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Cooling fan running with engine off?

I noticed that in some cars (even new ones) the cooling fan stays on for a minute or two, even after the engine has been turned off and the key removed.

This caused me a little bit of curiosity —keep in mind I'm not very car-savvy— so I wanted to ask: Why is that?

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  • When some engines are shut off, especially on a hot day, the temperature in the engine can actually increase due to "percolation". This is why the electric cooling fans can run for several minutes in order to cool down the engine. It's been this way for many years.
  • This is entirely normal. As missileman explained, the fans are removing excess heat from the engine's cooling system.

    Some cars allow the fan to run after the key has been removed, some cars don't. I like the ones that allow the fan to run.

  • If you look under the hood, you will see a warning label telling owners to expect this fan behavior. It's quite common in today's cars.

  • edited July 2011
    I see, but why not let the engine cool by itself?

    Because the way I see it, its temperature would drop slower and more gradually than with the fan on. Which wouldn't seem to matter since the engine wouldn't be generating more heat.
  • It's a function of the fan being controlled by a thermostat. There probably would be nothing wrong with letting the engine cool "by itself" (which is what happens if the design removes power from the fan when the ignition is off), but since the normal operation of most cars is to run the fan when the water temperature exceeds a set threshold, it does so even with the engine off. The rise in the temperature of the water after the engine is shut off is because of the residual heat in the engine and the lack of further airflow across the radiator - hence the running of the fan.
  • edited July 2011
    If your temperature gauge still operates when the ignition is switched off, as it does on some cars, you will see the needle rises after the engine has stopped. That is because it is measuring coolant temperature, not engine temperature. The engine is still much hotter than the coolant. Yes, the engine still puts out heat even when it is off.

    You want to remove excess heat. The operating fan helps out by blowing fresh air past the engine. Perhaps there are some cars that, on very hot days, may even boil were it not for the operating fan.
  • SteveF has it right...With all engine and cooling system designs, when you shut off the engine, you also shut off the water pump. Coolant trapped in the heads can boil as it absorbs the heat in the metal engine parts with no way of transporting this heat to the radiator. Running the fan for a few minutes can prevent this from turning into a boil-over and coolant loss...Car owners need to realize that 65-70% of the energy they buy in the form of gasoline is converted into waste heat by their cars engine...Car engines make a much better furnace than they do a producer of torque....
  • That makes a lot of sense! Thanks for clearing that one up for me.

    One more little thing —now that you mentioned it—, the temp gauge always measures coolant temperature, or it depends on the car?

    The next time I'm driving I'll make the experiment of checking the needle after I turn off the engine.
  • The engine isn't the only thing that gets hot. The fan helps keep the wiring and electronic junk from holding the maximum amount of heat. It can't hurt to have the fan run for a while unless you have an old battery, but that's easy to fix.
  • percolation? The fan stops cooling the radiator and the water pump quits pumping water. Yes. The first thing an engine does when you shut it off is get hotter
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