A sweet-smelling aroma and antifreeze smell could indicate that the car's heater core is leaking. Other common signs of a bad heater core include a foggy windshield, a water surging sound inside the dashboard and wet carpeting on the front passenger side. Heater cores are responsible for circulating heat inside the passenger cabin by extracting heat from the water/antifreeze mixture.
Heater cores look like tiny radiators. They heat passenger compartments by allowing hot coolant through small tubes with fins. Antifreeze odor from leaking heater cores is transferred with the heated air inside the car when the heater blowers are turned on.
There are several reasons why heater cores malfunction, but the common cause of leaks is continued temperature cycling from hot to cold. Rust buildup, corrosion and vibrations also cause pipes, tubes and fittings to leak.
Another indication of a busted heater core is insufficient heating. This electromechanical device is mounted higher than the automobile's cooling system, and low water and antifreeze mixture levels fail to reach the heater core, leading to clogging. Accumulated sludge sometimes leads to leaking that eventually emits antifreeze odor into the car.
According to the Streetdirectory.com Car Parts Guide, flushing a clogged heater core can improve efficiency, but replacing it is a better option.