A local guy years ago bought a new diesel pickup, those 6.2 gm's and on a really cold winters day, it wouldn't start, so he took a waste oil pan with some waste oil in it and lit it, slid it under the engine and waited for it to get warm enough to start. Turns out he used the waste oil from his old worn out car engine, that had far too much gasoline in it, as the flames came out around the engine compartment and lit the engine compartment and front tires on fire as he tried desperately to pull the oil pan back out, before he could get the water hose to put it out, his entire pickup was engulfed in flames as he watched it burn up. The only lucky part for him was that it was in front of his attached garage and not inside it or else he'd have burned his garage and house down as well. His next new pickup [the same year] was a gasoline instead of diesel. The investigators though it was gelled up as to the reason why it wouldn't start and stay running. Open flames with anything involving grease, diesel or gasoline probably isn't the best idea, but yet most of use salamander space heaters all the time to warm equipment up enough to start.
We've also taken huge 220volt tank heaters, 5000 plus watt units,[zero start and others sell them] plumbed in a hot water pump inline for forced circulation and used a welder generator to heat the engine that way, takes about 15 minutes on below zero days to warm most any engine up to 80 degree's or so, works for really fast safe starts in remote area's and saves dragging lp tanks for the hilton heaters or space heaters around, just pull up and fire up the welder and drag out a short 220 cord and wait a bit and shes warm and ready to start.
The Hilton cordless heaters, if we have a half dozen machines sitting on the job site, we use a couple of those heaters, have quick couplers set up in the lines ahead of time and have two machines warmed over night, then once there, we just uncouple those two heaters, carry them to the next machines, couple them up via the quick couplers and use the alligator clips to the battery, fire up the heater and while two engines are started and warming, we preheat the next two, so in a couple hours we have all machines warm and running with only two heaters and two 20 lb lp tanks, once done we just unhook them and put them back in the pickup to wait till evening to decide which two machines we'll need first the next day and hook them up to those machines that evening and repeat, saves the cost of having a heater on every machine I own, that way a couple heaters can be used on 20 machines or more, just as long as you don't need them all to start and run right away the next morning at the same time. Those heaters also come in two sizes, standard or high out, if I recall correctly the high out put version has more catalyst in them so they burn hotter, cost a couple hundred bucks more than the standard version, also heat a lot faster, so we bought the high output versions for use with the quick coupler use, standard version for the overnight use on smaller block engines. Its not a big deal, once the block was warm enough it would open the thermostat to allow cold coolant from the radiator to circulate through the engine anyhow. So with the high output version left on overnight, even the radiator would warm when you got there in the morning, worked great for the semi's, or anything with cab heaters on them, that way you had instant heater in the cab once the engine was running, seeing how the entire system was basically hot upon start up. Those heaters also had overheat shut downs built into them, so before they'd boil the coolant, the heater would shut down automatically.