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Engine Brake Q...

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by Whiteyprius, Feb 8, 2018.

  1. Whiteyprius

    Whiteyprius Active Member

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    Q for those of you that drive often in mountainous areas ... In my 2017 trim two, I often use the engine brake while descending a long hill, after the battery is full . Then when I shift back to drive and let off the gas, the engine sounds and feels like it is at least partially in engine brake mode. After several miles, things feel normal again. Any similar experiences? Thanks!

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  2. padroo

    padroo Senior Member

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    I haven't driven my new Prius in the mountains yet but don't remember my Gen 2 doing that. It just started coasting until I hit the gas.
     
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  3. Whiteyprius

    Whiteyprius Active Member

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    What I described never happened with my Gen 2 or Gen 3.

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  4. mark27lim

    mark27lim Active Member

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    i have similar rxperience. i think the computer is just doing its thing and being extra smart
     
  5. alanclarkeau

    alanclarkeau Senior Member

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    Yes, I'm told that's what it's designed to do to stop over-charging (and overheating) the battery. With Gen 4, "B MODE" is entered automatically going downhill after the battery is full. The kinetic energy from going downhill has to go somewhere once the battery is full, so it spins the engine, that's instead of wearing out the brakes.

    There have been several threads on this.
     
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  6. Whiteyprius

    Whiteyprius Active Member

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    I understand. But this occurs after the battery indicator has dropped a bar or two. Do you think the car hasn't decided to accept charging quite yet?

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  7. alanclarkeau

    alanclarkeau Senior Member

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    No idea - just an idiosyncrasy - someone put a sophisticated metering on it, and I think came to the conclusion that it's in "limbo*" for a couple of minutes till the battery temperature stabilises after a rapid charge.

    *limbo - a technical term which escapes my understanding.
     
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  8. Prodigyplace

    Prodigyplace Senior Member

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    That could be the case if the battery needs to cool further before safely charging. How long have you had the car? How many miles?

    Is this a recent change? If so, check the hybrid fan vent on the lower rear passenger seat. The manual says to just vacuum the grille, but if you remove it you can see a filter in behind that too. If that vent gets clogged with pet hair or other debris, the cooling fan needs to work harder and traction battery can get too hot.
     
  9. alanclarkeau

    alanclarkeau Senior Member

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    I tried checking mine when it first happened - and it was clean as a whistle, but check that nothing is over the vents. The main one is the one on the corner of the rear seat (left of the photo), we're not absolutely certain what the vent in the middle is (right in the photo), but I make sure it isn't covered.

    upload_2018-2-9_23-1-12.png
     
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  10. RRxing

    RRxing Senior Member

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    Are you sure about this? I don't see it mentioned in the Owner's Manual. I think you still have to place the shift lever in "B" mode, it's not automatic...
     
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  11. Lucifer

    Lucifer Senior Member

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    When descending a hill with cruise control on, the prius will maintain the speed the cruise control is set at, the engine brake can be heard, I prefer to let the computer control my speed when going down hill, and uphill and on the flat.
    The Gen2 manual warned about to much use of the (B) lever, I notice my Prime Advanced uses the engine brake frequently.
    I prefer to maintain the speed limit.
     
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  12. Prodigyplace

    Prodigyplace Senior Member

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    The same thing happens with Gen 4, even if it could charge the battery. On my trip to work I turn off cruise on the small downhill and brake manually to recharge the battery instead of wasting the energy.
     
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  13. Whiteyprius

    Whiteyprius Active Member

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    Thank you so much for bringing up the battery-cooling issue. I've had the car for several months and haven't noticed until looking for it today that the vent moved location for Gen 4 - And guess where I chose to put my little trash bin??? Right in front of the vent (that I didn't realize was there!) Thank you all for helping me to figure that one out!

    Sent using Hello Moto
     
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  14. Prodigyplace

    Prodigyplace Senior Member

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    I looked after somebody else mentioned the filter behind. It is surprising the manual does not mention cleaning or replacing that. @Elektroingenieur is the cooling fan filter easily replaced on Gen 4?
     
  15. kithmo

    kithmo Couch Potato

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    I may be wrong but I don't believe the HV battery indicator is all that accurate TBH.
     
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  16. alanclarkeau

    alanclarkeau Senior Member

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    I suspect not too - particularly at the top of the range. It's just an indicator. Like the EV light which lags often.

    Bear in mind that the battery WON'T go below about 20% and above 80% anyway, the HV indicator only theoretically utilises the area between. I've never noticed mine get as low as 1 bar, and only a couple of occasions has it got to the very top bar (after long downhill section).
     
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  17. Elektroingenieur

    Elektroingenieur Senior Member

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    The symptoms aren’t quite the same as those @Whiteyprius describes, but since he has a Prius Two with the NiMH battery, I wonder if it might be worth asking a dealer to do the HV ECU software update described in T-SB-0241-17, if it applies. This addresses other concerns “after repeated driving on long downhill slopes.”
    It does—for MY 2017, pages 83, 88, 573–574, and 667.
    Yes. The Owner’s Manual gives a procedure only for vacuuming the No. 1 HV battery intake filter in place, but removal is not difficult. The Repair Manual says to remove the battery service hole cover (a clip, then five claws) and the filter (a claw, then two more claws) and then clean the filter with compressed air or replace it. The part number is G92DH-47070; list price $11.35.
     
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  18. RCO

    RCO Senior Member

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    Australia obviously lacks the switchback dipping and winding roads of Cornwall! :ROFLMAO:
     
  19. davran

    davran Member

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    Going downhill, when the battery gets full on my '19 awd-e the ICE starts to rev higher, but B-mode make it rev a lot higher.
     
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