To understand what is wrong with a vehicle's exhaust system, it is good to understand what it does. The exhaust system (complete with muffler) is in place to move the hot exhaust fumes away from the engine, muffle the engine noise, and reduce polluting emissions from that exhaust.
While the muffler is part of the exhaust system, as its name indicates, its primary purpose is to muffle sound from the engine. The smoke coming out of the exhaust system, although it travels through the muffler, is coming out of the exhaust pipe itself and not specifically the muffler.
For vehicles that run on gasoline, black exhaust generally indicates that there is too much gas in the mixture. This is called an over-rich mixture. When the accelerator is pressed, a mixture of gas and air reaches the spark plugs to ignite the engine, and when this mixture is too heavy on gas, black smoke is produced. It is a good idea to check with a mechanic and see why this is occurring.
In diesel engines, there are several reasons why black exhaust might be produced. More simple reasons include low fuel pressure, a faulty diesel injector, restricted air intake, or problems with the high pressure pump. However, the presence of black smoke in and of itself does not narrow the possibilities very much for diesel engines; check with a trusted mechanic to diagnose the issue.
