I found this guys blog and thought it was interesting.
I can remember my grandfather going outside 10 minutes or so before he left for work to start the car. He'd crank it up, then come back inside and eat a banana or something while he let his bright yellow Lincoln warm up. Back in the day, this wasteful ritual was practiced regularly by drivers around the world. It was rooted in practicality. Old carbureted engines functioned much better warm than cold. They ran more smoothly and efficiently once warmed up. So letting an engine warm up at idle was actually not a bad idea at all.
Fast forward to this millennium and we've got a very different story. Carbs are history, our engines use fuel injection systems that carefully monitor efficiency both in engine power and fuel delivery. These systems employ subsystems that compensate for things like temperature by making minute external adjustments to fueling. This means it will operate at optimum efficiency regardless of engine temperatures. First thing in the morning, it's adding a little extra to the mix to get things warmed up. It's also making sure that when you step on the gas there's enough oomph to get you up to speed quickly. This means that any amount of time you let your car sit in the driveway idling you're wasting gas. You're also not doing the environment any favors because your emission control devices (like the catalytic converter) only function properly hot, and they heat up much faster by driving than sitting still in the driveway. So even if you're a grandpa, don't watch your car warm up through the kitchen window, get going!
Matthew's Auto Repair Blog
By Matthew Wright, About.com Guide to Auto Repair
