The modern diesel engine is very sophisticated and often difficult to diagnose even with the assistance of advanced diagnostic equipment. Most vehicles generate computerised fault diagnostics which don’t always isolate the exact cause or source of the fault or failure.

Below is a guide that may help point you in the right direction.


Possible cause                           Hard/non startlack of powerrough at idlediesel knockblack smoke
Low compressionx 
Low fuel pressurexx
Insufficient fuel deliveryxx
Fuel restriction / blockagexx
Contaminated dieselxx
Air in fuel systemxx

Black smoke is generally due to an imbalance in the air to fuel ratio. Either too much fuel is being delivered into the engine or there is not enough clean air.

Common causes are:

  • Faulty pump or injectors
  • Dirty Air Cleaner
  • Turbo or Intercooler faulty
  • Cylinder head


White smoke is often caused by the fuel being injected into the cylinder not buring correctly.

Common causes are:

  • Engine/pump timing not set correctly
  • Fuel starvation to the pump effecting the timing
  • Low engine compression
  • Water or petrol in the diesel system


Blue smoke is the result of engine oil burning

Common causes are:

  • Diesel in oil
  • Worn cylinders / liners
  • Over filling of engine oil
  • Cylinder head valves or seals

It is always advisable to seek the advise of a qualified technician as soon as possible.

Breakdowns and consequential costs and damages can most often be prevented.

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