Idling can actually damage your engine over the long term. Because idling engines don’t operate at their peak temperatures, the fuel passing into the engine does not completely combust, leaving residue in the engine that can contaminate engine oil and make spark plugs dirty. Excessive idling also allows water to condense in the vehicle’s exhaust, contributing to corrosion.
While older cars from the 1970s and early 1980s might have needed time to warm up, the norm since the mid-1980s has been fuel-injection vehicles which can be restarted frequently without engine damage.
When you start a car’s engine, a little bit of extra gas is used to get the combustion process started. That means a good rule of thumb for conserving gas is to idle for no more than 30 seconds, except, of course, if you’re just sitting in traffic.
If you have a hybrid car, it does the work of avoiding idling for you. While parked or sitting in traffic, hybrids shut off their gasoline engines automatically. See also: ::Citroën C2, Now With Stop & Start Technology