Both Mary and Deanna can have a similar problem. Frequently, furnaces are installed with return air ducts that are too small. When the fan runs it creates a low pressure area in the return duct and plenum. Any openings in the furnace casing and return duct sucks air from the space it is in.
In the case of Deanna, the ductwork in the garage and all connections to the furnace including the furnace door needs to be sealed. Very often the rack for the filter is open to the area as well, instead of having a metal cover that can be sealed away from the area.
If the fan creates a significant suction in the return duct, the ductwork needs to be increased in size so the suction is reduced. This would prevent fumes in the area from being drawn into the ductwork. This is also basic fire safety prevention to prevent fire and smoke from passing to living areas. If the furnace is in a radon area, radon can be passed to the occupants from air in the basement.
In these cases, although the heat exchanger is not passing fumes, the air around the furnace can draw fumes, especially in a tight basement where combustion air is drawn from the space creating negative pressure on the combustion flue draft. There are books available on combustion and draft at http://www.notaei.org