If your vehicle is showing steam billowing from under the hood, here are the most likely causes and the parts you'll need to fix each one.
Rubber cooling hoses degrade over time from extreme heat and pressure. A split or burst hose will spray pressurized coolant directly onto the hot engine block or exhaust, instantly creating a massive cloud of sweet-smelling steam.
Modern radiators use plastic end tanks that become brittle after years of thermal cycling. A sudden crack in the plastic will release a high-pressure jet of coolant, resulting in immediate steam billowing through the front grille.
If cooling system work was recently performed, a hose clamp that wasn't tightened properly can blow off under pressure. Alternatively, an unbled air pocket can cause the coolant to flash-boil and violently vent out of the reservoir.
A thermostat that fails to open blocks the flow of coolant to the radiator. The engine rapidly overheats, boiling the trapped coolant and forcing steam out of the reservoir overflow or cap.
The internal seal or bearing of the water pump can blow out, dumping large amounts of coolant down the front of the hot engine block and onto the drive belts, creating thick steam and often a squealing noise.
The radiator cap maintains system pressure to raise the boiling point of the coolant. If its internal spring fails, the coolant will boil at a much lower temperature and vent out as steam, even if the engine isn't technically overheating yet.
A severe head gasket failure can allow high-pressure combustion gases to enter the cooling system, over-pressurizing it and blowing hoses off, or it can leak coolant directly onto the exterior of the hot exhaust manifold.