White smoke: This is one of the most difficult symptoms to diagnose because a number of factors can point to two general causes: overcooling, whereby the cylinder head and combustion chambers operate at a temperature that's too low for proper combustion; and piston-ring blowby, which indicates low compression and poor combustion.
White smoke represents atomized fuel, very small droplets of fuel that form a fog of sorts. It's common, and quite normal, to see this when a cold engine is started and until it warms up. If, however, a preheat device such as glow plugs or an air-intake heater are malfunctioning, the production of white smoke may be excessive and longer lasting. In extreme cases, the engine may be difficult or impossible to start.
Fuel of poor quality, particularly fuel that's off spec or not properly formulated as Number 2 diesel, will burn poorly, which in turn may produce white smoke. Adding a fuel cetane booster may temporarily alleviate-and identify-this problem.
Other causes of white smoke are poorly adjusted valves or worn valve seats, a partially activated decompression lever, a blown head gasket, or a cracked cylinder head or cylinder liner. A mechanic with the proper tools can narrow down the suspects.