What is the cause of a big cloud white smoke when you first start your car up?

White smoke from a car exhaust on startup is usually caused by condensation in the exhaust system. Cars often have this white/water vapor smoke after sitting overnight or for an extended period. This water vapor is produced when the engine is cold, and the exhaust system condenses the water vapor into visible smoke.

If the white smoke persists after the car has warmed up or is accompanied by other symptoms, such as overheating or rough idling, it could indicate a more serious problem, such as a blown head gasket or a cracked engine block. In these cases, it's important to have the car inspected by a qualified mechanic as soon as possible to prevent further damage.