Sudden Drop of 10-15 MPGs After Brakes Repared

Discussion in 'Gen 3 Prius Fuel Economy' started by CMeNot, Nov 17, 2017.

  1. CMeNot

    CMeNot New Member

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    I bought this Car in the end of 2015 I drive 120 miles per day back and forth to work. I loved this car as it didn't seam to make any difference what speed I went on the highway as long as it was constant I got 48-53 MPG for over 43,000 miles. Last week I took my Prius in for its 45,000 mile checkup. The Dealer noticed that my breaks were really bad and so I got both the pads and rotors replaced. The next day I drove to work as normal and I only got 35.8 MPG. So I looked at the air pressure in the tires noticed they were all at 30 PSI so I put them back to 37 Front and 35 Back and the mileage did go back up a bit. The next 2 trip to work got me 39 MPG both ways. I took the car back to the dealer and they could not find anything wrong. I cannot believe that a car can just drop 10-15 MPG over night and there not be any reason for it when the only thing done was a break job, which I might add work really good now. Any ideas?
     
  2. Prodigyplace

    Prodigyplace Senior Member

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    My first guess is that the brakes may be binding. @Mendel Leisk is somewhat of a brake expert here.
     
  3. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    The rear brakes, with tricky integrated parking brake, are the "usual suspects", so start by checking them.

    First, after a drive, feel all 4 rims: are any more than slightly warm?

    Then, chock front wheels, raise rear with parking brake off, try spinning the wheels. The should have the only slight drag, easily do a revolution or two if you give a good push.

    If they're fighting, dealership has very likely misaligned the caliper piston.

    Check that first and post. In a pinch, you can do that rear check one side at a time, with just the scissor jack. Be sure to securely chock the diagonally opposite front wheel, since you need the parking brake off.

    (Kosher, heavy rubber wheel chocks are best, a good investment.)
     
    #3 Mendel Leisk, Nov 17, 2017
    Last edited: Nov 17, 2017
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  4. MelonPrius

    MelonPrius Active Member

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    You got both pad and rotors replaced at around 43k miles? I figured the rotors could have been resurfaced, at the least.

    I'm in a similar boat. I was told at 48k miles, my front pads are very low. I've been meaning to get a second opinion (without having to pay a diagnostic fee) because compared to other owners here, it' seems very early and I mostly use regenerative braking and coasting to slow down my vehicle.
     
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  5. bisco

    bisco cookie crumbler

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    especially in a dry climate.
     
  6. bisco

    bisco cookie crumbler

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    welcome!

    your dealer ripped you off big time, find a new one.
     
  7. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    This machining of rotors is way overdone. The rotors are typically fine, if you were to check them against spec, with dial indicator and micrometer.
     

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  8. bisco

    bisco cookie crumbler

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    and pads at 45k?
     
  9. MelonPrius

    MelonPrius Active Member

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    That's what has me scratching my head with my Prius at 48k. I purchased the car at 29k miles, so I've put on around 20k miles.

    I want to think my mechanic is pulling my leg saying that the front pads are very thin and needs replacing. But when I pull into my garage, I hear some faint whistling. I hear the recognizable squealing other times when I'm driving. I'm wondering if I am braking the wrong way? I brake like an annoying old man. BTW, I hate putting money into brake related work, so this sticks in my craw.
     
  10. bisco

    bisco cookie crumbler

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    isn't the faint whistling the pedestrian warning system?
     
  11. MelonPrius

    MelonPrius Active Member

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    No. It's just once in a while, but it's the sound all of my previous cars made when the pads needed changing. I'm going to have someone else eyeball it (to avoid a diagnostic fee), but I have a sinking feeling that the pads need changing.
     
  12. royrose

    royrose Active Member

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    Most brake shops where I live do a free brake inspection. Not in Vegas?
     
  13. fuzzy1

    fuzzy1 Senior Member

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    CMeNot, do check your wheel and hub temperatures. But before actually touching them, hold a hand close to see if major heat is radiating off of them.

    I've had this happen on two different cars. On the first, the shop incorrectly adjusted a replacement power brake booster, such that it started self-applying the brakes in hot July temperatures when fluids expanded. The shop kept insisting that disc brakes normally get hot, and that this wasn't a problem. They didn't believe me that my heating was excessive, until the car couldn't even maintain highway speed (55 in those days) without downshifting, and the brakes began smoking.

    The second was on a later car with replacement calipers. They rusted too quickly, leading to frozen pads that began continually dragging, again causing excessive wheel and hub temperatures and drag.

    Since your problem is with a fresh brake job, incorrect assembly is the first suspect. Please do Mendel's recommended temperature and drag checks.

    And consider having someone else do the work. Absent special circumstances (e.g. very heavy winter salt) or a heavy brake foot, this is quite early for a normal Prius. (Both my spouse and I get about 100,000 miles on the factory brakepads of non-hybrids, but that isn't the norm either.)
     
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