phinsil6:
also, why would you consider leaving a car to "warm up" while running idle a bad idea?
It is very bad for an engine to run cold - it causes poor lubricant distribution and accumulates acidic moisture all around the insides of the engine. It's desirable to warm the engine up as fast as you can, within the constraint that very high revs or load while it's cold are also bad for it. Leaving the engine running at idle gives you the slowest possible warmup which gives you the maximum time running cold, so it is a bad way to warm up an engine. Normal sensible driving is a happy medium which warms the engine up quickly without overstressing it while cold.