A brand new diesel engine running at full load will experience a little bit of blow-by upon startup. Blow-by is a condition where diesel fuel, air and vapor are pushed past the rings into the crankcase of the engine. Correct pressure should be maintained in the cylinder chamber in order for proper combustion to occur. In a new diesel engine the rings need time to seat properly and develop an air-tight seal. After a short period of break-in hours under load, the blow-by problem should correct itself. Consequently, a proper running diesel engine should produce no visible smoke from the exhaust. If there is smoke coming from the exhaust it could indicate a more serious problem with the engine. This article will help diagnose the underlying causes of diesel engine smoke.
Diesel engine smoke comes in three colors: white, black and blue. Consistent smoke coming from the exhaust most likely indicates a deeper internal problem with the engine. A small puff of smoke during quick acceleration is acceptable with older diesel engines due to a lag before the turbocharger’s air flow can match the increased volume of diesel fuel injected into the cylinders. Newer electronic diesel engines with common rail injectors simultaneously match the speed of the turbo with the metered flow of diesel fuel into the cylinder.
White smoke coming from the exhaust usually points to one point of failure: the injectors. Usually, white smoke indicates that the diesel fuel is not burning correctly. Unburned diesel fuel will make its way through the exhaust completely unused. Be careful of white smoke as it will irritate your eyes and skin. If white smoke occurs during a startup in freezing temperatures, then goes away, it usually indicates frozen deposits of soot which expanded around the rings then burned away once the engine warmed up. The use of glow plugs during cold starts and/or the use of a flushing solvent to remove engine sludge is recommended.
Common Causes of White Smoke:
• Damaged Injectors
• Faulty Injection Timing
• Damaged Crankshaft Keyway
• Damaged Timing Gear
• Low Cylinder Compression
• Damaged Rings or Cylinder Liners
• Water mixed in the Diesel Fuel (Cracked Head Gaskets, Cylinder Head or Block)
• Damaged Fuel Lines
• Low Fuel Pressure to the Fuel Pump
• Damaged or Incorrect Fuel Pump Timing
Black smoke, unlike white smoke, contains a high concentration of carbon exhaust particles. The combustion of diesel fuel in the cylinders breaks down the long chain of carbon molecules to smaller and smaller molecular chains. When the exhaust leaves the engines the byproduct is a combination of carbon dioxide and water. If something goes wrong during combustion the chemical reaction taking place is not as robust, causing long tail hydrocarbons to be left completely intact and then expelled in the form of smog or soot. Partial burning of diesel fuel results in large carbon dioxide particles as well as greenhouse gasses which contribute to air pollution. The advent of the Selective Catalytic Converter, Diesel Exhaust Fluid and Diesel Particulate Filter all helped to regenerate exhaust back into the combustion chamber to further break down particulate matter.
Black smoke is the most common smoke color coming from a diesel engine and most likely indicates something is wrong during the combustion of the diesel fuel. When diagnosing the problem the first place to look at is the mixture of air and fuel flow into the cylinders. The engine could be delivering too much fuel, not enough fuel, too much air or simply not enough air.
Common Causes of Black Smoke:
• Clogged Air Cleaner
• Damaged Injectors
• Bent Injector Nozzles
• Incorrect Injector Timing
• Clogged Air, Fuel or Oil Filters
• Damaged Injection Pump
• Damaged/Clogged EGR Cooler
• Damaged Turbocharger
• Damaged Intercooler
• Over-Fueling the Engine
• Wrong Blend of Diesel Fuel For Temperature
• Cracked or Clogged Valves in Cylinder Head
• Improper Valve Clearance
• Low Compression due to Damaged Piston Rings
• Excessive Engine Sludge Build Up
Blue engine smoke is the rarest type of smoke emanating from a diesel engine. The presence of blue smoke is an indication of burning oil. Blue smoke should not be ignored but is common when starting an engine in a cold weather. The oil thins out when it is cold and some could escape into the cylinder and be burnt. Cold temperatures can cause older more worn rings to unseat just a bit due to deposits found around the rings or cylinders. Cylinder glaze, or the smooth deposits left behind from the piston going up and down, can also build up over time and burn. The seal between the combustion chamber and crankcase should be completely sealed after the initial break-in period. The use of Lubriplate 105 or Molybdenum Disulfide during the engine rebuild will help the rings to seat properly during initial startup as well as burn off any carbon deposits.
Common Causes of Blue Smoke:
• Damaged or Worn Piston Rings
• Damaged or Worn Cylinders
• Damaged or Worn Guides
• Damaged or Worn Stem Seals
• Overfill of Engine with Oil
• Damaged Lift Pump
• Fuel Mixed with Oil
• Cylinder Glaze Burning
• Wrong Grade of Oil
No matter the color of the smoke it is not something you should ignore. A properly working and maintained diesel engine should produce no visible smoke. Make sure to shut down the engine immediately if you encounter excessive smoke as further heat or load could severely damage the engine further.