
    28
    13
    0
    Location:
    Austin
    Vehicle:
    2008 Prius
    Model:
    Base
    How's your mpg now?
     
    #18 Ozzbo, Aug 27, 2023
  19. Jon Good

    Jon Good New Member

    Joined:
    Oct 27, 2022
    20
    3
    0
    Location:
    Seattle
    Vehicle:
    2008 Prius
    Model:
    Two
    Between 42.3 and 42.5mpg
     
    #19 Jon Good, Aug 27, 2023
    Ozzbo likes this.
  20. Jon Good

    Jon Good New Member

    Joined:
    Oct 27, 2022
    20
    3
    0
    Location:
    Seattle
    Vehicle:
    2008 Prius
    Model:
    Two
    Honestly the mpg was bad for a few months until I wanna know 300 mile road trip..
     
    #20 Jon Good, Aug 27, 2023
    Ozzbo likes this.
(You must log in or sign up to post here.)