Colored haze should be reserved for beautiful sunsets or not-so-beautiful smog – not a diesel engine. Whenever you see any color or haze from a diesel exhaust, that engine is telling you something about the deterioration of its internal condition.

After a diesel engine reaches its running temperature, the exhaust smoke should be colorless. It is best to do a test on a cold engine on a sunny morning, and look at the smoke into the sun. Generally speaking, color means trouble. Decoding whether the smoke is blue, black, white or gray can give you clues to what is happening in the power plant.

BLUE SMOKE

Blue is bad. This is the color you do not want to see. Blue smoke is burning engine lube oil, which may be caused by several conditions.CAPT. ALAN R. HUGENOTYou may see gray for a minute after starting the engine (above), but exhaust should clear quickly (below).

CAPT. ALAN R. HUGENOT

The most common is piston blow-by, in which lube oil finds its way into the combustion chamber by getting up past the rings or down the valve guides and stems. In both cases an engine decarbonizing may be needed.

Another cause is that the oil seals in the turbo charger may be failing. On turbo-charged engines, if the seals leak on the turbine shaft, oil will find its way into the inlet manifold, then into the combustion chamber and it will show up in the exhaust.

Excessive crank case pressure is another culprit. A clogged crank case vent can cause excessive crankcase pressure as the trapped air heats up. This can force oil up past the piston rings. The same thing may happen if too much oil is added to the crank case sump.

Occasionally you will see a very light, almost invisible, blue haze coming from the exhaust of a recently rebuilt engine. This engine may not have been run hot enough yet for the rings to expand into place. If this engine is taken out and run for a while at full speed under load, then the blue haze will usually disappear.

Blue smoke always indicates burning lube oil. It is just a matter of finding out why.

BLACK SMOKE

Black smoke indicates unburned carbon particles from the fuel being blown out of the exhaust.

It is often caused by overloading the engine or by poor fuel injection. When an engine is overloaded, the governor will compensate by opening up until more fuel is being injected than can be burned in the available oxygen. The unburned fuel appears as black smoke.

There are several causes of overloading. For example, the propeller may not be properly sized. It is often the practice to fit the most powerful propeller the engine can handle, but this can tax the engine even under normal operating conditions. If you see black smoke on a new boat, look to the propeller. Too much auxiliary equipment may cause the same problem.

Dirty air filters can cause insufficient air to reach the engine. Similarly, a very high inlet air temperature can cause less air to get into the cylinders. Both can result in unburned diesel. High exhaust backpressure, prevalent on turbo-charged engines, can cause the turbo charger to slow down with the same result: less air is pushed into the engine than is needed and fuel remains unburned.

Poor fuel quality -- heavier diesel fuels are harder to burn and do not burn clean – can cause black smoke. So can defective fuel injection. If fuel injectors have poor nozzles, causing improper atomization, the result can be unburned fuel. Improper injection timing can cause the same problem if fuel is injected too late to burn completely. On many older engines, any sudden attempt to accelerate will cause a cloud of black smoke as the fuel rack opens and the engine responds slowly. Once the engine reaches the new speed setting, the governor eases off on the fuel rack and the smoke immediately ceases.

Unfortunately, black smoke that does not go away when the engine warms up usually points to general engine deterioration: compression is most likely falling, the injectors need cleaning and the air filter needs replacing. If the engine is otherwise performing well, you have no immediate cause for concern. The engine is just telling you that a complete service -- new filters, reset injectors and de-carbonization -- may soon be needed.

If smoke persists when the load eases off, the engine is crying out for immediate attention.

WHITE OR GRAY SMOKE

White smoke is always caused by water vapor, or unburned, atomized fuel.

Unburned fuel indicates that one or more cylinders are failing to fire. Misfiring can be caused by lack of compression. Another occasional cause for white smoke is water condensation. For every gallon of diesel fuel that is burned, about one gallon (air volume) of water vapor is formed. Some of that condenses on colder surfaces and finds its way into the lube oil sump. When starting the engine, this water is pumped through the lube oil circuits of the engine and finds its way into the combustion chamber, causing white smoke. Later, as the engine reaches normal operating temperature, this water is vaporized out of the lube oil and the white smoke disappears.

There are other suspects as well. Water may be entering the combustion chamber due to a leaking head gasket, cracked head or cracked cylinder wall, which allows cooling water into the combustion chamber. It may also be that you got a tank full of dirty fuel, contaminated with water. Air in the fuel lines can yield white smoke, as can a lack of compression or problems with the valves.

If the smoke is a little gray, it usually means that the engine is cold, causing condensation that prevents the diesel fuel from getting a complete burn. It is common to see this for the first minute or so after lighting most diesel engines.