[throwawaaarrgh]

The fan is triggered by a relay, triggered by the ECU, which uses coolant sensors (the thermostat sensor that is iirc not always the mechanical thermostat) to adjust when to cool the radiator. That temp is much higher than the ambient air will ever get, so if the fan comes on after the car is turned off then either it was just about at the temp that would have turned on the fan, or something could be slowly going wrong with the car to trigger the fan unnecessarily. A car is a system of systems so many different things can cause it.

If it happens a lot (when turned off) I'd think about having an inspection done. Worst case scenario, engine ends up overheating, warps or explodes, and you need a new one. Coolant system is one of those "oh shit" systems that you need to pay close attention to.

[sgc]

Car engines heat up when you park, because the air cooling and circulation of the coolant keeps it cooler while driving. It seems completely within the common laws of physics that when you are at not so uncommon temperatures, the coolant closer to the engine heats up well above standard operating temperatures once the car is off, and then convection currents bring enough of this overheated coolant to the sensor to trigger the fan. I have seen this behavior on many cars over the years, without any underlying concern in any of them. If it is not running hot while driving, I personally would not worry about it.

[JasonSage]

This is 100% correct.

For those curious, the coolant around the engine block can boil and flow upwards after the engine is off, and the coolant circulation system is designed to handle this pattern. You can observe this yourself if you overfill your coolant and take it out on a hot day, the excess hot coolant will bubble up and run out for a good while after the engine is off.

I second the advice that if the engine isn’t overheating while running, everything is likely working as intended.

[londons_explore]

It's also a common cause of breakdowns... The fan normally cools the radiator, but the temperature sensor is on a coolant hose. Since the engine is stopped coolant isn't flowing so the fan isn't cooling the same thing the sensor is sensing, which means it can end up running for 30 mins or more. Since the fan can be 20 amps or so, you can easily kill old car batteries due to this effect. User comes back after an hour shopping to find their car battery is dead.

If I was a car manufacturer, I would wire the fan into the ignition circuit so it can never run with the ignition off - it's fairly useless to have a fan blowing on a radiator when the coolant isn't flowing anyway.

[pests]

Isn't that feature the exact one being discussed in this threads in order to not overheat the engine though?

[londons_explore]

Yes, but other posters are mistaken that it is a 'clever' feature to prevent overheat during heat soak. It is instead more an accidental byproduct of the fact it is wired direct to the battery and not to the ignition circuit. Thats done to save a couple of cents on the ignition switch (no need for a large high current fan to be powered through it).

[epcoa]

No you’re mistaken.

> Thats done to save a couple of cents on the ignition switch (no need for a large high current fan to be powered through it).

The fan must be powered by a relay in most cases so this is utter bullshit.

In most any modern car the fan is controlled by the ECM (a computer). Furthermore this fan relay will come on in anticipation of high heat load such as when the AC is turned on as well in cars without dual fans.

There is so much more complexity here than just wiring through the ignition circuit (which wouldn’t cost any more anyway as noted) that thinking this is a cost saving issue is utterly clueless.

And the temperature sensor that controls it is invariably located in the engine block, not the radiator hose. Modern cars may have multiple sensors but they need one in the block.

Cooling fans in most cars don’t draw 20 amps while running normally (they should be closer to 10), and while anything can fail the cooling fan in these setups should only run for a few minutes. Draining the battery like you claim is not a “common” cause of breakdowns.

You clearly don’t know much about modern (last >30 years) cars to be commenting on them so authoritatively, tone down your arrogance.

https://acurazine.com/forums/3g-tl-2004-2008-93/wiring-diagr...

This is a typical setup (and it’s 20 years ago). Debunking two of your claims. First note that the condenser fan is wired through the ignition, debunking the notion that it would be costly to do so and that running the main fan bypassed isn’t intentional. Also note that the actual fan control is the lower right black box PCM/ECM (engine computer)

[throwawaaarrgh]

If it goes on for 15 minutes or more, it could be one of seven different problems: https://mindofmechanic.com/fan-still-running-when-car-is-off...

Cars are like people. Sometimes a quirk is normal, and sometimes it means your bottom end is gonna fall out. Gotta get regular checkups and hope you catch it in time.

[pests]

> Car engines heat up when you park, because the air cooling and circulation of the coolant

A further reason is no movement = no wind speed. I used to have an old Pontiac grand am with the ram air. I had overheating issues sometimes that were literally solved by going faster.

[bb123]

I don’t think this is true - this is completely normal behaviour. The cooling fan is coming on due to a phenomenon called heat soak which is where the engine temperature increases above normal for a time after it has been switched off due to the lack of coolant flow. The fan is coming on to aid cooling the engine/engine bay to reduce the effect of heat soak.

With carburettor fed engines it is sometimes advised by the manufacturer to allow the engine to idle for some time after running it at high RPM. This is because the effect of heat soak can be significant enough to boil all the fuel off in the carb, making the engine hard to restart when hot.