Washington Post Wonkblog: The biggest winter energy myth: That you need to idle your car before driving
This article got a level of hate usually reserved for political pieces. The thing is, it's correct, though it did genuinely overlook one thing — idling your car to activate the defroster so you can see through the windshield is sometimes necessary. I've had to do this on numerous occasions, usually when I've also had to scrape ice or snow off the windshield, and it's usually defrosted by the time I'm finished.
Otherwise, most of the objections people raise are moot.
(Full disclosure: I have never lived north of Reno, so I can't speak for the subzero tundra of the Great White North. But given the anti-idling laws in Minneapolis and elsewhere, it can't be that radically different.)
My car's owner's manual has this to say about driving with a freezing engine:
The idea, which I know to be true in my car from eight years of driving it, is that the engine warms up more efficiently when it's operated normally instead of humming at idling speed. On the few occasions I've let it idle for an extended period of time — usually when a foot of snow was involved — the blue cold-engine light was still on afterward, and the cabin not much warmer.
Which brings me to the most common objection to this article: driver comfort. "I don't idle the engine for the car, I do it for ME!" There's a nice picture — millions of cars idling and burning greenhouse gases solely so the driver isn't inconvenienced for a minute, but it's not even a good tradeoff. It stands to reason that if a car runs more efficiently when it's being driven normally, so will its heater. It'll take the same amount of time (if not less) to warm up while driving, so why wait?
Most of the other objections involve outdated notions of the internal-combustion engine. Maybe you needed to warm up a carburetor, but unless you're a classic-car aficionado or really need a raise, you have fuel injection, which doesn't need warming up. Maybe you're concerned about oil viscosity, but most motor oils today are formulated to account for extreme temperatures. Maybe you're worried about your antifreeze, but most are formulated to start your car short of a blizzard in Antarctica. Maybe your well-meaning uncle offers you tips courtesy of his 1971 Buick, but perhaps you should just read your owner's manual.
(Scratch that — you should definitely read your owner's manual. Not every car plays by the same rules, and you might find plenty of surprises. I still do after eight years.)
In any case (deicing aside), there is no reason to start a car under 25 years old and leave it idling for an extended period of time. If your car was made in that time frame (or even a few years before), you're driving a computer. Don't treat it like a typewriter.