Poor Fuel Economy After Brake Change

Discussion in 'Gen 3 Prius Fuel Economy' started by Tim Jenkins, Jul 15, 2021.

  1. Tim Jenkins

    Tim Jenkins New Member

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    Hello,

    I just had my rear brake pads and rotors changed on my 2010 Prius, before the change, i was averaging 40-42 mpg, now the best I can get is 31-32 mpg. I have read other posts where others have had this experience. It seems like I am driving with the brakes slightly pressed. Now when I accelerate, my dash energy bar goes to 'power' section almost immediately, and when I let off the accelerator, even going up hill, the energy bar goes into the "charge" section. Anyone that knows what the fix is, please let me know, I have taken my car to 3 mechanics, including the local Toyota dealership and everyone says that nothing is wrong. FYI, the brakes/rotors are not smoking or glowing, with the car on a lift the rear wheels turn freely. Thanks!
     
  2. bisco

    bisco cookie crumbler

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    who did the brake job?

    how are you measuring mpg? manual calc at fill up is the only reliable way

    put her in neutral, and see if she glides okay

    if they disconnected the 12v, it just needs to relern everything
     
  3. ChapmanF

    ChapmanF Senior Member

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    Have you reviewed any of @Mendel Leisk's posts on the rear caliper piston orientation?
     
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  4. Tim Jenkins

    Tim Jenkins New Member

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    In neutral there is ZERO resistance, but as soon as I put in Drive, the resistance returns.
     
  5. Tim Jenkins

    Tim Jenkins New Member

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    Brake job done by ASE certified mechanic that I have used for 25+ years
    MPG comparison is Prius display on both, both before and after brake job, but I am confident it is accurate as I am getting 80 fewer miles on full tank of gas
     
  6. Tim Jenkins

    Tim Jenkins New Member

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    No, I am new to this site, how do I view the relevant posts?
     
  7. Mendel Leisk

    Mendel Leisk Resonant Resident

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  8. Tim Jenkins

    Tim Jenkins New Member

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  9. bisco

    bisco cookie crumbler

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    i can't see any reason why not. of course, when you put it in drive, you get regen braking
     
  10. bisco

    bisco cookie crumbler

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    if the rear wheels spin freely, i don't see how the brakes could have been installed incorrectly
     
  11. bisco

    bisco cookie crumbler

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    when you let off the brakes, the bar should go to regen, even uphill, and the car should slow down. that's not an issue.
     
  12. Tim Jenkins

    Tim Jenkins New Member

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    ok, so I just drove 440 miles (90% interstate) and averaged 42 MPG, so I cannot 'really' complain, however, when I let off the accelerator and coast going downhill and not using cruise control (btw, I was in the mountains of Tennessee and Kentucky), the 'Hybrid System Indicator' goes 1 to 2 bars into the Charge area and slows down. Before the brake change, when I let off the accelerator going down theses same hills (mountains), the car would gradually gain speed. Also, I checked the heat on all 4 wheels and rotors @ 350 miles and there was almost no heat -- YAY! I agree with you that the brakes seem to be installed correctly. Is there anything that can be adjusted to change when the regenerative braking system engages; mechanical or sensor? Not sure if you saw my post above, but when I put the car in Neutral, there is ZERO resistance/engine load and the bars in the Charge section disappear, but in Drive and B gear, the resistance returns immediately.
     
  13. Grit

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    42 mpg driving at 90 mph? Impressive!
     
  14. bisco

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    no adjustments or settings that i can remember from my gen2 days. i wonder if coincidentally, the hybrid battery took a turn for the worse at the same time the brake work was being done
     
  15. Tim Jenkins

    Tim Jenkins New Member

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    Haha, not quite that fast, but 75-80 most of the way
     
  16. TheLastMojojomo

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    Check your tire pressure. This could be the cause of you not being able to roll as easy.

    Higher tire pressure = less rolling resistance.

    Posted via the PriusChat mobile app.
     
  17. Tim Jenkins

    Tim Jenkins New Member

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    Tire pressure has not changed... 38 PSI front / 36 PSI rear. There is something causing the regen brake system to engage all the time, during acceleration, I am going into 'Power" mode on the dash 'hybrid system indicator' almost immediately, and when I let off the gas pedal and coast, the system indicator goes 1 to 2 bars into 'charge' mode.. Like I said, it's like the regen brake system is on all the time, FYI, when I put car in neutral, all of the regen brake resistance goes away completely.
     
  18. TheLastMojojomo

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    I don't understand then.

    It's completely normal for low grade regenerative braking to occur when you let off the gas pedal.

    This is partially to deliver a charge to the HV Battery, and partially to simulate how compression of the engine slows a vehicle in a standard ICE/automatic transmission vehicle.

    When you shift in to neutral regenerative braking ceases.... that's exactly what's supposed to happen.

    I own a Gen 2 so I can't speak to the exacts of your Gen 3. But did it not go into regenerative braking everytime you let off the gas before the brake job on the energy monitor? Because it should have been.

    Here is a definition I wrote of braking in the Prius:

    There are two braking modes in the Prius.

    Friction brakes through the brake pads/rotors using direct input from the brake pedal through hydraulic lines, assisted by an electruc brake pressure accumulator to help with braking power. This accumulator will make a buzzing noise frequently when pushing the brake pedal.

    And regenerative braking... which uses electric current from the Hybrid Battery and magnetism to slow the vehicle. During regenerative braking, electric current from the Hybrid Battery is applied to a hollow coil of wire, inside of which Motor Generator 2 (MG2) is housed. MG2 is a rotating assembly that contains lots of magnets and is directly connected to the wheels through the transmission. When MG2 rotates, the wheels rotate. When current is applied in a certain way to the coil of wire while the wheels are spinning, it slows the vehicle through electromagnetism by acting upon MG2's magnets. This has a secondary affect of simultaneously generating an electric current to recharge the Hybrid Battery. That's why it's called regenerative braking. The Prius also scales this affect. The harder you push on the brake pedal, the more it increase the regenerative braking affect. this slows the vehicle quicker and regenerates more energy for the Hybrid Battery. This effect continues until you reach a speed of under 7 mph, the battery fully recharges, or you brake really hard. then the hydraulic brakes take over.

    It's also the same principle in reverse that drives the vehicle forward in electric mode. Current is applied to the coil in a way to spin MG2 forward and in turn drive the wheels forward.

    You will hear a high pitched electric whirring noise when the vehicle is braking in this mode and when current is being applied to drive the vehicle forward in EV mode.



    Posted via the PriusChat mobile app.
     
  19. Tim Jenkins

    Tim Jenkins New Member

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  20. bisco

    bisco cookie crumbler

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    perhaps the gas pedal interface is worn, and not sending the proper signal to the motor generator when you apply acceleration
     
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