White smoke emanating from a vehicle's exhaust pipe is an abnormal sign indicating that some type of mechanical problem exists within a car's engine. While not always a sign of catastrophic mechanical trouble, white exhaust smoke is a sign that a car needs to be mechanically evaluated and/or serviced.
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Bad Piston Oil Rings
Bad or worn piston oil rings allow engine oil to seep into the main engine combustion area, an area where the engine oil gets burnt and produces smoke, which gets emitted from a car's tail pipe in the form of white smoke.
Worn Valve Seals
Valve seals prevent circulating engine oil in a car's valve-train from reaching the main engine combustion area of a car's engine. Worn valve seals allow engine oil to seep from a car's valve-train directly into a car's engine cylinders, where the oil gets burnt and emitted from a car's exhaust pipe as white smoke.
Bad Piston Compression Rings
Piston compression rings work much like piston oil rings, in that they help to keep circulating engine oil in an engine's lower section from flowing into the main combustion areas. Worn piston compression rings, in addition to allowing engine compression gases to escape, allow engine oil to enter a car's engine combustion area, where the oil gets burnt and emitted from a car's tailpipe as white smoke.
Blown Head Gasket
A blown engine head gasket, which helps to seal a car's head to the main engine block, can allow circulating engine coolant in a car's head to flow into the engine combustion chambers, where the coolant gets burnt and emitted out of a car's exhaust pipe in the form of white smoke.
Worn Valve Guides
Valve guides, which are rubber inserts that surround each individual valve assembly in a car's engine, can allow engine oil to leak from a car's head and flow into the engine cylinders, where the oil gets burnt and emitted as white smoke from a car's exhaust pipe.