diesel truck black coloured smoke

Colour in Your Exhaust Smoke

Modern diesel engines (’08 and newer) equipped with exhaust after-treatment systems (DOC, DPF, SCR) should emit NO smoke AT ALL during engine operation.  Older engines that are not equipped with exhaust after-treatment systems should emit very little smoke.

A properly maintained diesel engine using good quality fuel may emit some white smoke when cold.  The white smoke should disappear once the engine reaches its normal operating temperature.  This is the only time that you should see any smoke from the exhaust.  Smoke at any other time is not considered to be normal.

The various types of exhaust smoke indicate different problems.  Correctly identifying the smoke can help to quickly diagnose a poor running engine.  Lets look at the common smoke issues.


Black smoke is also referred to sometimes as gray smoke or dirty smoke.

Black smoke is created by partially burned fuel being blown out of the combustion chamber with the exhaust gas.  The reasons for the fuel being only partially burned often relates to one of the following problems:

  • Excessive fuel in the combustion chamber (too much fuel).
  • Restricted air inlet (too little air).
  • Advanced injection pump timing (fuel entering too soon).

Black smoke can be caused by too much fuel, poor fuel quality, not enough air or not enough time to burn the fuel.  Black smoke is not considered normal and is often related to low power complaints or poor fuel economy problems.  Usually the repairs pay for themselves by improving engine performance and fuel economy.


White smoke is caused by particles of fuel passing through the combustion chamber without burning, and exiting with the exhaust gases.  Fuel not burning is often related to low combustion chamber temperatures (cold engine).  During light loads, the temperature in the combustion chamber temps may drop down to 500 F.  This lower cylinder temperature delays the combustion process causing some fuel to remain unburned, and blown out with the exhaust gases.


Blue smoke is an indication of engine oil burning in the combustion chamber.  Blue smoke is usually accompanied by excessive oil consumption complaint.  Any of the following mechanical conditions can cause excessive oil consumption:

  • Worn piston rings.
  • Failed valve sleeves.
  • Failed Turbo Charger seals.

If the engine health is suspect, we recommended starting with a compression test to confirm that the engine is building at least 350PSI and has the ability to compress and fire the cylinder charge.

If the compression test passes on all cylinders, then start looking into the turbo intake piping to check for oil in the piping.  When a turbo fails, from age (or a lack of service) often the turbo shaft seals can leak oil into the intake – the vacuum of the turbocharger compressor wheel literally sucking the oil out past the turbocharger shaft seals and creating a blue smoke complaint.

Smoke from a diesel is not a good thing, but knowing what the smoke means can help you get to the source of the problem more quickly and get your customers back on the road – smoke free.

Get back on the road smoke free. Shop high quality diesel parts.