Why YSK: People seem to, on average, think that a car takes a lot of fuel to start up. In reality, it takes on the order of a few millilitres of fuel to start an engine. That means if your car isn’t equipped with an automatic start/stop system to stop your engine instead of idling, it saves fuel to turn off your engine and start it back up when you need it.
Caveat: air conditioning and radio might not work with the engine turned off.
Scenarios where this might be useful include waiting for trains to pass at rail crossings, waiting for food at drive-throughs, dropping off or picking people up on the side of the road when they need to load stuff, etc. May not be a good idea to use this while waiting at a red light because starting the engine does take time which would annoy drivers behind you when the light turns green.
Some cars are equipped with systems that will automatically stop the engine when you are idling for a while (e.g. waiting for a red light). If yours is, then manually turning off your engine will probably result in reduced fuel savings compared to just relying on the car to do it for you.
The few seconds is bare minimum, but you can improve on it. Again, if you live in a warm climate, this doesn’t apply. I give my engine a maximum of 5 seconds of idle time in the summer too, because I know the oil isn’t too viscous at those temperatures and I don’t need to demist my windshield. But in the winter, you can tell from the sound the engine’s making that it’s not happy at -30C, even in a fairly new car with good oil. So you start off idling a bit and then start driving gently. I let it idle till I can see the mist starting to disappear from the windshield on its own, then drive real gentle - luckily I usually drive on low-traffic roads in the morning, so I don’t have to gun it to the speed limit in one second to keep everyone else happy.
There is truth to the damage from excessive cold idling, which is why I don’t recommend doing it too much on newer cars either - in direct injection cars in particular, I believe you can wash oil film off the cylinder walls with excess cold idling. But a minute or two is fine. We’re talking about temperatures that people from warmer climates might consider deadly cold. Not just mildly freezing temperatures
