One reason is that unless you get all the parts up to proper operating
temperature, then you're not evaporating any of the nasty stuff that
collects in the oil and exhaust system, so repeatedly starting the car just
causes the crankcase and exhaust to get contaminated with raw fuel, water
vapor, and various (other) corrosive substances. Carbureted cars tend to wet
down the cylinder walls with raw fuel as well, either during startup or
shutdown, and that dilutes the oil in a place where you really need it. The
camshaft lobes are lubricated mainly by oil splashing from the crankshaft,
and there is not much of that going on when the engine is just sitting there
idling, especially if the oil is cold. So if you want to run the engine in
the winter, then the best thing is to wait for a nice day when the roads are
clear, then take the car out for a hard run. Winter tires are pretty much a
must-have for anything like that...
Theo
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