What causes black smoke from exhaust of petrol cars?

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Black smoke from the tail pipe of a petrol engine car is a sign that the air-fuel mixture is rich and there is incomplete combustion.

A mixure that is rich contains more proportion of fuel than it should normally contain. Whatever that can cause a rich mixture can potentially cause black smoke emission from the exhaust.


Causes of black smoke from tail pipe

1. Dirty/restricted engine air filter

A restricted engine air filter starves the engine of sufficient air, thereby reducing the proportion of air in the air-fuel ratio stoichiometry. Due to this, the mixture becomes rich (excess fuel) resulting in incomplete combustion.

2. Bad fuel pressure regulator

A FPR is meant to regulate the pressure of fuel that enter into the cylinders through the injectors from the fuel pump. A faulty FPR which does not regulate can cause excess fuel which in turn causes black smoke emission.

3. Bad coolant temperature sensor

From cold start until when the engine reaches a closed loop state, the engine computer makes adjustments to enrich the mixture in order to attain engine operating temperature quickly. The ECU is able to achieve this through the coolant temperature sensor that sends the coolant temperature signal to it. If this sensor is faulty, it can cause continuous enrichment of the mixture which will cause black smoke to come out from the exhaust.

4. Bad Air/fuel or Oxygen sensor

An air-fuel ratio or oxygen sensor is responsible for controlling the air to fuel stoichiometry for efficient and complete combustion. A bad one might cause excessive fueling which can cause black smoke.

5. Bad or dirty Mass airflow sensor

A bad or dirty MAF sensor will read the the amount of air entering the engine wrongly. This will send a wrong signal to the ECU/ECM which might command more injector pulse that will amount to more fuel and black smoke.

6. Bad Manifold absolute pressure sensor

This sensor does the same function as the MAF and can similarly cause black smoke.

7. Blocked, leaky or dirty injectors

A leaky injector causes too much fuel to enter the cylinder. Also, when one injector is blocked, the ECM increases pulse duration of other injectors. All these can cause black smoke.

8. Bad Exhaust gas recirculation

EGR is used to reroute some exhaust gases into the engine to curb the emission of NOx gases. When it goes bad, it can cause black smoke.

9. Carbon deposits

As engines age, carbon deposits begin to accumulate. When an engine is revved and black smoke is seen from the exhaust after which it clears again, it might be a sign of accumulated carbon in the engine.

10. Wrong engine timing

An over advanced engine timing can cause poor performance and black smoke from the tail pipe.

11. Bad ignition coil

When an ignition coil is faulty, there is fuel but no spark in that cylinder except for some cars that are smart to also cut out the injector pulse of that cylinder. This causes fuel entering that cylinder to not combust.

12. Bad spark plug

Just like a bad ignition coil, a bad spark plug will not generate spark that ought to be used to burn the mixture, causing black smoke.

13. Blocked intake manifold

When intake manifold is blocked, sufficient intake air will not enter the engine for combustion. This happens to vehicles with variable intake manifold too, when the flap inside it gets stuck in a closed position.

14. Faulty wiring

Wiring of sensors like MAF, MAP, ignition coil, A/F or Oxygen sensor and so on can cause malfunction of the sensors.

15. Bad Engine control unit

A bad ECU also known as ECM (Engine control module) might not communicate with one or more sensors or components which can affect the air to fuel mixture ratio and cause black exhaust smoke.