In the old days, it was believed that you would save gas by leaving your car running if you planned to use it again within a few minutes. That has probably become less true over time as cars have gotten more and more efficient over time. Apparently, idling your car for even something like 10 seconds uses more gas than turning it off and on again. This is caused by the use of fancy fuel systems that cars use now. This technology is, of course, leveraged in hybrids which turn their internal combustion engines off and on as needed.
Anyway, here is an infographic that provides the details. The simple answer is, no, idling the vehicle is generally not a good idea, so stop doing that.
Transport for London has a similar campaign, which I first saw advertised on the back of a bus in outer London.
Just after I got my 2005 Prius, I took a young motorhead friend of mine for a ride -- I wanted to see if anything in the Prius would offend his motorhead sensibilities. But the first time we stopped at a light, and the gas motor shut off, he immediately said, "Why doesn't my car shut itself off when it's stopped?"
the chief reason folks let their car idle is because they're afraid it won't start again
@chris heinz: There are plenty of non-hybrid cars that do that as well now- a lot of manufacturers introduced it over the past few years.
Hans, I'm not sure that is the main reason. Here in Minnesota people idle their car in the winter to keep it warm, and a lot of people I know (mostly older folks) have the belief about gas. But you are correct that lots of people certainly do what you suggest. For years I drove a truck that didn't have a starter so I'd park it up hill, but sometimes I just left it running for few minutes (that was an old diesel, so the mileage thing may have applied there.
AlphaGamma: I did not know that, very cool.
Greg, my dad lives in WI, and I grew up in New England. I just don't buy that bit about cars needing 30 seconds to warm up :-) I have never, ever had that work. Most of the time when I tried to start anything but a brand-new car after a cold night it was a struggle. When the car starts in he winter with no problems, I say "praise the freaking Lord" because I never expect it.
Now, maybe it's because the cars I have experience with are relatively old. I'll certainly buy that.
That said, the rest of the thing about idling I do think is right on, and I always wondered if in heavy, Long-Island Expressway-is-a-parking-lot traffic it might be better to turn the car off (assuming I am driving one whose starter I trust :-) )
I think the need to warm up is highly variable by car, and how cold it is. The engines probably need very little warm up, but under some conditions, things can break if it is too cold. I'm told many cars in Alaska lack numerous plastic levers and dials because they froze and broke when someone tried to use them! My fetish is additives to the gas, the stuff that eats water once or twice during the winter, fuel injection cleaner whenever I get an oil change.
@Jesse: Starting the car is a separate issue from warming it up. In places where temperatures of -40 or lower are common, such as Fairbanks, cars have electric engine block heaters installed so that you can start it in cold weather. When I was in Fairbanks the recommendation was to "plug in" your car whenever the temperature was below +20 F (which was most of the time in winter months, as average January temperatures there are about -10 F). But once you got the car started, the engine would warm up reasonably quickly. I did observe some drivers in Fairbanks who idled while filling up their cars (which is a Really Bad Idea for obvious reasons), presumably because they feared not being able to restart the engine after they were done refueling. But that never made sense to me either (assuming your starter isn't iffy), since it takes the engine quite a bit longer than that to cool down, even when it's -40.
In temperatures that cold it is actually not unusual for engines to overheat. This happens because the belt that drives the coolant pump becomes brittle and falls off, and without coolant running through the engine block the engine temperature spikes upward. For this reason we generally travelled in convoys when going outside of the Fairbanks/College urban area*, and the rental car people at the Fairbanks airport are quite understanding--you just tell them where you had to abandon their car, and (as long as you weren't on one of the roads the contract specifically forbids) they'll close out that rental and give you another one.
*Peculiar Alaskan traffic law: you are required to stop and offer assistance to any motorists whose vehicle is disabled. The obvious reason is that situations that would be merely annoying elsewhere are potentially life-threatening in Alaska.
The engine may cool down slowly in the winter, but the *cabin* sure cools down quickly! On a really cold day, I can definitely see leaving the engine running just so the heater stays on. But this isn't to save gas, of course, it's to save the passenger who is waiting for Spouse to finish a shopping run or whatever. And in the summer, you might keep the car on for the A/C so any passengers left behind are comfy. So idling the engine is totally fair, as long as you're not doing it in the delusion of saving gas. ;-)
On a really cold day, I can definitely see leaving the engine running just so the heater stays on.
I can't speak for you specifically, but I, and most people I know, usually dress appropriately for the weather when driving somewhere. In most cases you have to venture outside the car at one or both ends of the trip, and during cold snaps moving about (i.e., getting out of the car) helps you stay warm. Not to mention the risk (small, I'm sure, but not zero) of carbon monoxide poisoning. I'm slightly more sympathetic to the air conditioning issue, but even then, unless there is a mobility issue, why are you staying in the car?
Japan has a big campaign called "Idling Stop".
Even the city buses will kill their engines at stop lights, unless safety dictates otherwise (on some of the rather steep hills, for example).
Diesel engines use very, very little fuel when idling, but there is really not much reason for the average driver to leave a diesel idling other than the desire to act like a big-rig trucker. And now even they are being encouraged to shut down when stopped. There are various approaches to providing cabin heat/cooling when parked long-term other than the main engine, and some truck stops apparently are providing air conditioning and other plug-in services to truckers. Diesel engines use so little fuel when idling that the engine won't reach operating temperatures at idle(or takes a very long time) from a cold start, and can overcool so much that fuel condensation can wash oil off the cylinder walls and cause premature wear.
" but there is really not much reason for the average driver to leave a diesel idling other than the desire to act like a big-rig trucker. " <-- exactly.
If you have a turbo diesel and its been on the road traveling for sometime it is best to leave it idle a few minutes before turning the engine off, It will reduce wear on the turbo unit. Likewise when starting a turbo diesel idle for a short time for the same reason.
I got this information from the manual to my turbo diesel,
@Makeinu: there may be cars that cut engines whilst stopped, but drivers do not. I've never seen so many cars idling in any weather. The air is very bad here at ground level because at a minimum, 20% of cars stopped are idling, usually for at least 10 minutes, often for hours.
Individuals have no understanding about air quality, but still will whinge about China polluting the air.
Biggest cognitive dissonance I've seen in a rich, educated country.