[H. L. Chauvin]

FWIW:

Always difficult to solve from miles away .... all above replies very possible causes.

With windshield open, and "if" your gas tank cap is loose, rising gasoline evaporation vapors in hot weather could enter through the open windshield while traveling ........ or when stopped with engine fan blowing air out of sides of hood ..... mix this with engine blow-by out of the oil filler cap and you may have a "recipe" for a "mixed" odor.