We can generally understand what is wrong with a diesel engine by the colours of smoke emitted from the exhaust. There are three basic colours - black, white and blue.
This is due to a air to fuel ratio imbalance, either the fuel system is delivering too much fuel into the engine or there is not enough clean air (oxygen ) a few things to look for :
Faulty injectors, defective or incorrect atomisers. (injectors need attention at about 100.000 to 120 000 Kms)
Faulty injector pump
Incorrect fuel injection pump timing or incorrect valve timing.
Poor engine compression
Dirty air cleaner
Turbocharger or intercooler faulty, poor boost pressure
Problems within cylinder head, valves clogged up due to faulty EGR (exhaust gas recycling unit)
Exhaust system restriction
Defective "Thermostart" (Cold starting aid)
Incorrect valve clearances.
Normally means that the fuel injected into the cylinder is not burning correctly. The smoke will burn your eyes.
Engine/pump timing out
Fuel starvation to the pump causing the pumps timing not to operate correctly
Low engine compression
Water/petrol in the fuel
The engine is burning engine oil
Worn cylinders or piston rings
Faulty valves or valve stem seals
Engine over full with engine oil
Faulty injector pump/lift pump allowing engine oil to be mixed with the diesel
Incorrect grade of oil used.