There are multiple reasons that smoke may be exiting your exhaust system. The most likely sources are oil being burned or combustion problems that are causing excess gas to be consumed.
The solution may not be readily apparent without performing some tests on various components to determine the source of the problem.
The most obvious sign that oil is being burned along with gas is a drop in the oil level in your vehicle without any obvious leaks. Oil leaks will usually present themselves as dark puddles on the ground under the engine compartment of your vehicle.
However, leaks from compromised gaskets may just spill oil onto the engine. This will be recognized by dark stains on the engine and an acrid burning smell, possibly accompanied by black smoke, from under the hood of the vehicle.
Oil flows inside your engine to lubricate and protect the moving parts inside the engine. However, if a gasket or internal component becomes worn, oil can mix with the fuel inside the combustion chamber and produce black smoke from the tailpipe.
One way to test for this problem is to perform an oil change, filling the vehicle with the proper amount of oil, then checking the oil level after 1000 miles. If the oil level drops a quart without any visible leaks, it's likely that engine repairs need to be performed.
Failure to perform the necessary repairs may eventually cause the oil level to drop so low that it can no longer protect the engine's internal parts, resulting in engine failure.
Burning Excess Fuel
The exhaust from your vehicle will be darker and more odorous if the engine is using excess fuel in the combustion process. A proper mix of fuel and oxygen is needed for optimal combustion. Additional fuel is required for combustion when the oxygen level is the chamber is too low.
This issue may result from a clogged engine air filter, which cleans contaminants from the intake air before it reaches the combustion chamber. When the filter reaches a saturation point, air can no longer flow freely to the engine.
Checking and replacing the engine air filter may solve the problem. However, other factors may be at play in denying air to the engine. The oxygen sensor, which is part of the emissions system, regulates the flow of gas into the combustion chamber by measuring the amount of oxygen in the exhaust.
The mixture may become too rich, or use too much fuel, if the oxygen sensor becomes impaired. The check engine light on the vehicle's dash usually remains lit when any component of the emission system goes awry, so then oxygen sensor is a likely suspect if the light is on. Oxygen sensors are a primary subject of emission test repairs when a vehicle fails the test.
Dark smoke coming from your tailpipe is not only hazardous to your vehicle, but also to its occupants and the surrounding community, so take care of the problem as soon as possible. You'll breathe a lot easier when you do.