[edit] BTW there is a legend here that in Siberia people actually keep a fire under engines to warm it up. I even found a photograph of lorries to support this: http://www.marxists.org/history/ussr/art/photography/workers...
Not sure if I would do it with gasoline engines. With diesel engines it's quite understandable, particularly if you have summer-grade diesel fuel that starts to congeal in -15C.