Idling is bad for an engine.
This is because the engine is not running under a load, which results in incomplete combustion of the fuel, and the fuel in turn condenses on the cylinder walls. This not only causes extra wear of the cylinder walls(because the fuel washes the lubricating oil off the walls), but it also results in unburned fuel flowing down the walls and contaminating the oil in the crankcase. Other byproducts of incomplete combustion also find their way into the crankcase oil as well, including water, acids, and carbon particles. And these engine damaging conditions are worse during the engine warm up period when the engine is fed a richer than normal idle fuel mixture in order to prevent it from stalling. Idling also drops the temperature of the spark plugs, leading to dirty plugs, which can increase fuel consumption.
The best way to minimize this contamination and resultant engine damage is to warm the engine as quickly as is reasonably possible. And that means driving the car, which places a load on the engine and causes it to generate more heat. 15-30 sec. of engine idling is sufficient to get the oil pressure up and flowing to the critical engine areas, unless the temps drop to around 0 F. Then, idling for a minute or so is recommended. If one lives in a location where the temperature frequently dips way below 0 F, then an engine oil heater should be installed.
Finally, and even more important than the above, is to drive conservatively and stay out of the loud pedal until the engine is fully warmed up.
Idling is bad for the exhaust system.
Water vapor that's produced during combustion forms into water droplets that are deposited inside a cold exhaust system. The longer a cold engine runs at idle, the longer it takes the exhaust system to warm up, and thus more water will be deposited inside the exhaust system causing it to rust quicker.
Idling is bad for the environment.
Long periods of cold engine idling causes the engine to produce a lot more pollutants, because it takes the catalytic converters longer to reach their normal operating temperature when they work much more efficiently.
