Diesel Exhaust Smoke: Causes & Solutions

Diesel engines have been refined vastly since their agricultural beginnings, but older or faulty cars will produce exhaust smoke. In this article we will address the symptoms, causes and solutions of diesel exhaust smoke and look in to what the various colours of smoke can tell you. We will look at black, blue and white smoke commonly expelled from diesel exhausts. This guide will help you discover if this exhaust smoke is normal or a cause for concern.

What Causes Black Exhaust Smoke?

The most common diesel exhaust smoke we see is black, but what causes this?

Black smoke is formed when there is too much fuel for combustion, this causes an imbalance in the air-fuel ratio of the engine. This is known as fuel-rich combustion, it occurs when there is not enough air to completely burn the fuel and excess fuel is left unburned. 

This fuel has still been exposed to the high temperature and pressure environment of the combustion chamber. These conditions cause carbon to form and this is what gives the smoke its black colour.

6 of the most common causes of black smoke are:

  • DPF (Diesel Particulate Filter)
  • Dirty air filter
  • Turbocharged engine with a boost leak or faulty turbo
  • Build-up of carbon deposits
  • Faulty injectors
  • EGR Valve fault
Twin Exhaust
Image Credit: Unsplash

Could black smoke be caused by my DPF?

A diesel engine blowing black smoke could mean a few things.

Firstly if the car is an older car without a DPF (Diesel Particulate Filter) fitted to the exhaust system. Then a small amount of black smoke is normal.

However, a lot of Black smoke would indicate a problem with either the intake or fuelling system.

If the car is newer and fitted with a DPF then there should be no visible black smoke from the exhaust. So if you can see a lot of Black smoke then there is a problem.

This thick black smoke will be more present under heavy loads and acceleration, it will also have a sooty smell. If the car is smoking a lot, then there will also be some soot deposited onto the rear of the car. Most visible on the rear bumper area & around the exhaust pipe.

Diesel Smoke
Photo Credit: Unsplash

How do I know if my air filter is dirty?

A dirty air filter causes the engine to struggle with taking in air efficiently.

If the engine can’t get enough air then this will cause the fuel-rich combustion we have been talking about. A tell tale symptom of a dirty air filter is a loss in power and performance combined with black smoke. 

The air filter is the first thing you should check and is the easiest cause of diesel smoke to fix. Simply replace the filter with a new, clean filter. If this was the cause of the diesel smoke, the car will stop producing excessive smoke once it’s replaced.

Air filter
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How do I know if I have a boost leak?

On a turbocharged diesel engine, you have boost pipes and an intercooler connecting the turbo output to the intake manifold. This is to cool down the pressurised air to create more efficient combustion. It’s an airtight system and any leaks here can cause black diesel smoke.

This is because the engine is still accounting for the full amount of air being sent to the intake manifold. So it supplies the engine with the necessary fuel to burn properly. However, if some of the air is escaping before entering the combustion chamber due to a boost leak. There will be a fuel-rich environment in the combustion chamber.

Along with Black smoke, other symptoms will be a loss of power, and you may also hear a loud whooshing sound from the engine bay.

When checking the boost pipes you should look for any disconnections or splits in these pipes. Getting a garage to check this system can help you to find the problem and fix it. If no fault is found with this system then it can be eliminated as a potential cause of black diesel smoke.

What are the signs of a bad turbo on a diesel engine?

A faulty turbocharger will not be generating pressurised air to supply to the engine. The turbochargers job is to force more air into the intake manifold so that more fuel can also be injected and therefore create more power. If the turbo fails it will not be able to pressurise the air going into the intake manifold.

In the best case, the turbo will just stop producing boost and this will cause black diesel smoke. This is because the engine is set up to inject the correct amount of fuel for the increased air supply from the turbo. This will give us a fuel-rich environment and cause black diesel smoke.

In the worst case, the turbo will fail and fall apart and some of its components can get sucked into the engine and cause a catastrophic engine failure. 

Turbo diesel
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Does my diesel engine have carbon build-up?

Carbon build-up on diesel engines is normal in small amounts, sometimes carbon can accumulate to excessive amounts. The main reasons are poor servicing, poor quality fuel or it has been driven excessively at a low rev range. 

Using a fuel additive will help to clear out the carbon built up and also help to clean other components such as injectors. Using more of the engines rev range will also prevent carbon from building up. Taking the car for a more spirited drive where you rev the engine harder can help to blow out this built-up carbon and can restore lost performance caused by it. There will be more black diesel smoke as you do this, but once the engine is cleared it should stop producing smoke from the exhaust.

How do I know if my diesel injectors are bad?

Faulty injectors cause diesel smoke if they are injecting too much fuel. If the injector supplies too much fuel when it is spraying then this will cause a fuel-rich environment, and thus, cause black diesel smoke from the exhaust. Replacing the injectors will solve this issue and will also prevent further engine damage from being caused. An injector cleaning fuel additive might be worth a try first. 

Can a bad EGR valve cause Black smoke?

An EGR valve that is stuck open can cause black diesel smoke. This is because the exhaust gasses are constantly being recirculated into the engine, causing a low oxygen environment.

This causes incomplete combustion of the fuel and this gives us the black smoke. The EGR valve may be stuck open due to excessive carbon build-up or mechanical failure. Removing and cleaning the valve may solve the issue. However, it is better to replace the EGR with a new unit. This will solve the problem of excessive black diesel smoke.

Diesel Engine Maintenance
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Do All Diesel Engines Make Black Smoke?

All diesel cars will produce some amount of smoke. Older diesel cars will make more than newer cars. On older cars, it is normal to see a small amount of black smoke from the exhaust under heavy load or heavy acceleration.

Newer vehicles with DPF’s should produce so little that it is not visible to the naked eye, even during a DPF regeneration.

During this process, the particles trapped in the filter are burned away. This empties the filter and allows it to start collecting particles again. This process burns at very high temperatures, so there should still be no visible smoke while this is happening.

Some high-performance diesel cars may have had their ECU remapped by a 3rd party. This can change the amount of fuel and air being injected into the engine. This is usually done to increase performance and can cause black smoke. But in this case, it does not indicate a problem.

Is black smoke bad?

In older cars, a small amount of black smoke is perfectly normal and healthy as they don’t have particulate filters to catch the carbon. However, excessive Black smoke is bad and indicates there is a problem with the car.

Newer cars should not produce any visible smoke so anything here is a sign that something is wrong with the car and should not go without investigation.  

What Causes White Smoke?

White exhaust smoke typically shows that something is seriously wrong with a diesel engine. However, a cold engine on startup can produce a small amount of white smoke whilst it gets up to temperature. This is because things like condensation in the exhaust evaporate as the engine heats up. If white smoke is related to a fuel fault then the smoke will smell like hot tar. If the white smoke is related to a fault with the cooling system then it will have a sweet smell. We are going to cover the main causes of white smoke on a diesel engine.

Diesel smoke from car exhaust
Photo Credit: Pixabay

Improper combustion

Improper combustion of the fuel will cause white smoke from the exhaust. This should not be confused with fuel-rich combustion that causes black smoke. Improper combustion is when the fuel does not burn correctly and this can be caused by low compression due to damaged pistons, piston rings or valves.

Water in your diesel fuel

Water in your diesel can cause improper combustion and thus white smoke to be emitted from the exhaust. Contaminated diesel will also be harmful to other components such as the fuel pump which relies on diesel fuel for lubrication. Water can cause these components to fail prematurely. Draining the tank, flushing it out and refilling it with fresh uncontaminated fuel will solve this issue. You may also need to replace your fuel filter. 

Faulty injectors

If you have a faulty injector that is injecting fuel at the wrong time then this will cause white diesel smoke. This is because if the fuel is being injected at the wrong time so it doesn’t get properly burned. Therefore causing the engine to produce white smoke. A faulty injector can also lead to severe engine damage. If you see white smoke from the exhaust of your diesel engine then you should get it checked out immediately. 

Diesel Engine
Photo Credit: Pixabay

Coolant leaking into the combustion chamber 

If you have coolant being burned in the combustion chamber then this causes white smoke from the exhaust. However, this smoke will have a sweet smell. Different to the smell of hot tar from a fuel related issue. Coolant in the combustion chamber is a serious issue and can be caused by a faulty head gasket or a crack in the cylinder head or engine block. Both these issues will also cause the cooling system to become over pressurised. These faults can be quickly identified by a garage but the repair costs are usually quite high. A cracked engine block could also result in the engine being scrapped.

What Causes Blue Exhaust Smoke?

Blue smoke is caused by oil being burned and will have a distinct pungent oily smell. Blue smoke also normally indicates a serious problem with the engine. However, there are some cases where the fixes are cheap and easy. Here are the main reasons why you might be seeing blue smoke from your diesel engine exhaust.

Diesel Exhaust
Photo Credit: Unsplash

Worn engine components

When components like piston rings, valve stems and cylinder bores wear out. Oil will pass by the previously well-sealed components and enter the combustion chamber. Here it gets burned off and produces blue smoke. You may also notice the oil level dropping and a reduction in engine performance. These worn-out components mean the engine will not be operating as efficiently as it used to. Replacing these components is usually costly and time consuming. 

Overfilling the engine with oil

General engine maintenance such as topping up with oil is always good practice, however too much can cause issues. When the engine oil is overfilled, oil can weep past the piston rings and into the combustion chamber. Here it gets burned and produces blue smoke. Checking the oil level on the dipstick will quickly let you know if this is the cause of the blue smoke. If the engine is overfilled them some oil will need to be removed so that the level is within the specified limits for your engine.

Engine Oil
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Faulty turbocharger

Oil from your engine is circulated through your turbocharger to lubricate its components. If there is a fault with your turbocharger then oil can enter the intake air stream or the exhaust. This could be due to a faulty shaft seal on the turbocharger. If oil gets into the intake stream it will go through the engine and get burned. When oil gets into the exhaust the high temperatures of the exhaust will cause the oil to burn and produce blue smoke. Here a turbocharger rebuild or replacement is required to fix this issue.



There are many reasons why your diesel car is producing smoke. Some are an easy fix and others are more serious. We have covered the different colours of smoke, their causes, fixes, and the severity of each of these situations. Whether it be a slight wisp of black smoke from your old diesel car, to plumes of white or blue smoke. Hopefully this article has helped you on the way to finding and fixing the problem. 

We hope you gained the knowledge and information you needed from this article. Please share this article with anyone else who could benefit from it and leave us a comment down below. 


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