YSK: A car takes uses about seven seconds worth of fuel to start up. If you are going to idle for a period of time longer than that, you'll save fuel by turning your engine off.
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.
When it's cold it can be beneficial to let the car idle for about 15 seconds before moving, to get the oils moving. They don't usually need much more than that, unless it's very cold of course.
Also cars will warm up faster driving around minimizing wear while engine is cold. Just don't stomp on the gas go easy on it
Indeed! It's a balancing act. It's good to wait a while right at start, but then it's better to go than wait on idle.
Yeah, I wouldn't recommend long idling*, but I also wouldn't recommend shutting down the engine for short stops when it's cold. Just keep it running if you're stopping for less than 5 minutes.
* Where I live, the winters can get to around -30C, though normally it won't get colder than -25. You'll want to let your car idle for a few minutes because otherwise it's not going to be blowing any warm air at the windshield, and your visibility will be shit.
How cold are we talking? It routinely falls below 0F where I live in the winter. Are we talking anything below 32F (0C) requires more than 15 seconds?
I've heard mixed information from the car people in my life.
There are no hard rules, cars are different and so are oils, but I personally would give it a few minutes under 0F and maybe up to a minute if it's just a bit under the freezing point, but not too cold.
Of course, something like a 0w30 oil will get pumped around the engine much faster in the cold than a 10w30 or 10w40 and therefore won't need as much time.
Also, idling isn't terribly good for your engine either, you should only do it enough to get the oil flowing and the blower to start putting out a tiny bit of warmth. Then you're better off driving, because that warms up the engine faster.
Everything I've read says that newer engines (like last decade or two) only need a couple seconds. All the warming up for a minute plus advice was literally from carburator-era engines. Newer engines will be damaged from running cold far more than anything else, so it's best to give them a second or two and then drive to get them warmed faster. That DOESN'T mean to push the engine or red line it, but easy driving.
But I'm not a mechanic, just a super type-A scientist that looks into things at a semi-ridiculous level for the sheer joy of knowledge.