A plume of white smoke is generally most noticeable at diesel startup, particularly when it's cold. This is due to the fact that colder air, which is more dense than warm air, lowers temperatures in engine cylinders at the end of the compression stroke. This colder air leads to reduced combustion of the fuel that has been injected into cylinders. These unburned fuel droplets are exhausted as a white smoke.
White Smoke Signals
It is acceptable for all but the most modern diesels to experience white smoke at startup. But this should disappear after the engine warms up.
White smoke from older, mechanically governed, pump-line-nozzle (PLN) engines will take longer to clear up than from electronically controlled power units, which provide tighter injection timing. But if the engine continues to churn out white smoke at operating temperatures, this could indicate misfiring cylinders caused by improperly timed injection pumps or malfunctioning injectors.
Bob Brady, a diesel technician from Burnaby, British Columbia (Canada), suggests that one way to check PLN injector timing problems is to loosen the fuel line nut one half turn. If the engine operating sound changes and its speed slows down, then that injector is firing. If there is no operating change, then that injector is faulty.
Other causes of white smoke could include a plugged crankcase breather. Look to see if the breather is plugged. Or remove its valve cover while the engine is running. If the engine exhaust clears up, replace the breather.
Here are other possible causes of excessive white smoke:
• Either poor-quality fuel or inco-rect fuel grade.
• Air in the fuel system because of loose fittings.
• Faulty intake manifold air tem-perature sensor signal (particularly if the engine has been idling a long time).
• Malfunctioning atmospheric pressure sensor.
• Faulty coolant temperature sensor.
Black Or Gray Smoke Signals
On the other hand, if the diesel is pumping out excessive gray or black smoke, first inspect the engine for a plugging air inlet. If the airflow is unrestricted, then double-check your fuel's quality and grade.
Next, set about checking out other causes, which may include a plugged crankcase ventilation system, incorrect valve adjustment, defective injectors, a faulty atmospheric pressure sensor, leaking turbocharge oil seal, mechanical pump injector timing, or faulty automatic timing advance.
A common misconception is that the presence of gray or black smoke indicates a powerful engine. Actually, it's a sign of lack of power.
Blue Smoke Signals
While white and black smoke are related to fuel problems, blue exhaust is a sure sign that your engine is burning excessive oil. This could be caused by worn or broken piston rings, glazed or worn liners, or worn valve guides. Perform a cylinder compression check to isolate the cause. Also, check for a leaking turbocharger seal on the inlet side of the charger. A restricted turbocharger drain line or high crankcase pressure could be at fault as well.