by 
The Curious Bag
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The combustion of fuel in an automobile engine, or any internal-combustion engine, creates exhaust. The exhaust is made up of various chemicals, some of which are harmless, others of which are toxic and polluting.
Coloration
Most engine exhaust is invisible, but under certain conditions, a cloud of visible exhaust will appear. This cloud can be white, blue, or black in appearance.
Conditions
Black exhaust is created when fuel enters the combustion chamber and is not completely burned. This is also known as an engine running too rich.
Results
An engine running too rich will reduce mileage and often cause a strong odor. Black smoke can indicate a range of problems, some minor and some more serious.
Causes
Your air filter may be plugged, or the carburetor, which controls the mixture of fuel and air entering the combustion chamber, may be poorly adjusted and allowing an excess of fuel into the combustion chamber. The fuel pump or fuel injector may need adjustment. In many cars, the settings on these parts are controlled by onboard computers.
Repair
Check the air filter and then the engine oil for fuel contamination. A mechanic can then check the settings on your carburetor, fuel pump or fuel pressure regulator.
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