crankcase type fumes

2002 Ram 4.7L - When the temp goes below -15 to -20C (0 to -5 F) the truck cab begins to fill with burnt oil/gas smell when idling and heater blower is on. All connections under the hood appear connected. It's like there some kind of engine blow-by valve that's freezing open (shut?). I'm not sure how everything hangs together under the hood (emissions stuff etc). Any ideas?

Randy