50F Thermal Cycling in Ready and Higher Charge Voltage in Park

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by rjparker, Dec 15, 2020.

  1. rjparker

    rjparker Senior Member

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    Well, I have a real snoozer 17 minute video here for the seriously disturbed. It does have a Schubert sound track live from a Prius v.

    Two unrelated but frequently debated Prius behaviors are captured in the video.

    1. Does a Prius exhibit serious thermal cycling of the engine?
    This has been offered as a reason Prius 1.8L engines have more head gasket problems then Corollas with the same engine but no EV off states when running. I suggest they do thermal cycle by as much as 50F (eg 150-200F) especially on long "idles" (EV) or city driving with a healthy hv battery. Others say no.

    The "Car Scanner" screen capture below starts with a fully warmed up 2012 Prius v after about 10 minutes in Ready but waiting outside for the wife. The coolant temp is 150F, then increases to 170F on level roads and peaks at 200F as we approach the house after driving up a fairly steep hill. Then, as a test, it is Park (Ready mode) for ten minutes. The temp falls back to almost 150F. Thermal cycling seems to occur. Selected data points below.

    2. A different issue concerning aux battery charging is captured in the same video.
    In Ready Park, the aux voltage is 14.4v. In Drive it almost immediately goes to 13.4v. Park again and it goes back to 14.4v. This behavior is repeated several times throughout the test including ten minutes at the end while in Park. What is going on? Is more electrical power diverted to the migs while in drive resulting in less for the hv battery?

    The aux battery has been charged and exhibits about 12.7 when Ready is off. At the beginning of the video the car is in Ready and in Park. Zero on the speed indicator. Aux charge voltage is about 14.4v. Soon after placing it into drive, the voltage drops to around 13.4v and stays there. You can hear the door locks operate as it goes from Park to Drive or back to Park. I stop several times in the video and go to Park. Voltage goes up. Start driving, voltage goes down. The last ten mintes, I am sitting in the driveway with Ready Park. The voltage stays up for that ten minutes.

    So driving is 13.4v. Park Ready is 14.4v. Apparently some sort of system logic?

    Time Temp Volts Ready Mode (Selected)
    0:00 150F 14.4 Park
    0:10 155F 13.38 Drive
    1:50 170F 13.38 Drive
    4:00 180F 13.4 Drive (uphill)
    5:55 200F 13.4 Drive (top of hill)
    7:05 185F 14.4 Park (home)
    11:00 172F 14.4 Park (home)
    13:00 167F 14.4 Park (home)
    17:00 158F 14.4 Park (home)


    Video is raw data without narration primarily for reference.
     
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  2. rjparker

    rjparker Senior Member

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    Bump on Aux Battery Charging question above:

    2. A different issue concerning aux battery charging is captured in the same video. In Ready Park, the aux voltage is 14.4v. In Drive it almost immediately goes to 13.4v. Park again and it goes back to 14.4v. This behavior is repeated several times throughout the test including ten minutes at the end while in Park. What is going on? Is more electrical power diverted to the migs while in drive resulting in less for the aux battery?
     
  3. davecook89t

    davecook89t Senior Member

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    I would say the conditions that produced the 50F temperature difference in your coolant temperature are not really typical. How many taxi drivers (the group cited in the original video arguing that thermal cycling causes head gasket failures) drive up a steep hill, then let the car idle, then drive up another steep hill multiple times a day? In a city like San Francisco, the car may be driving up a steep hill several times a day, but there wouldn't be rapid fluctuations in the thermal temperatures as originally postulated because there would be downhill portions stretching out the interval between extreme coolant temperatures. Most cities are fairly flat, so I don't think many taxi drivers would see these kind of fluctuations anyway.
     
  4. jdenenberg

    jdenenberg EE Professor

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    Thermal cycling and vibration can loosen up the head bolts in an engine. It is good practice to re-torque head bolts after a break-in period (at least it was when I was a bit younger).

    JeffD
     
  5. rjparker

    rjparker Senior Member

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    My example may be the maximum delta t over a short period but does occur. I live outside of San Antonio which is flat over a large geographic area. To the north and west is the Texas Hill Country.

    In our hot and long summers, freeway and ac use keep the coolant temps around 195-200f, while a traffic jam or wait period will quickly drop the temp to 150f. Everytime. Even two or three minutes at a long light will cause a change. So thermal cycling is common but the car has no temperature gauge that would make it common knowledge. It is unlikely to be hundreds of times a day for normal drivers as suggested by the Gasketmasters video.

    Just offering data rather than pure conjecture.
     
    #5 rjparker, Dec 16, 2020
    Last edited: Dec 16, 2020
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  6. davecook89t

    davecook89t Senior Member

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    Gasketmasters are making their case by comparing the Prius engine to the Corolla engine. Is that a fair comparison, given that the Prius uses the Atkinson cycle, whereas the Corolla does not? How would coolant temperature extremes differ between the Corolla and Prius?
     
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