Black Smoke From Your Exhaust - What It Means & How To Fix It

Ellie Dyer-Brown, 9 months ago

5 min read

  • Diagnostic
  • Exhaust
Black exhaust smoke from a grey car.

FixMyCar explains why you might be seeing black smoke from your exhaust.

You probably don’t need us to tell you that black smoke billowing from your exhaust is a bad sign. But what you might not know is that - despite how it looks - it usually isn’t catastrophic as long as you act fast. The cause is pretty simple: your car is burning too much fuel.

Knowing why your car is excessively burning fuel isn’t so straightforward. There are a few possible explanations, all of which are covered in this guide.

Contents:

What makes exhaust smoke black?

Why is there black smoke from my exhaust when accelerating?

Six causes of black smoke from the exhaust

Black smoke from the exhaust of a diesel car

How to get rid of black exhaust smoke


What makes exhaust smoke black?

Black smoke is caused by an incorrect air-fuel ratio. Usually, the mixture of fuel and air in your engine is carefully controlled by sensors, which provide information allowing the fuel injectors and intake valves to create the ideal conditions for combustion.

When one part of this system is faulty, it can lead to too much air in the combustion chamber, making your car run lean. Symptoms suggesting your vehicle is running lean include stalling, loss of power, an illuminated check engine light and trouble starting the engine.

On the other hand, a faulty fuel system component may result in too much fuel - when this happens, it’s called running rich. In addition to black smoke, you may also notice sooty spark plugs, engine performance issues, low fuel efficiency and unusual idling.

Black exhaust smoke from blue car

Why is there black smoke from my exhaust when accelerating?

It doesn’t matter whether the black smoke comes from your exhaust when idling or accelerating; either way, it has the same cause. You’re more likely to encounter the problem when accelerating because your car burns more fuel to get up to speed.

Six causes of black smoke from the exhaust

Here are the components that could be to blame for black smoke.

1. Faulty oxygen sensor

The oxygen sensor is a likely culprit since it regulates the air-fuel ratio by sending information to the engine control unit (ECU) about how much unburned oxygen is in the combustion chamber. Its job is to inform the ECU whether the engine is running lean or rich.

When the sensor becomes damaged, it can’t do its job correctly, resulting in your car burning more fuel than needed. Without enough oxygen, there will be incomplete combustion, causing an outpouring of black smoke from the exhaust.

2. Defective MAF sensor

The mass air flow sensor - MAF sensor for short - measures the air entering the combustion chamber. If it gets dirty, it may tell the ECU to put an excessive amount of fuel in the chamber, creating black smoke.

3. Dirty air filters

Dirty or clogged air filters won't allow the correct amount of air into the combustion chamber; as less air reaches the chamber, a higher concentration of fuel is burnt, producing lots of dark smoke.

You should replace your air filters every 15,000-30,000 miles. Alternatively, you can use age-based intervals, replacing them every one, two or three years as per the manufacturer’s recommendations.

4. Damaged piston rings

Piston rings are designed to stop engine oil from entering the combustion chamber. They apply the correct pressure to the cylinder wall or liner to ensure a thin film of grease coats the working surfaces. 

When they become damaged, the oil may enter the combustion chamber. As you can imagine, burning engine oil and fuel together creates a mess of thick black smoke. To find out whether your piston rings are damaged, it’s best to arrange an exhaust diagnostic test.

5. Engine deposits

A combustion engine provides the right conditions - heat, oxygen and fuel pressure - for forming carbon deposits. These deposits can interfere with performance when they build up and make their way into vital components like the fuel injectors.

You can use specialised fuel additives to clean and protect the engine from damage.

6. Leaking fuel injector

Fuel injectors are located at the head of the engine block. They allow fuel to flow directly into the cylinder head, so if one of them starts to leak, you could end up with too much fuel in the cylinder, resulting in black smoke.

Mechanic testing diesel injectors wearing blue latex gloves.

Black smoke from the exhaust of a diesel car

One of the most common reasons diesel engines produce black smoke is a clogged exhaust gas recirculation (EGR) valve or diesel particulate filter (DPF). The DPF can be cleaned at some specialist garages, but replacing an EGR valve is often an expensive job, costing £534.77 on average when booked through FixMyCar. 

That’s why it’s a good idea to take your diesel car on a reasonably long motorway drive at least once a week, allowing the EGR to get hot enough to burn off excess carbon. You could also use an engine-cleaning fuel additive every so often as a preventative measure.

The two components work together, so when one fails, it has a knock-on effect on the other. Consult a mechanic to determine whether a blocked EGR valve or dirty DPF is to blame for your car’s black smoke.

How to get rid of black exhaust smoke

The following fixes might help you eliminate your car’s black exhaust smoke. However, you must get an accurate diagnosis before you can work out which repair is suitable.

Unfortunately, there’s no silver bullet for black car smoke. You have to match up the problem with the appropriate repair, and doing so is likely to require the expert knowledge of a mechanic.

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