The genesis, I think, is from seeing unoccupied long-haul trucks with their engines running, and the practice does have some basis in fact. Diesels do burn very little fuel at idle because unlike gasoline engines, their throttles do not restrict the amount of air they ingest. As for truckers, the practice seems to have stemmed from the need to keep diesel fuel from getting too cold in winter and gelling. In any case, letting your diesel idle for anything more than a short duration is a bad idea because while the engine will use little fuel, what fuel it does burn will not combust completely because the operating temperature is too low. Unlike gasoline engines, diesels need to be under load to reach optimum operating temperature—if they’re not, unburned fuel can cause needless pollution and even can dilute the lubricating oil, increasing wear.