Is idling a car engine for extended periods - over 6 hrs - harmful?

Answer 1

Yes, ideling your engine for extended periods of time is not recommended, due to the fact that carbon deposits can build up on the head of the pistons and CFC's (unburned gases) emitted out the tail pipe are more concentrated and harmful to the environment.

Engine idling does have a couple of effects on the engine, the most severe is to the connecting rod bearings. Because of the relatively slow speed of the engine, more pressure is exerted on the bottom center and top center of the bearings. The faster the engine turns, the load gets spread out more evenly over the bearing surface. Prime vehicles for this kind of damage is police cars. Police cars spend an inordinate amount of time idling writing tickets, filling out reports and stake outs. I have replaced many a connecting rod bearing in police car engines because of this. The other effect is not so drastic. It is an idling engine may not get hot enough to start the catalytic convert working or hot enough to run efficiently. It may also load up the catalytic converter with fuel that could lead to an early demise of the catalytic converter.