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I have a 2005 manual transmission Toyota Corolla. Just this winter, it's been doing this weird thing in the morning: I'll start the car and start driving it, and the engine starts warming up. Once it's warmed up (temperature gauge in the middle), I'll turn on the heater if the windshield isn't fully defrosted. However, it won't start blowing warm air right away. While it's blowing cold air, the temperature gauge will creep up until it's close to the red, where it'll remain for a few seconds. After that, I can hear something... shift? behind the dashboard, and hot air will start blowing, and the temperature gauge will start going down.

It only does this when it's cold, and it won't overheat on the rest of my commute. When I drive home at the end of the day, this doesn't happen.

To give context to what I mean by "cold", I live in northern California, so this winter the temperature range in a given day is from 30ish to 60ish degrees F (

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    Does the alleged overheat happen if you don't turn on the defroster?
    – fred_dot_u
    Commented Jan 14, 2022 at 9:41
  • I... don't think so?
    – John Doe
    Commented Jan 14, 2022 at 15:30
  • Did you check coolant level? Some cars have a low coolant light while others do not. If the coolant container is empty, air may enter the cooling system resulting in erratic temperatures.
    – F Dryer
    Commented Jan 15, 2022 at 15:46
  • As far as the "shifting" is concerned there could be an issue with a stuck blend door actuator, linkage, or blend door itself that occurs when cold. This of course doesn't explain the overheating issue. Perhaps a stuck thermostat when cold? This will of course take some diagnosing.
    – Jeremy
    Commented Jan 27, 2022 at 16:15
  • @FDryer I topped off the radiator with water, and that seems to have helped, but the behavior is still occurring.
    – John Doe
    Commented Jan 27, 2022 at 20:16

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