[spike021]

I have a relatively analogue car (Subaru BRZ) and there are times when you shut it off on a warm day and then randomly about ten mins later a radiator fan kicks on. I guess it's to help cool down but I don't remember exactly what the manual says.

Basically it's cool how cars do things largely on their own sometimes to stay within certain parameters.

[bb123]

This is 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.

[op00to]

I had an old Volvo that would “vapor lock” on short trips. It’s amazing how reliable cars are now compared to 20 years ago.

[rootusrootus]

In my experience it was more than 20 years ago that this happened. It was when most cars switched to fuel injection. Late 80s, early 90s. Treating a car as an appliance and having it 'just work' became so much easier when fuel injection was invented.

20 years ago was 2003, which is practically yesterday [in my mind...].

[bratwurst3000]

To my knowledge it is because the the oil and water needs the motor on to move through the engine and cool certain parts . If the motor is shout down to early and those can’t circulate anymore following could happen

-turbocharger overheats

-oil overheats And the things you stated and much more …

Better to let the motor run while standing is to run it the last kilometers in low rpm

[l33tman]

same issue with projector lamps, the fan is kept on for a while after you shut them off

[HPsquared]

And ovens.

[almostnormal]

And some PC power supplies.

[krallja]

My portable induction stove runs its fan for a minute after I shut off the coil.

[actionfromafar]

Also turbo charged engines can greatly benefit from running idle for a minute. I learned the hard way. (Had to replace the turbo.)

[rainbowzootsuit]

There have existed "turbo timers" that keep the engine running for a short time after the key is off to help avoid this. Not sure if that works with modern cars as easily.

[rootusrootus]

Modern turbo designs (most anything in the last 25 years, at least) use water cooled turbos and convection keeps the water flowing after the engine is turned off. This is why we don't use turbo timers any more.

[gambiting]

And also even back then it affected almost no one, there were loads of tests done on this by automotive magazines of the era and found out that usually the last 1-2 minutes of gentle driving to your driveway/garage is more than enough to cool down the turbo properly, the only people affected were those who due to either aggressive driving or circumstances of their location would drive at high rpm, stop, then immediately shut down their engines.

[actionfromafar]

Towing something heavy uphill, then stopping to get some gas and a sandwich. The SAAB killer move.

[FireBeyond]

I have a 2021 twin turbo car (Audi RS 5). It runs the radiator and pumps adaptively on engine shut off to bring things down.

[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).

[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.

[kiksy]

I have the same car, it does this too. As have most if not all of my other cars I've owned. I've always put it down to the fact there is no cooling from the forward motion of the car passing air towards the radiators so the engine slowly heats from the already generated heat from driving. The temperature hits the limit the sensor has set and the fan turns on until it's below that set temperature.

[c0nsumer]

Because it was designed with Toyota I don't know if the BRZ does it, but a number of other Subarus will kick on an recovery pump ~6 hours after ignition. It sounds sort of like a small/quiet compressor.

Before I looked up what it was, I kept hearing a gentle noise coming from the garage in the late evening. Figured it was noise from outside... Then one day I was out there and heard it start, realized it was the car, and looked it up.

[rootusrootus]

That is the evap leak testing pump. It pressurizes the tank briefly to test the effectiveness of the cap.

[twisteriffic]

This confused the hell out of me the first time I heard it. Taking the garbage out in the middle of the night and hear whirring coming from under a vehicle that hasn't moved in hours.

[rpmisms]

My Kawasaki Ninja 500 will do the same thing, and it's a bike with very old tech. It is air-cooled, so it's probably a safety feature. Still cool.

[akira2501]

Supposedly one of the arguments for lane splitting in California was the propensity for bikes to overheat out here when stopped in traffic. My 650 did not have a fan but it did a good enough job heating up my crotch to a nearly unbearable degree on a hot day without it.

[IgorPartola]

I have certainly experienced overheating due to traffic on my air cooled bike. Now I am thinking about adding a fan to my oil cooler that would either be on a switch or even on a proper thermostat.

[rpmisms]

I have as well, and it's why I'm getting a liquid-cooled bike.