If your vehicle is showing white steam billowing from under the hood, here are the most likely causes and the parts you'll need to fix each one.
A rubber radiator hose has ruptured due to age, or slipped off because a hose clamp was left loose during recent maintenance. This sprays pressurized hot coolant directly onto the hot engine.
The plastic end tanks on the radiator have cracked from repeated heat cycling over the years. This allows pressurized coolant to escape and instantly vaporize into steam.
The radiator cap was not fully tightened after a recent fluid top-off, or its internal pressure spring has failed. This allows boiling coolant to vent out and create a massive steam cloud.
The plastic thermostat housing has warped or cracked, or the gasket was pinched during a recent replacement. This creates a major leak point for hot coolant.
A heater hose or its plastic quick-connect fitting has broken near the firewall. This sprays coolant onto the hot exhaust manifold, creating thick white steam.
The water pump's internal shaft seal has blown out, causing coolant to pour from the weep hole. The drive belt then flings the hot coolant all over the engine bay.
A catastrophic failure of the cylinder head gasket is allowing high-pressure combustion gases to enter the cooling system. This violently forces coolant out of the reservoir or blows off a hose.