What Causes Black Smoke from Exhaust?

Black smoke from the exhaust is one of the easiest smokers to recognize and fix as well. That’s why mechanics usually advise car owners to inform them when they spot this type of smoke as soon as possible. The reason behind this is that most car owners tend to ignore the smoke claiming it will disappear on its own.

It is true that sometimes it might be just a burst of smoke when you start your car, and then it clears out in seconds. However, if the smoke doesn’t clear up, you should know there is a problem and make the necessary steps to take it to the auto shop. Kindly remember that this type of smoke is usually caused by several problems such as faulty injectors, a leaky valve, a damaged air filter, or a bad turbocharger among others.

But there is good news if any of the problems are detected early, it is very fixable. Therefore, make sure you don’t ignore the smoke when you see it.

black smoke from exhaust diesel engine

What Does Black Smoke Mean?

Experiencing black smoke simply means your car’s engine is burning too much fuel. By this, we mean either the fuel has been contaminated or has mixed too heavily with oil in the engine. Note that although it’s the fuel that is supposed to mix with the oil, it has to be the right balance. Affecting both diesel and petrol engines, it is unfortunate that most diesel engines have a higher rate.

For instance, when we look at the diesel car, most times, we will get black smoke being emitted from the exhaust is the first moment the engine is switched on.  Another time is when the weather is cold or when hard acceleration is applied. That said, in these three cases, the black smoke that comes out of your exhaust is often very thick clouds.

When we look at gas cars, although it’s not more popular as most diesel engine cars, the smoke you notice it is likely due to fueling just like the case of diesel cars. Mostly experienced in cars that are turbocharged, this turbocharged acts as an exhaust-driven pipe that pumps extra air into the engine and is joined by additional fuel to give increased power. “Note that the extra fuel will only be burnt properly if enough air is supplied, and that means the turbocharger has to be in good shape”

Unfortunately, if the turbocharger happens to be worn out or has a leak from its pipework, then it won’t supply enough air for the extra fuel to be properly burnt.  This will definitely lead to the extra fuel being disposed of through the exhaust. Don’t forget, if you ignore the frequent smoke, then you will risk damaging your car’s sensors, and the catalytic converter.

What are the Main Causes of Black Smoke from Exhaust?

There are several issues that will lead to black smoke from the exhaust.  These can be a result of a clogged air filter, blocked manifold, malfunctioning fuel injection, or other issues. When we look at both diesel and petrol cars, we know black smoke is emitted when the engine is burning fuel.

However, petrol engine car is known to also emit dark gray smoke caused by a rich fuel mixture. But wait!  There are some issues found in both diesel and petrol cars while others are just for a specific engine. So, without wasting time, let us look at these causes in detail starting with petrol cars.

black smoke from exhaust when accelerating

Petrol Engine

Dirty Filters

One of the main causes of black smoke from exhaust petrol is a dirty filter. This is because the dirt prevents clean air from reaching the engine, thus affecting the emission control systems. By reducing the airflow, a rich fuel mixture is formed which usually results in damaging the spark plug among other parts.

Bad oxygen sensor

Another cause of black smoke emission is a bad oxygen sensor.  Its main function is sending information to the engine control unit thus enabling it to run your car efficiently as well as to prevent any emissions. When you have an oxygen sensor that is bad, your car will definitely run less efficiently which leads to not only smoke but also other problems. These are usually high fuel consumption, poor idle, hard starting problems, and so on.

Faulty Auto Computer

A faulty auto computer can also contribute to the emission of smoke from your car. The faulty computer may cause the car to have several problems, such as black smoke from exhaust and stalling as well as misfiring. Note that the symptoms may come and go; therefore, it is good to be keen when you spot them.

Diesel Engine

Clogged Air Filter

As you know, a normal engine is known to burn all of its fuel, only giving out Co2 and water. However, when the fuel gets partially burned, black smoke will eventually come out of the exhaust. When this happens, you should check out the air filter to see if it is clogged due to being dirty.  If it is surely clogged, all you have to do is replace it with another one.

Faulty Fuel Injector

A fuel injector is an important part of your car’s fuel system found at the head of the engine. It is specifically located at the intake manifold near the intake valve, and its purpose is to make sure the fuel flows directly into the cylinder. Furthermore, it also ensures the fuel is sprayed toward the engine inlet valve. That means if the fuel injector becomes faulty due to getting clogged with debris, it doesn’t do its job properly. Therefore, the extra fuel forms a solid carbon residue that is emitted as black smoke when you accelerate your car.

Bad Mass Airflow (MAF) Sensor

When we look at the mass airflow sensor, its main purpose is to make sure that the correct amount of fuel is mixed with air coming into the engine. Nevertheless, that’s not always the case especially if the MAF is bad. This is because it will register more airflow coming in the system thus resulting in excess fuel flowing into the engine. Finally, the end result will be unburned fuel which will be emitted as black smoke.

Dirty Exhaust Gas Recirculation (EGR) Valve

Just as the name suggests an EGR is there to re-circulate the small amount of engine exhaust back into the combustion chambers so as to decrease the combustion temperature. This process is necessary since it reduces the formation of nitrogen oxides. However, when the incorrect amount of carbon (IV) oxide is reintroduced to the combustion chamber, the EGR valves tend to malfunction since it’s not able to control the increase of nitrogen oxides.  Therefore, before you decide to replace the EGR valve, it’s important you first clean them and see if the black smoke still comes out of the exhaust.

Read: Common Signs and Symptoms of a Bad EGR Valve: How to Test, Fix, and Change

Excessive Carbon Deposits

Mostly affecting cars that have covered more miles on them, the carbon deposits are usually accumulated in critical areas that lead to them not functioning properly. Since diesel-run longer than petrol and is also crude fuel diesel engines are easily prone to carbon buildup.

How Do You Fix Black Smoke from Exhaust?

 Most cars experience black smoke now and then. However, this does not always indicate any serious problem. For instance, if your car emits a little smoke at startup then clears as the engine warms up then there is nothing to worry about. This is usually in the case of white smoke from the exhaust.  But hey! If it continues after the car warms up, then there is cause for concern and that’s why we are looking at the best ways to fix the issues causing the smoke.

Therefore, with no further ado, it’s time to state the best ways to fix both petrol and diesel engine smoke emission issues.

Petrol Engine

Clean the Air Filters

It is common for air filters to get dirty due to the buildup of oil or debris. If this happens, you can have them cleaned by a professional auto mechanic. This is important since the filters need to be removed from your car’s hood and it is likely you don’t have the right mechanic’s toolbox. After the filter is removed, it is vacuumed thoroughly for some minutes then clean by first applying a cleaning solution outside and inside. Leave it for like ten minutes then rinse it. When it is dry, put it back to its rightful place.

Replace the Oxygen Sensors

Even if they are small devices found on your vehicle, oxygen sensors are very important when it comes to measuring and balancing the oxygen-to-fuel ratio. Being part of the emission system, they sometimes get clogged or stop working altogether which can cause your car to stop unexpectedly. Although it is possible to clean your oxygen sensor to get a few more extra miles out of it, replacement is the best solution.

Repair/Replace the Auto Computer (ECU)

Repairing cars computers or ECU can be very costly depending on the extent of the damage. Fortunately, if it is only the power supply, then a skilled mechanic will repair it in no time. However, if the problem is serious then the computer needs to be replaced.

Diesel Engine

Clean the Filters and Combustion System

Your car’s internal combustion requires the right amount of air intake to burn the fuel completely.  Unfortunately, if there is no enough air coming into the engine then the fuel won’t burn fully thus leading to black smoke from your exhaust.  Fuels need to burn fully for it to emit water and CO2 which is invisible to the naked eye. That’s why to keep your car from emitting black smoke you need to check your filters and combustion system to see if they are dirty or clogged. If they are then you should clean or replace them if necessary.

Check the Piston Rings

Important engine components, piston rings are there to seal the combustion so as to minimize the loss of gases to the crankcase. So, when these components get damaged or worn out, you will definitely find black smoke being emitted from the exhaust when accelerating. Therefore, ensure you check them and replace them if necessary to get rid of the smoke. Remember this; you will need to seek out the services of an auto mechanic as the engine will need to be removed to access them.

Invest in the Common-Rail Fuel Injection System

Unlike the old diesel cars, most new diesel-powered automobiles use a common rail fuel injection system which is great at feeding fuel directly to the solenoid valves. The good news is that this high-tech injection system makes sure your vehicle won’t experience any black smoke. Therefore, when buying a diesel vehicle, just pick the one using the common rail fuel injection and you won’t have to deal with the disturbing smoke.

Use Fuel Detergent Additives

Do you know that combustion deposits and debris are known to gradually build in the fuel injectors and cylinder chambers? That’s right they do but that’s not good for the fuel flowing inside the engine since it brings about engine problems.

One of the problems is the reduction of fuel economy as well as lowering the performance of the engine. This will surely result in the emission of black smoke. Luckily, you can fix this issue by mixing the diesel fuel with a detergent additive to get rid of the deposits which will make the smoke disappear in a few days. Keep in mind there are several detergent additives in the market today that will take care of your car’s needs with no fuss.

Frequently Asked Questions (FAQs)

Q: Why is My Car Blowing Black Smoke When I Accelerate?

There are several reasons why your car may be blowing black smoke from the exhaust when accelerating. One of them is from a leaky fuel injector, faulty Oxygen sensor, or MAF sensor, or too much fuel pressure. Furthermore, your car may have an engine computer fault, especially in diesel engines or the black smoke might be a result of hard acceleration.

Q: What Color Smoke is Bad?

Although there is no good color smoke when it comes to your car, blue smoke is one of the colors you should be concerned about when you noticed it being emitted from your exhaust pipes. The reason being is that there is likely an oil leak that makes your engine burn oil. Mostly this is caused by damaged piston rings, worn-out cylinder walls, or leaking valve seals.  Keep in mind, another color to look for is grey smoke from the exhaust. Also an indication of burning oil from your car, these two colors can surely cause a lot of issues.

Q: Can a Bad O2 Sensor Cause Black Smoke?

If your car is functioning properly, the Oxygen sensor (O2) cannot cause your engine to emit any smoke. However, if the sensor is broken in any way, your car will run either rich or lean. What does running rich mean? This simply means too much fuel is entering the combustion chamber.  On the other hand, less fuel than the recommended amount entering the combustion chamber will cause a running lean car.

So, if your car is running excessively rich, you will definitely see black smoke coming out of your exhaust pipe. Also, if your car runs excessively lean for a long period of time, it could cause engine failure. This will result not only in black smoke but also in white or blue smoke from the exhaust Pipe.

Q: Can I Drive My Car with Black Smoke?

It all depends; if it’s just a burst of smoke that clears after a few minutes, then there you can drive your car. However, if the smoke continues pouring out of your exhaust, then it’s time to pay a visit to your mechanic. Apart from being the easiest smoke to recognize and fix, the burning of unnecessary fuel will truly affect your fuel economy. Therefore, don’t ignore the signs since the smoke might be due to the poor functioning of the sensors, fuel line, or injection system. In addition, you will be saving yourself the cost of repairing a damaged engine.

Q: Can a Bad Catalytic Converter Cause Black Smoke?

Yes, a bad catalytic converter can cause black smoke. This happens when your car engine is burning oil too much or from a clog caused by fluids from the engine coolant. Note that, apart from the dark smoke being emitted from the exhaust, here are other ways to recognize your catalytic converter isn’t functioning properly is:

  • The smell of sulfur or simply smelling rotten eggs from the exhaust
  • Sluggish engine performance
  • Excessive heat under your car
  • Reduced acceleration

Keep this in mind; some of the above symptoms can be caused by other parts of the car’s emission system. Therefore, it’s essential you let your mechanic check out the car to determine if it’s really the catalytic converter causing the smoke.


We hope whatever we have covered in this article will be of big help to you. Personally, no car owner wants to experience any black smoke from their exhaust pipes. Therefore, it is crucial you deal with this type of smoke or any other type immediately you notice it to prevent any damage.

Apart from that, it is also important to take your car for servicing now and then since your mechanic will be able to check beforehand your fuel system, air filters, computer system, as well as other areas. So, are you ready to visit your favorite auto shop?

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Hi there, I am R. Hasan Tito, a mechanic, and owner of this website. My friend and I created this website to share our knowledge, expertise, and experience with our fellow mechanics' community and car users. I am a specialist and certified automotive mechanic (Both Heavy Commercial and Private Cars). I worked as a Mechanic and Mechanic Supervisor for over fifteen years at Global Rebound Automotive companies - Toyota, TATA, BMW, Nissan, TVs, and Others. Now, I enjoy my new role of leading a team of automotive experts (in their respective fields) and publish new content on a regular basis on my website and social media.

3 thoughts on “What Causes Black Smoke from Exhaust?

  1. would redex put in the diesel tank any or is that just a myth also i have a citron dispatch diesel engine, bad starter so i started spraying easy start into the air intake and then its fine all day,but without this it will not start, will this damage the engine in any way

  2. I love your site please keep it up, my am experiencing black smoke on my Volvo v40

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