RPM Slowly goes down when coasting

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by labradorPrius, Aug 3, 2015.

  1. labradorPrius

    labradorPrius Junior Member

    Joined:
    Jan 20, 2014
    5
    0
    0
    Vehicle:
    2010 Prius
    Model:
    IV
    I have noticed something about my 2010 Prius. Lately when I am driving the car for about an hour, the engine slowly reduces RPMs (based on sound) when I take my foot off the gas. The MPG indicator slowly goes from near zero to 100. Like VERY slowly. And that is just street driving below 45 mph. The car has almost 60k miles. Tires are correct and in very good shape.

    *Edited because it is just a normal prius
     
    #1 labradorPrius, Aug 3, 2015
    Last edited: Aug 3, 2015
  2. alekska

    alekska Active Member

    Joined:
    Feb 14, 2010
    435
    138
    0
    Location:
    Atl
    Vehicle:
    2005 Prius
    I think it's a normal operation when the engine is not fully warmed up. Also ( but this is only once per drive cycle ) Prius performs idle calibration at stop, which is slow reduction of the RPMs and then shut down. If you interrupt this calibration by driving off before shutdown, car will try to do it again at the next stop

    Alex
     
    #2 alekska, Aug 3, 2015
  3. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    111,169
    50,602
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    i have not noticed this.
     
    #3 bisco, Aug 3, 2015
  4. labradorPrius

    labradorPrius Junior Member

    Joined:
    Jan 20, 2014
    5
    0
    0
    Vehicle:
    2010 Prius
    Model:
    IV
    The engine is fully warm. It happens after driving about an hour.
     
    #4 labradorPrius, Aug 3, 2015
  5. peterjmc

    peterjmc Ping pong in Ding Dang...

    Joined:
    Dec 5, 2010
    917
    189
    0
    Location:
    Clarksburg, MD
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Advanced
    This occasionally happens with mine. I thought it was really odd the first time it happened. My car is fully warmed up and has been operating for at least 40 min.

    It's as if the motor is free reving. Very similar to driving a manual transmission and pushing in the clutch and hearing the engine free reving until the motor slows. Really odd.

    LG-H811 ?
     
    #5 peterjmc, Aug 3, 2015
  6. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    111,169
    50,602
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    have your mpg's changed?
     
    #6 bisco, Aug 3, 2015
  7. peterjmc

    peterjmc Ping pong in Ding Dang...

    Joined:
    Dec 5, 2010
    917
    189
    0
    Location:
    Clarksburg, MD
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Advanced
    Im not sure about op, but I know that I have a hard time getting anything over 44mpg these days

    LG-H811 ?
     
    #7 peterjmc, Aug 3, 2015
  8. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    111,169
    50,602
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    a/c use?
     
    #8 bisco, Aug 3, 2015
  9. electrontechnik

    electrontechnik Member

    Joined:
    Mar 2, 2012
    117
    31
    21
    Location:
    USA
    Vehicle:
    2018 Tesla Model 3
    Model:
    N/A
    Mine does this too. Seems normal.
     
    #9 electrontechnik, Aug 4, 2015
  10. Den49

    Den49 Member

    Joined:
    Oct 25, 2010
    498
    269
    0
    Location:
    Maryland
    Vehicle:
    2010 Prius
    Model:
    IV
    Dirty throttle body? It can cause the throttle plate to hang open or close slowly.
     
    #10 Den49, Aug 4, 2015
  11. peterjmc

    peterjmc Ping pong in Ding Dang...

    Joined:
    Dec 5, 2010
    917
    189
    0
    Location:
    Clarksburg, MD
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Advanced
    That might be a contributing factor but I'm gonna blame it on carpooling with passengers and the added weight. :D

    I'll have to give that a look. That sounds like a reasonable path to look at. The symptoms of a stuck throttle plate are the exact type of symptoms I'm seeing occasionally. Thanks!
     
    #11 peterjmc, Aug 4, 2015
  12. electrontechnik

    electrontechnik Member

    Joined:
    Mar 2, 2012
    117
    31
    21
    Location:
    USA
    Vehicle:
    2018 Tesla Model 3
    Model:
    N/A
    I cleaned my throttle when I had the engine apart for the headgasket job.

    It still did it this past weekend. Doesn't happen all the time, maybe 1-2 times in 1000 miles, and when the engine is warm.
     
    #12 electrontechnik, Aug 4, 2015
(You must log in or sign up to post here.)