If you own a machine that uses diesel fuel, this article is for you. Seeing smoke while your machine is running means there is a story being told. The ending of this story could be good or bad. So, what story are you being told?

What’s the Story?

The story of diesel smoke is not a pleasant tale. Generally, if you see smoke upon starting your machine or while idling, a problem is present or developing. This problem could lead to a shortened engine life and unnecessary out-of-pocket costs. At the sight of smoke, it is time to take measures that can fix your problem.


Diesel smoke comes in three colours. Each colour is an indicator of what the problem could be. Taking notice of what colour the smoke is will help you develop a plan of action to mend your machine.

Colour Indicators

  1. Black Smoke — The most common of the three, black smoke indicates a poor and incomplete combustion is occurring. There are many reasons this could happen.
  • Timing not correct
  • Worn or dirty injectors
  • Injectors sticking
  • Too much fuel or poor fuel quality
  • Faulty turbocharger
  • Incorrect valve clearance
  • air/fuel ratio is off
  • Low cylinder compression
  • Dirty air cleaner
  • Induction system becoming restricted
  • Intake manifold is carboned up.
  • Excessive carbon build up in combustion and exhaust spaces
  • Cool operating temperature

The story of black smoke can be changed quickly and easily. Make sure not to purchase fuel from lacklustre places. Regular maintenance such as oil changes and tune-ups can prevent lack of smoke from happening.

  1. Blue smoke can become noticeable when oil is being burnt. Reasons for blue smoke are as follows:
  • Valve seals or guides become worn
  • Power assemblies are worn
  • Cylinder glaze
  • Piston ring sticking
  • Turbocharger seals become faulty
  • Incorrect oil grade use
  • Dilution in the oil by fuel


Blue smoke will be seen at a cold start. This means there are most likely fuel deposits that are reducing the efficiency of pistons and cylinder. In order to prevent this, regular maintenance and cleaning is a must. It must be noted that some engines will have a good sound combustion, have no visible blue smoke, and still need attention.


  1. When white smoke is seen blowing from the exhaust, there is an indicator that diesel fuel is not being burned properly. The white smoke is raw diesel fuel. Factors causing this white smoke include:
  • Damaged or faulty injectors
  • Injection timing is incorrect
  • Cylinder compression is low

When white smoke from engine is detected at a cold start and disappears as the engine warms up, you can diagnose the common cause. This happens when fouling deposits build up on piston rings and cylinder grazing.

If water is entering combustion spaces, white smoke will appear. This is caused by head gaskets gone bad or a cracked cylinder head. If this is the problem, mechanical repair is the only solution.

  1. Blowby smoke is the most critical. It indicates an urgent response and most likely an expensive engine rebuild. This type of smoke is found in the crankcase ventilation system. This could be happening due to the following causes.
  • Sticking piston rings
  • Worn cylinders
  • Worn or broken piston rings
  • Cracked pistons

Anytime smoke is seen exiting a diesel engine, it is time to listen to the story. Check the main characters and plan a course of action accordingly.



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