Some diesel engine owners think it’s good, even necessary, to let their engines idle for twenty or thirty minutes after startup or before shut down. Truth is, besides being wearisome to neighbors excessive idling can be harmful to your engine.
If running hard or pulling up hill, the engine may need to be run as long as 5 minutes at idle to uniformly cool internal components and reject heat away from turbo bearings; running slowly off the Interstate ramp and driving at low power levels to the fuel stop or campground counts for some of this time, but if running on the highway, it’s a good idea to idle for a couple of minutes before shutting down.
Engine Manufactures do NOT recommend excessive idling of the engines, since it can cause excessive carbon buildup on the pistons, piston rings, injector tips, valves, etc.
Read more about diesel engine idling at RVbasics.com