White smoke occurs in a diesel engine when the diesel fuel goes through the engine and reaches the exhaust without having been burned. This typically occurs due to the engine being too cool to burn the fuel, often resulting from low compression in one cylinder, problems with the fuel injection timing or a defective fuel injector.
Other causes for a diesel engine failing to burn the diesel fuel and producing white smoke include poorly sealed piston rings, burnt-out glow plugs, poor fuel quality or a clogged air filter. Extreme engine problems, such as a cracked block, a cracked cylinder head, leaking valves or a blown head gasket, can also cause the problem.
Sometimes white smoke only appears when the engine starts cold, going away as the engine warms up. When this occurs, it is typically due to deposits around the piston rings. Products designed to flush carbon away from the pistons often cures this problem. If the white smoke is due to the engine being too cool, adding an automatic pre-heater may eliminate white diesel smoke.
Diesel engines also sometimes produce black or blue smoke, both of which are also signs of problems within the engine. Black smoke indicates poor combustion of the diesel fuel, and blue smoke is a sign of oil burning within the engine.