Automotive Systems

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Exhaust Smoke Color


One of the easiest methods to use when troubleshooting an engine for a performance complaint is to monitor the color of the smoke coming from the exhaust stack visually. There are four basic colors that may exit from the exhaust system at any time during engine operation—white, black, gray, or blue. The color of the smoke tips you off to just what and where the problem might lie.

White smoke is generally most noticeable at engine start-up, particularly during cold conditions. As the combustion and cylinder temperatures increase during the first few minutes of engine operation the white smoke should start to disappear which indicates the engine is sound. However, if the white smoke takes longer than 3 to 5 minutes to disappear a problem exist. The problems white smoke may indicate are as follows:

  • Low cylinder compression from worn rings
  • Scored piston or liner
  • Valve seating problems
  • Water leaking into the combustion chamber
  • Faulty injectors
  • Use of a low cetane diesel fuel.

Black or gray smoke generally is caused by the same conditions—the difference between the colors being one of opacity or denseness of smoke. Black or gray smoke should be checked with the engine at operating temperature of 160F.  Abnormal amounts of exhaust smoke emission is an indication that the engine is not operating correctly, resulting in a lack of power, as well as decreased fuel economy.  Excessive black or gray exhaust smoke is caused by the following:

  • Improper grade of diesel fuel
  • Air starvation
  • High exhaust back pressure
  • Incorrect fuel injection timing
  • Faulty nozzles or injectors
  • Faulty automatic timing advance unit
  • Faulty injection pump Incorrect valve adjustment clearances

Blue smoke is attributed to oil entering the combustion chamber and being burned or blown through the cylinder and burned in the exhaust manifold or turbocharger. Remember always check the simplest things first, such as too much oil in the crankcase or a plugged crankcase ventilation breather. The more serious problems that can cause blue smoke are as follows:

  • Worn valve guides
  • Worn piston rings
  • Worn cylinder walls
  • Scored pistons or cylinder walls
  • Broken rings
  • Turbocharger seal leakage
  • Glazed cylinder liner walls due to use of the wrong type of oil


With the engine stopped, the condition of the pistons, rings, and liners on a two-stroke cycle Detroit diesel engine can be checked visually by removing an air box inspection cover on the side of the engine block and accessing the components through the cylinder liner ports.

Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

Copyright 2001-2004 SweetHaven Publishing Services
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