only when the vehicle is at a stop, or warming up
smell of exhaust when heater runs on 1995 Mercury Villager
Check for an exhaust leak - at the engine to exhaust manifold, or somewhere in line on the exhaust system. Your heater is likely picking up the exhaust leak as it takes in air form the outside to circulate in your van.
Probrbly has at least one broken/missing exhaust manifold bolt.