Smoke and smells
Black smoke from the exhaust: causes and what to do
Black smoke comes out when there is too much fuel or too little air to burn it: the mixture is too rich. On diesels it usually points to the turbo, the EGR valve, the air-flow meter or the injectors; on a diesel with a particulate filter, visible black smoke can also mean the filter is damaged.
Check without hurrySmoke and smells
Reviewed by Daniel Morales, founder of Reprorace SL, on
Is it serious? Can I keep driving?
Usually not urgent, but do not ignore it. It rarely means you must stop, but it raises consumption and emissions and can clog the particulate filter or damage the catalytic converter. Have it checked soon, and sooner if the engine light comes on or the car loses power.
A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.
Most likely causes
- Clogged air filter: the simplest thing to rule out.
- Boost leak: a leaking hose or intercooler leaves the engine short of the air it expects.
- Air-flow meter: if it reads too low or too high, the mixture goes wrong.
- EGR valve: open when it should not be, it takes oxygen away from the combustion.
- Injectors: a dripping injector or poor spray leaves fuel unburnt.
Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.
To understand it better: What is a mass airflow sensor and what symptoms does it cause when it fails? What is EGR and what happens when it fails? How does a turbocharger work and what does P0299 mean?
Car system: Fuel and air
What you can check yourself
Check the air filter or when it was last changed.
With the engine cold, see whether the turbo hoses are secure and free of oil.
Note whether the smoke only appears at full throttle or at idle too.
On a diesel with a particulate filter, visible black smoke is not normal: mention it at the garage.
Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.
Related fault codes
- P0172Mixture too rich (bank 1)
- P0175Mixture too rich (bank 2)
- P0101Mass or volume air flow sensor A, circuit: range/performance
- P0299Turbocharger/supercharger A: underboost
- P0401EGR A flow: insufficient flow detected
- P0088Fuel rail/system pressure: too high (bank 1)
Generic codes that often come with this symptom. Seeing one does not confirm the cause: it is where the diagnosis starts.
How the ECO-LITE helps diagnose it
Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.
What to look at here: air flow, requested and actual boost pressure, fuel trims and EGR position, depending on the car.
Frequently asked questions
Is a little black smoke normal on a diesel when accelerating?
Older diesels without a particulate filter could show a brief puff at full throttle. A modern one with a filter should not show visible smoke.
Does black smoke use more fuel?
Yes: it is fuel that is not being used. It usually comes with higher consumption and less power.
Can it fail the MOT?
Yes: diesels have a smoke opacity test at the periodic inspection (the MOT in the UK), and black smoke can make them fail it.
Keep reading
- Fuel and air: how it works and what fails
- Car loses power: what limp mode is
- Car uses more fuel than normal: causes and what to check
- Diesel particulate filter (DPF) warning light: what to do
- Car symptoms: what they mean and what to do
- Check engine light on: what to do
- How to read and clear your car's fault codes
- OBD fault codes: what each one means