from Natural resources Canada for fuel efficiency techniques:
Avoid unnecessary idling – turn off your engine when you park for more than 60 seconds, except in traffic. The average vehicle with a 3L engine wastes 300 ml (more than 1 cup) of fuel for every 10 minutes it idles.
Contrary to popular belief, excessive idling is not an effective way to warm up your vehicle, even in cold weather. The best way to warm it up is to drive it. With today's computer-controlled engines, no more than 2-3 minutes of idles is usually enough warm-up time before starting to drive – even in cold winter days. (but make sure that windows are free from snow and properly defrosted before driving away).
Under tests in a cold chamber and 3 vehicles, each was cooled to -18ÂşC (0ÂşF) and driven over a simulated urban driving cycle using first a 5 minute warm-up and then a 10 minute before driven. Test results on 5 min warm-up fuel consumption increased by 7-14%, with a 10 minute it increased from 12-19%.
You can help reduce the impact of cold starts – and reduce idling times – by using a block heater on a timer. This device warms the coolant, which in turn warm the engine block and lubricants. The engine will start more easily and reach its proper operating temperature faster. Also, it will not have to work as hard to pump oil through the block. At -20ºC (-4F), block heaters can improve overall fuel economy by as much as 10%. For a single short trip at -25ºC (-13F), you fuel savings could be in the order of 25%. To limit the amount of electricity used to operate your block heater, put in on a timer so that it runs 2 hours before you have to leave.
What's often forgotten is that idling warms only the engine – not the wheel bearings, steering, suspension, transmission and tires. These parts also need to be warmed up, and the only way to do that is to drive the vehicle. Until the engine temperature begins to rise, it's a good idea to avoid high speeds and rapid acceleration.