The colour of your diesel smoke tells a story

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Even if you are not at all mechanically minded, there are certain indicators in the smoke your diesel engine emits that indicate certain anomalies that need attention. Excessive smoke from any modern diesel engine is indicative of a problem, but knowing what different smoke colours mean will help to indicate the nature of the problem, and what part of the motor needs urgent attention.

Marine diesel motors will always emit a bit of smoke on start up, but that smoke should disappear quickly. If the smoke becomes visible after the motor warms up or when the throttle is opened, it often indicates a problem that needs fixing. This is where noting the colour of the smoke will assist in determining the problem. There are basically three types of smoke that are emitted from a diesel engine and these are blue, black and white. Leisure Boating delves into the fume and smoke-filled world of diesel engines to get the facts.


There are usually two main causes of white smoke being emitted from a diesel engine. These are either an internal coolant leak within the engine or excess fuel being burned. A simple way to determine if excess unburned fuel is making it out of the exhaust is to put your hand near the exhaust without blocking it and let the warm exhaust fumes coat your fi ngers. If a smell of your hand gives off a strong smell of diesel, then it is safe to determine that there is unburned fuel being passed out of the motor and this could mean a fuel injection problem. If there is no diesel smell, simply touch one of your fi ngers on your tongue. If there is a sweetish taste of antifreeze found in the coolant, you could well have a blown head gasket or a cracked cylinder head. In this case, the motor would need to go to a qualifi ed mechanic for repair at once.


Blue smoke generally indicates oil burning within the engine’s combustion chamber. Blue smoke was often present in old two stroke motors, but with modern developments there should be no blue smoke being emitted. If you have a modern engine that has undergone high usage, blue smoke might be indicating that the piston rings and valve seals are worn, allowing the engine oil to get past the seals and pistons and form part of the combustible material. The blue smoke often indicates a complete motor refurbish by an experienced mechanic or complete replacement if the cost of repair is prohibitive. Have you overfilled the crankcase oil? Check the dipstick and if the crankcase is overfilled, it will need to be drained to the correct level to prevent damage to crankshaft seals due to increased crank pressure. Blue smoke should always be attended to as soon as possible. On a low hour motor, blue smoke is usually indicative of an overfilled crankcase.


Diesel Smoke
Diesel Smoke

Black smoke can either be caused by an inadequate supply of air to the motor or unburned fuel passing through the system. The air to fuel ratio is incorrect and this is the cause of the black smoke. A little black smoke is deemed normal on a diesel motor, but there should be no excessive amounts of black smoke. If there is line fouled on the propeller, the motor will determine that the boat is under greater load and consequently supply more fuel to deal with this load, causing excess black smoke. If the black smoke abates, the fouling might have cleared itself from the motor, but if it continues, you might need to take a trip overboard, or trailer the boat to remove the fouling. In some cases, the extra load placed on the motor is due to the cutlass bearing beginning to seize. Air flow restrictions in diesel engines also cause a loss and increased emissions of black smoke. Is your air filter clogged? Is your ventilation hose faulty? If there is poor engine performance coupled with an uneven idle, you might need to get a mechanic to check the injectors.

Keeping an eye on the colour of the smoke being emitted from your boat’s motor is one of the easiest visual methods to determine if there is a problem. Your boat is one of your favourite investments and provides you, your family and friends with much enjoyment. You wouldn’t want that enjoyment to go up in smoke now, would you?

Know the colour of your diesal smoke
Know the colour of your diesal smoke

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