Stephen Ciatti, a former drag racer who has a PhD in mechanical engineering from the University of Wisconsin-Madison and now oversees all of the combustion engine work at the Dept. of Energy’s Argonne National Laboratory, explains how idling hurts engines, excerpted below from the article: “Stop hurting your engine by idling the car when it’s cold out” http://www.businessinsider.com/heres-what-idling-your-car-in-the-morning-is-doing-to-your-engine-and-its-not-good-2016-1
Under normal conditions, your car engine runs on a mixture of air and vaporized fuel, gasoline in this case. When that mixture enters a cylinder, a piston compresses it, which — at the risk of oversimplifying — generates a combustion event, powering the engine. But when it’s cold outside, gasoline is less likely to evaporate. Your car compensates for this initially by adding more gasoline to the air-vapor mixture — what Ciatti calls running “rich” — and that’s where the problem begins.
“That’s a problem because you’re actually putting extra fuel into the combustion chamber to make it burn and some of it can get onto the cylinder walls,” Ciatti said. “Gasoline is an outstanding solvent and it can actually wash oil off the walls if you run it in those cold idle conditions for an extended period of time.” Over time, that washing action can “have a detrimental effect on the lubrication and life of things like piston rings and cylinder liners,” which are critical to running the cylinders and pistons that breathe life into your engine, Ciatti said.
The bottom line: Contrary to popular belief, idling your car does not prolong the life of your engine; rather it shortens it.