Black smoke coming from your Subaru Legacy’s exhaust is definitely a concerning problem. Different colored exhaust can mean different things. Black smoke is typically of greater concern than white smoke.
Dark black smoke coming from the exhaust of your Legacy indicates that there could be a serious engine problem, and indicates that the air/fuel mixture is rich.
Black Exhaust Smoke Causes: Subaru Legacy
There can be a number of things that can cause black exhaust smoke in your Subaru Legacy. Most of the time, black smoke indicates that the air/fuel mixture is “rich”. A rich condition means that there is not enough air in relation to the amount of fuel.
Before even so much as opening the hood, we recommend checking the engine trouble codes if there are any. If the check engine light is on, there will be some stored in your Legacy’s computer. These codes can provide valuable clues as to what is going on with your Legacy’s engine.
Here are some of the most common causes of black smoke in your Legacy:
One of the most commonly overlooked automotive maintenance items is the air filter. If the air filter is ignored long enough it will eventually clog to the point that not enough air is getting into the combustion chamber.
We always recommend taking a look at their filter first. They are really cheap, easy to replace, and often fix the black smoke problem.
The EGR valve recycles exhaust smoke back into the combustion chamber to help reduce engine emissions. When the EGR valve goes bad these gasses have nowhere to go but out through the exhaust, and they appear as black smoke.
Bad Piston Rings
Piston rings are a series of rings that create extremely tight clearances between the piston and the cylinder wall. These rings keep oil from getting into the combustion chamber.
When they fail, your Legacy will emit black smoke from the exhaust as oil is burnt along with the air and fuel in the cylinder.
Bad Fuel Injector(s)
Your Legacy’s fuel injectors use high fuel pressure to atomize the fuel into a fine mist. This mist is the most combustible way to burn gasoline.
Injectors can go bad in a few different ways. They can become clogged, open or close on time, or they may not have enough fuel pressure going to them. Here’s more on this: Subaru Legacy: Bad Fuel Injector Diagnosis.
Mass Airflow Sensor
The mass airflow sensor measures the amount of air passing to the intake manifold. This is one of your Legacy’s most vital sensors. When it is faulty, it can lead to changes to the air/fuel mixture and make your Legacy run rich. Black exhaust color is also common.
Here’s more on diagnosing the MAF sensor.
Good luck diagnosing the black smoke coming from your Subaru Legacy’s exhaust. If there is anything that you would like to add, please feel free to leave a comment below.