Burning oil smell
Leaking valve cover gaskets, oil filter seals, or overfilled oil can drip onto the exhaust manifold. It smells acrid and may produce light blue-gray smoke from under the hood.
Burning rubber
A slipping serpentine belt, a seized pulley, or a loose hose resting on the exhaust smells like hot rubber. Dragging brakes can also smell similar.
Sweet, syrupy smell
That's ethylene glycol coolant. It can come from a radiator, heater core, or hose leak. If you smell it inside the cabin with foggy windows, the heater core is likely leaking.
Burning plastic or electrical
Melting wire insulation or a failing blower motor resistor smells like burning plastic. Electrical fires are dangerous — pull over and shut off the engine.
Hot brakes or clutch
A sharp, chemical smell after driving downhill or in traffic often means overheated brakes or a slipping clutch.