Why would my Prius V suddenly be getting 26mpg

Discussion in 'Prius v Care, Maintenance and Troubleshooting' started by Rick F, Jun 30, 2022.

  1. Rick F

    Rick F New Member

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    Why would my Prius V suddenly be getting 26mpg coincidentally after an oil change, no check engine lights, no codes, 150k miles. Our mechanic couldnt find anything wrong (independent shop). Wife says when it gets hot it doesnt seem to shift correctly, high revs sticking before shifting. When I looked at it at one point the engine was shaking. Now or at least cold no shaking or noise.
     
  2. bisco

    bisco cookie crumbler

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    might have blown the head gasket, that's pretty common. any loss of coolant?

    who did the oil change? is the level correct?

    i would put it up and make sure there's no brake drag, especially parking brakes.

    no trouble lights?
     
  3. Mendel Leisk

    Mendel Leisk Underfoot

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    Wouldn't hurt to change transaxle fluid. An easy DIY, or if dealership does it, anything over a $100 is a rip.

    Transaxle fluid change info in my sig.
     
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  4. rjparker

    rjparker Tu Humilde Sirviente

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    It should not be getting hot. It should not feel like its shifting, ever. It should not have a shaking engine. All likely head gasket issues which occur frequently on 2010-14 Prius models. The earlier models happen first. A borescope test at the dealer can verify. Don't wait, the engine has been known to literally blow holes in the block.
     
  5. Rick F

    Rick F New Member

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    Oil seems to be good, coolant seems right no white smoke indicating a blown head gasket, no check engine lights, no codes. Drove it today and babied it got up to 41 mpg, sat in car 20 minutes with car on waiting for my kid ac running, mpg dropped to about 36mpg, engine didnt run the whole time. On the drive home mpg continued to go down, finally at 27 mpg, 10 mile drive.
     
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  6. Rick F

    Rick F New Member

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    Oil seems to be good, coolant seems right no white smoke indicating a blown head gasket, no check engine lights, no codes. Drove it today and babied it got up to 41 mpg, sat in car 20 minutes with car on waiting for my kid ac running, mpg dropped to about 36mpg, engine didnt run the whole time. On the drive home mpg continued to go down, finally at 27 mpg, 10 mile drive.
     
  7. TheLastMojojomo

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    Do you live in an extremely hot climate? If so this is likely the reason you're getting low mpg's and I will explain why.

    First the Prius technically never shifts... it varies its output through two electric motors and the combustion engine all connected to one output drive gear. If you notice the engine revving higher... there is usually a simple explanation relating to why the Hybrid Sytem is dictating the use of the engine over EV mode assist.

    The combustion engine in many scenarios acts simultaneously as a generator of electricity and a direct driver of the wheels which can cause it to rev very high in certain non-ideal situations with the feeling of little power output.


    As far as i can tell... you're experiencing completely normal behavior... the high revving is likely occuring for multiple reasons...

    1.) HV Battery is Low.

    When the HV Battery is low... the Prius limits EV mode usage.

    This creates a 3-fold state where the combustion engine will Rev significantly higher without feeling like much power is being delivered. This is because the combustion engine alone is simultaneously generating electricity for the HV Battery, driving the wheels down the road, and doing this while recieving no help from the HV Battery for power output because it is low.


    2.) HV Battery is overheating.

    When the HV Battery overheats... regenerative braking and EV output is completely stopped. This leads to all output Torque and HV demand being delivered by the internal combustion engine. You will notice much higher revs and hanging revs going from a high throttle input to lower throttle input. If temps are in the 90's or 100's at your location... HV Battery overheating occurs fairly easily... especially in direct sunlight.

    3.) AC use.

    Heavy AC use can cause high combustion engine revs as well because it is a major draw on the HV Battery. AC use basically adds a 4th factor to point #1 and a 2nd factor to point #2.

    The combustion engine now not only has to generate electricity for the HV Battery and drive the wheels.... it also has to generate electricity to power the AC in point #1 while recieving no assist from the HV Battery.

    In point #2... since there is no electric assist from the HV Battery due to it overheating... the engine now has to drive the wheels by itself and generate electricity for the AC since it can't draw on the HV Battery.

    So basically it puts more electrical load on the combustion engine that can't be handled by the HV Battery in both scenarios... which causes it to rev even higher.

    As I stated in the beginning... the Internal Combustion Engine acts simultaneously as an electricity generator and a wheel driver. The combustion engine output is constantly varied between doing those two things through the Hybrid System which causes a difference in rev response related to throttle input and the power the car has driving down the road in relation to engine revs.

    The total power output your recieving to the wheels is roughly the same in these scenarios... the computers are just telling the Prius to compensate for a loss in HV Battery Torque by revving the combustion engine higher instead.

    As for the engine shaking.... need more details on that to make an educated guess.


    Posted via the PriusChat mobile app.
     
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  8. Air_Boss

    Air_Boss Senior Member

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    All of the above, in Summer heat, with high loads.
     
  9. Mendel Leisk

    Mendel Leisk Underfoot

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    Maybe worth noting: the car knows the ambient temp, through a sensor in the front grill, and it's programming undoubtedly has variations depending on this. I've noticed for example, when driving on a hot day, the engine didn't shut down when at a stop, under conditions it normally would. There was sufficient hybrid battery charge, and we weren't even using AC as I recall.
     
  10. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Keep in mind that the battery is cooled by cabin air- meaning air conditioning.

    If you start out with a cool car (parked indoors or shaded) and keep the AC running throughout the trip, you're not likely to see much penalty.

    If the car (the whole car) is allowed to get particularly hot before the trip even starts, the whole system is going to work harder and as illustrated above that means more fuel burnt.



    tl;dr: Start with a cool battery and you'll burn less fuel cooling it down. Keep the battery cool and it will help you burn less fuel driving around.
     
  11. bisco

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    if this is new behavior in a car the o/p has owned many years, i would say something is wrong.

    none of my pri have ever gotten 26mpg.

    if the car is relatively new to the o/p, and it is extremely hot in upland, ca, and o/p is looking ar the dash for mpg, i would say everything might be normal.
     
  12. Rick F

    Rick F New Member

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    Interesting, since this didnt happen last year even in the worst California heat. I did notice it not wanting to drive in ev mode because of the heat but the mpg drop was not this drastic. I have noticed a drastic ev battery charge drop and charge full. So battery coolant change? Check battery cells? Seems odd I went from 41 mpg one way to 27 the other. But I guess it would make sense if half the system is stressed over the heat. Whats the solution here?
     
  13. bisco

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    there is no battery coolant. your battery is probably just old and tired, so doesn't stand up to the heat like it used to.
    you can run a health check with dr. prius.
     
  14. bisco

    bisco cookie crumbler

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    also, have you cleaned the cooling fan and intake grille?
     
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  15. rjparker

    rjparker Tu Humilde Sirviente

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    Problem is you don't get shuttering feeling like shifting or obvious visual shaking from a low hv battery even when it is failing. Which it easily could be doing. The shuttering and shaking are typical head gasket issues, also very common. Without coolant in oil or obvious loss of coolant until a blind man could see it.
     
    #15 rjparker, Jul 1, 2022
    Last edited: Jul 1, 2022
  16. Air_Boss

    Air_Boss Senior Member

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    In high heat, all you need is something on the seat, hanging down over the HV battery fan intake, and that will stress the HV battery.
     
  17. Rick F

    Rick F New Member

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    Where is that located , what seat? Is that the floor vents under driver and passenger seats?
     
  18. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Back seats. I'm not certain of the exact location in a V but it will definitely be the rear seats. If not under, look next to the seatbacks near where your elbow would be while sitting in back.
     
  19. Rick F

    Rick F New Member

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    So the issue is definitely heat related. After about 90 degrees outdoor heat the mpg takes a dive. Drove to the beach yesterday and averaged 37mpg. Going to tint the windows to try and help offset the ac working so hard. Also guessing the batteries have diminished in the last year because it was this drastic a difference last year. Changing the trans fluid tomorrow (toyota only fluid). Anything else we can do to help in this CA heat?
     
  20. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    reflective sunshade on the windshield, every single time you park. It really does help.
     
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