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.

Black smoke

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.

White Smoke

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