Should I Shut Off the Motor When I'm Idling my Car?
Here's the rule of thumb: If you're in a drive-through restaurant/business line or waiting for someone and you'll be parked and sitting for 10 seconds or longer... turn off your car's engine.
Why? For every two minutes a car is idling, it uses about the same amount of fuel it takes to go about one mile. Research indicates that the average person idles their car five to 10 minutes a day. People usually idle their cars more in the winter than in the summer. But even in winter, you don't need to let your car sit and idle for five minutes to "warm it up" when 30 seconds will do just fine.
But you're not going anywhere. Idling gets ZERO miles per gallon.
The recommendation is: If you are going to be parked for more than 30 seconds, turn off the engine. Ten seconds of idling can use more fuel than turning off the engine and restarting it. And when you start your engine, don't step down on the accelerator, just simply turn the key to start.
An alternative to idling is to park your car, walk inside, do your business and then go back to your car.
Here are some other Myths associated with idling.
Myth #1: The engine should be warmed up before driving. Reality: Idling is not an effective way to warm up your vehicle, even in cold weather. The best way to do this is to drive the vehicle. With today's modern engines, you need no more than 30 seconds of idling on winter days before driving away.
Myth #2: Idling is good for your engine. Reality: Excessive idling can actually damage your engine components, including cylinders, spark plugs, and exhaust systems. Fuel is only partially combusted when idling because an engine does not operate at its peak temperature. This leads to the build up of fuel residues on cylinder walls that can damage engine components and increase fuel consumption.
Myth #3: Shutting off and restarting your vehicle is hard on the engine and uses more gas than if you leave it running. Reality: Frequent restarting has little impact on engine components like the battery and the starter motor. Component wear caused by restarting the engine is estimated to add $10 per year to the cost of driving, money that will likely be recovered several times over in fuel savings from reduced idling. The bottom line is that more than ten seconds of idling uses more fuel than restarting the engine.
Courtesy of: http://www.consumerenergycenter.org/myths/idling.html