What the Colour of Your Diesel Exhaust Smoke is Telling You

black diesel exhaust smoke

Although diesel engines manufactured prior to 2008, which do not have after-treatment exhaust systems (SCR, DPF, DOC), may emit a bit of smoke during normal engine operation, vehicles with engines from 2008 to the present should be smoke-free. So what does it mean if you see diesel exhaust smoke?

Any vehicle with a diesel engine will emit white smoke and vapor in colder temperatures when it is first turned on.  This occurs even with proper maintenance and high-quality fuel.  When the engine reaches its proper temperature, the white smoke should disappear.  If you see any at other times, there is a problem that needs to be identified.

Many problems in the engine can be diagnosed by looking at the smoke that is emitting from the tailpipe.  Identifying what the different colours of smoke mean can help.  Read on to learn about the most common smoke indicators.

What does black smoke mean?

What is called black smoke can also appear dirty or gray and happens when the combustion chamber emits partially burned fuel mixed with the usual exhaust.  If this is occurring in your vehicle, the culprit is usually one of these problems:

  • Extra fuel entering the combustion chamber
  • Not enough air flowing in due to an obstruction
  • An error with the fuel injection pump timing

Black or gray smoke indicates a problem in the diesel fuel injection system.  There may be too much fuel, the fuel may not be high enough quality, or it may not have enough time to burn.  All of these problems also result in a sluggish car or truck that gets lower than usual MPGs.  Repairs will improve how the vehicle runs and uses the fuel.

What does white smoke mean?

When the combustion chamber in your diesel vehicle’s engine is too cold, the fuel will not properly ignite and burn and it will be blown out the back as white smoke.  Whenever the chamber registers at less than 500°F, this white smoke pluming may occur.  The reasons for this usually include cold weather and the engine not working as hard as usual due to low freight weights.

What does blue smoke mean?

Blue smoke only occurs when engine oil is being burned along with the diesel fuel.  The driver may also notice an increase need to add oil to the engine sooner than scheduled maintenance indicates.  The following mechanical problems may result in oil mixing and blue smoke:

  • Piston rings need to be replaced
  • Valve sleeves failure
  • Turbocharger seals no longer working

If smoke in the exhaust indicates an engine or maintenance problem, the first step is to perform a compression test.  Diesel engines need more than 350 PSI to compress fuel vapor and fire the cylinder.  If all cylinders are operating normally, next examine the turbocharger system.  All piping and seals should be checked.  Older vehicles and ones with spotty service records may experience leaking shaft seals that allow oil to be sucked into the intake.  The turbocharger compressor wheel’s vacuum action sucks oil readily through broken, damaged, or worn seals.

Anytime smoke emits from diesel engines, maintenance or a visit to the mechanic may be in order. Knowing what the colour of your diesel exhaust smoke means can help diagnose the problem and get it back in operation quickly.

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