Several months ago some kind of sensor was replaced on my 1991 Honda Accord. This was the case where the fan would sometimes go on after the engine was turned off. And to turn off the fan it was sometimes necessary to turn on the ignition.
What led up to all this was that one time I was driving up a fairly steep incline to a park. When I turned the engine off that is when the fan turned itself on.
Well, I drove up to that park the other day and the same thing happened (after the whatever was replaced). One thing different though, I did detect a faint whiff of what smelled like burning rubber when I exited the car (to check on the fan).
Your Accord has a cooling fan that will turn itself on and off as necessary to cool down the engine even if the ignition is off. This is necessary because the coolant will “percolate” and get hotter before the engine cools down with the engine turned off. The sensor was replaced because the fan would not shut off by itself.
I’ve had to replace that sensor on my Corolla a couple of times. But like said above, on Hondas the fan can turn on and off with the engine in “off”. On my Corolla the key has to be “on” for the fan to spin.
What happens when you turn the engine off is that the heat energy contained in the mass around your engine’s cylinders, in its exhaust manifold, etc. propagate into the engine’s coolant, heads, etc. The combustion temperatures, exhaust manifold temps, etc. being far higher than the 210+/- temperatures that your engine coolant normally runs, it causes a rise in the engine’s average temperature once the coolant stops flowing through it.
In addition, this heat ultimately radiates out the sides of the engine, heating up the engine compartment.
Honda’s cooling fan system is designed to turn on to move air over the engine and through the engine compartment after you turn the key OFF to mitigate the temperature rise. It’s a good system, but takes some getting used to if you’re used to cars that shut everything down when you turn the key OFF.
You’d think a hunk of metal like an engine block couldn’t get any hotter than what it was when the engine was turned off. So there’d be no point to run the fan then. But like Mt Bike and Missileman say, that would only be true if the entire block, inside and out, was at the same temperature.
“Heat soak” most adversely affects the top of the engine. This is where the fuel handling bits are normally kept cool by fuel circulation when running.
In.the “good old days” (here I go again) when cars had non-electronic temperature gauges (back in the 1940s and 1950s), after the car had been run, when you shut off the engine, the temperature gauge would rise from the normal position. The water pump would not be circulating the coolant. If you immediately restarted the engine, the temperature gauge would then fall back to the normal reading because the coolant would again be circulating. This happens on today’s cars as well, but today’s cars have electronic gauges that go to the cold end of the scale when the key is turned off.