Is it Okay to Idle or Warm Up My Car?

Exhaust and red light
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To be honest, the question of whether it's good or bad to leave a car idling for an extended time has never crossed my mind. This letter from Deb made me think about it:

As I read the comments about warming up your car, I keep looking to see if it is actually bad for your engine to leave your car idling too long. I've been told that recently, and just wondered if it was true. Thanks for any help and advice you can give! Deb

So, is it bad? I say no. An idling engine that is properly tuned is an efficient machine. If your engine is at operating temperature, your fuel injection is metering fuel correctly, and your exhaust pipe doesn't have a banana in it, there's nothing to be harmed by letting it run.

The Idling Myth

So how did we end up with this longstanding myth that engine idling can be bad? Myths come from all over, and some statements that I will clearly describe as myth others will swear are proven facts. But when it comes to engine idling, I'm pretty firm in my beliefs. That said, I can certainly understand how and why people started to think that idling was bad. In fact, there probably are some cases in which you'd be better off shutting the engine down than idling it.

For example, let's say you drive a car that is so highly tuned for speed that it's practically a race car. When idling, these engines are running at their least efficient. Some will barely run at all! They're not fully burning the fuel that is coming into the engine (usually through a carburetor) and as a result they can stand to build up all sorts of gunk in the engine, especially around the valves. I'd imagine a '60s Ferrari falls into this category, which is also why the myth of "blowing the carbon out of the exhaust system" has been around for so long. It's based somewhere on fact, but has no practical application in the present day. 

While we're asking questions about engine idling, we have to ask why are you letting your car idle for a long time in the first place? I can understand a little morning warm-up in hideous weather, but try to do this as rarely as you can stand. It's bad for the environment to pump all those extra hydrocarbons into the atmosphere and you're wasting precious, not to mention expensive, gas. I don't think you'll do your engine any harm, but why do it? Give nature a break and turn it off.