Symptom
Black smoke from exhaust
Sooty exhaust indicating unburnt fuel — too much fuel or too little air. On diesels a normal sight under hard acceleration; sustained black smoke at steady speed is not.
Also described as: black smoke, sooty exhaust, smoking, smoky exhaust, dark smoke, coal smoke, دود سیاه از اگزوز, دود سیاه, humo negro, humo negro escape, sale humo negro, دخان أسود, دخان أسود من الشكمان, عادم أسود
Some causes mean you should stop driving
8 of the 105 codes that produce this symptom indicate a fault where continuing to drive risks damage or is unsafe. If the symptom is sudden or severe, treat it as one of those until a scan says otherwise.
What this usually means
Sooty exhaust means unburnt fuel — too much fuel, or too little air, and on a diesel those two are worth separating because the repairs are unrelated.
On a diesel, a puff under hard acceleration is normal and always has been. Sustained black smoke at a steady speed is not, and that distinction is the whole triage: an engine smoking only when working hard is over-fuelling briefly at the limit of its air supply, while one smoking at cruise is over-fuelling all the time.
Too little air is the larger population and it is mostly restriction. A blocked air filter, a boost leak that means the promised air never arrives, a variable-geometry mechanism coked shut, an exhaust gas recirculation valve stuck open displacing fresh air with inert gas, or a loaded particulate filter throttling the whole exhaust. The engine measures the air it expects and fuels for it, so anything that removes air after the measurement produces exactly this.
Too much fuel is the narrower one: a leaking or poorly atomising injector, excessive rail pressure, or a temperature sensor lying about how cold the engine is so enrichment never tapers.
On a petrol engine black smoke is unusual and more serious, because the mixture is normally controlled by the oxygen sensor — so visible soot means the correction has been overwhelmed. Read fuel trims before anything else; they say directly which direction the error runs and roughly how large it is.
Sustained black smoke destroys the aftertreatment it passes through, so a filter or catalyst replaced without curing the smoke is replaced twice.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Diesel, brief puff under hard acceleration only | Normal — momentary over-fuelling at the limit of the available air | Note whether it clears the instant the throttle is eased. Brief and load-related is the design working. | No repair. Investigate only if it has grown or now appears at steady speed. |
| Sustained black smoke at steady cruise | Over-fuelling all the time, or a permanent air restriction | Compare live air mass against the expected figure at wide-open throttle, and read fuel trims or injector correction values. | Repair the air restriction or the fuelling fault the readings name. Do not renew aftertreatment first. |
| Smoke with poor power and a boost fault | Air never arriving — a charge leak, a coked variable-geometry mechanism, or a loaded particulate filter | Compare requested against actual boost and pressure-test the charge system. Read filter soot and differential pressure. | Repair the leak, free the vanes, or regenerate the filter. See the power-loss brief for the ordering. |
| Smoke with rough running and sooty plugs on some cylinders | A leaking or poorly atomising injector over-fuelling one cylinder | Read injector correction values across the set and inspect the plugs. An outlier names the cylinder. | Renew or recondition the injector. Check for fuel dilution of the oil while it is apart. |
| Air filter heavily fouled | The simplest restriction, and the cheapest to exclude | Inspect against the light. This is a minute's work and it is skipped constantly. | Renew the filter and check its housing seals and ducting for splits. |
| Petrol engine, visible black smoke | Serious over-fuelling that has overwhelmed the oxygen sensor correction | Read fuel trims first — they name the direction and rough size of the error. Check fuel pressure and the coolant temperature signal. | Repair the injector, regulator or sensor found. Black smoke on a petrol engine is destroying the catalyst as it happens. |
-
Diesel, brief puff under hard acceleration only
- Likely cause
- Normal — momentary over-fuelling at the limit of the available air
- How to confirm it
- Note whether it clears the instant the throttle is eased. Brief and load-related is the design working.
- What fixes it
- No repair. Investigate only if it has grown or now appears at steady speed.
-
Sustained black smoke at steady cruise
- Likely cause
- Over-fuelling all the time, or a permanent air restriction
- How to confirm it
- Compare live air mass against the expected figure at wide-open throttle, and read fuel trims or injector correction values.
- What fixes it
- Repair the air restriction or the fuelling fault the readings name. Do not renew aftertreatment first.
-
Smoke with poor power and a boost fault
- Likely cause
- Air never arriving — a charge leak, a coked variable-geometry mechanism, or a loaded particulate filter
- How to confirm it
- Compare requested against actual boost and pressure-test the charge system. Read filter soot and differential pressure.
- What fixes it
- Repair the leak, free the vanes, or regenerate the filter. See the power-loss brief for the ordering.
-
Smoke with rough running and sooty plugs on some cylinders
- Likely cause
- A leaking or poorly atomising injector over-fuelling one cylinder
- How to confirm it
- Read injector correction values across the set and inspect the plugs. An outlier names the cylinder.
- What fixes it
- Renew or recondition the injector. Check for fuel dilution of the oil while it is apart.
-
Air filter heavily fouled
- Likely cause
- The simplest restriction, and the cheapest to exclude
- How to confirm it
- Inspect against the light. This is a minute's work and it is skipped constantly.
- What fixes it
- Renew the filter and check its housing seals and ducting for splits.
-
Petrol engine, visible black smoke
- Likely cause
- Serious over-fuelling that has overwhelmed the oxygen sensor correction
- How to confirm it
- Read fuel trims first — they name the direction and rough size of the error. Check fuel pressure and the coolant temperature signal.
- What fixes it
- Repair the injector, regulator or sensor found. Black smoke on a petrol engine is destroying the catalyst as it happens.
Codes that produce this symptom (105)
Listed with the codes that most typically present this way first — the likeliest place to start.
- P0103 Mass or Volume Air Flow Sensor A Circuit High Typical of this code High
- P0149 Fuel Timing Error Typical of this code Stop driving High
- P0172 System Too Rich — Bank 1 Typical of this code High
- P0175 System Too Rich — Bank 2 Typical of this code High
- P0216 Injector/Injection Timing Control Circuit Typical of this code Stop driving High
- P026D Fuel Injection Quantity A Higher Than Expected Typical of this code Moderate
- P04DA Closed Loop Exhaust Gas Recirculation Control At Limit — Flow Too High Typical of this code Moderate
- P2192 System Too Rich at Higher Load Bank 1 Typical of this code Moderate
- P2194 System Too Rich at Higher Load Bank 2 Typical of this code Moderate
- P2288 Injector Control Pressure Too High Typical of this code High
- P2563 Turbocharger Boost Control Position Sensor A Circuit Range/Performance Typical of this code High
- P2587 Turbocharger Boost Control Position Sensor B Circuit Range/Performance Typical of this code High
- P2599 Turbocharger Boost Control Position Sensor A Performance — Stuck High Typical of this code High
- P2672 Injection Pump Timing Offset Typical of this code High
- P2673 Injection Pump Timing Calibration Not Learned Typical of this code Moderate
- P2674 Injection Pump Fuel Calibration Not Learned Typical of this code Moderate
- P2696 Injector Data Incompatible Typical of this code Moderate
- P003A Turbocharger/Supercharger Boost Control A Position Exceeded Learning Limit Moderate
- P0046 Turbocharger/Supercharger Boost Control A Circuit Range/Performance High
- P0088 Fuel Rail/System Pressure — Too High Bank 1 Stop driving High
- P00AF Turbocharger/Supercharger Boost Control A Module Performance Moderate
- P0108 Manifold Absolute Pressure/Barometric Pressure Sensor Circuit High High
- P0118 Engine Coolant Temperature Sensor 1 Circuit High High
- P0132 O2 Sensor Circuit High Voltage — Bank 1 Sensor 1 Moderate
- P0152 O2 Sensor Circuit High Voltage — Bank 2 Sensor 1 Moderate
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Others with this symptom
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about Black smoke from exhaust.