If your vehicle is showing sweet, syrupy smell from the engine or cabin vents, here are the most likely causes and the parts you'll need to fix each one.
The heater core is a small radiator inside your dashboard. When it leaks, sweet-smelling coolant vapor is blown directly through the cabin vents, often leaving a greasy film or fog on the inside of the windshield.
Rubber coolant hoses degrade over time, or their clamps loosen, allowing coolant to drip onto hot engine components. The sweet-smelling steam is then pulled into the cabin through the fresh air intake at the base of the windshield.
Plastic radiator end tanks become brittle and crack with age and heat cycles, or a faulty cap vents coolant prematurely. The resulting steam smells sweet and wafts into the cabin air intake.
When the water pump's internal bearing seal fails, coolant leaks from a designed 'weep hole'. The coolant hits the hot engine block or drive belts, creating a sweet smell that enters the cabin.
Plastic thermostat housings frequently warp and leak, or the sealing gasket fails. This is also a common installation error if the seal is pinched or the bolts are over-torqued during a recent thermostat replacement.
The gasket sealing the intake manifold can degrade, allowing coolant to leak externally onto the hot engine block. This is highly prevalent on certain V6 and V8 engines.
A severe engine failure where the head gasket allows coolant to leak externally onto the exhaust, or internally into the combustion chamber (producing sweet-smelling white exhaust smoke).