Most common causes of Smoky Vehicles

Most common causes of smoky vehicles

When a vehicle is emitting excess levels of smoke, chances are that it is not properly tuned or maintained. When a vehicle is poorly tuned or maintained, the equipment on the vehicle designed to control the level of pollutant emissions also may not function properly.

Smoke from petrol engine vehicles (most cars) is mainly due to excessive wear.

Diesel vehicles (most trucks) may emit smoke from poor injector maintenance, excessive fuel delivery rates or poor driving technique (for example, lugging which is labouring the engine in too high a gear).

Smoke emissions mean that the vehicle is wasting fuel and engine damage is probably occurring. Routine servicing will eliminate many problems that cause smoke emissions and save you time and money.

Problems can and do occur after vehicle servicing or in vehicles in an apparent state of good repair, and even in relatively new vehicles.

The following problems may be the cause of smoke from a vehicle:

Petrol-engine vehicles

Four-stroke or rotary petrol and LPG engines may emit blue/grey smoke or black smoke. Blue smoke normally means engine wear or damage. Black smoke results from an excessively rich fuel mixture. Where this occurs, the following components may be at fault:

Air cleaner

A rich mixture can be caused by excessive build-up of dirt or oil. Replace the filter element at regular service intervals. Rags or paper caught in the air cleaner intake pipes can also cause black smoke emissions.

Fuel system

Carburettor and engine management system faults leading to black smoke include:

  • choke butterfly unable to open fully
  • carburettor flooding
  • incorrect grade of oil in dashpots
  • incorrectly adjusted or faulty automatic choke
  • air cleaner winter/summer lever set in wrong position
  • manual choke operated incorrectly or when the engine is warm
  • worn or loose jets or needles
  • sticking diaphragm
  • faulty engine management system
  • faulty oxygen sensor or other engine management sensors
  • faulty fuel injector.

Other engine problems

Blue/grey smoke may be emitted from the exhaust pipe or the crankcase breather pipe. It is emitted from the exhaust pipe when oil finds its way into the combustion chamber and is burnt.

This may happen in a number of ways:

  • via worn or broken rings, pistons, rotor seals, worn valve guides or faulty valve stem seals
  • via the inlet manifold through a faulty brake-booster assembly
  • via the inlet manifold through a faulty auto transmission vacuum diaphragm
  • via an overfilled oil-bath air cleaner or overfilled carburettor dashpot.

When worn or broken rings or pistons allow gases from the cylinders to pass into the crankcase, ‘blow-by’ occurs. In older cars this causes blue-grey emissions from the crankcase breather. Modern cars use positive-crankcase ventilation. This prevents emissions of ‘blow-by’ fumes to the atmosphere by passing them from the crankcase to the induction system, so they are burnt in the combustion chamber.

Positive crankcase ventilation valve

A faulty PCV valve can cause blue smoke emissions from the vehicle’s exhaust.

Two-stroke engines

Two-stroke engines may emit a light blue haze from the exhaust pipe due to oil in the inlet charge. Excessive smoke from two-strokes is usually the result of excessive oil in the inlet charge. Where fuel and oil are mixed in the fuel tank, the minimum proportion of oil recommended by the vehicle manufacturer should be used. For oil injection engines, the oil pump should be set to the manufacturer’s specifications.

Exhaust pipes, mufflers and ports on two-stroke engines should be cleaned according to manufacturers specifications. Excessive build-up of carbon causes loss of power and excessive smoke emissions.

Diesel-engine vehicles

Blue smoke from diesel engines normally means engine wear or damage. Black and grey smoke results from incomplete combustion and may be caused by many factors that can usually be rectified during routine maintenance.

For example:

  • maximum engine speed too high caused by faulty or incorrectly adjusted governor
  • air filter blockage causing restriction in intake system
  • incorrect valve/tappet adjustment or worn camshafts
  • damaged fuel pump or injectors caused by dirty fuel filters or water traps contaminated with dirt, grit, water or fuel wax. Water traps need to be cleaned regularly
  • incorrect fuel pump timing or worn or badly calibrated fuel pump and injectors
  • poor cylinder compression indicating leakage past valves or piston rings
  • excessive back-pressure in exhaust system
  • fuel contaminated by algae, water, grit or fuel wax
  • faulty electronic engine management system.

Maintenance is best carried out in a fully equipped garage or service centre by qualified service personnel. Service manuals supplied by the vehicle manufacturer provide information on the control of smoke through good maintenance practices and should be studied when planning preventive maintenance schedules. The fuel-injection pump or fuel injectors should only be repaired by the manufacturer, its agent or a reputable specialist.


Overfuelling is a common cause of smoke emissions. Adjusting the fuel system in an attempt to increase the power output of an engine which may be too small for the job, or to compensate for power loss in a worn or poorly maintained engine, will lead to more fuel being injected into the combustion chamber than the engine can efficiently use. The excess fuel that cannot be burnt is then emitted as black smoke.

If fuel settings have been increased beyond those specified by the manufacturer, readjustment to the proper settings should be made.

Care should be taken when selecting a new vehicle to ensure that the engine capacity is adequate for the intended use. Engines that are too small for the job suffer more wear-and-tear and are generally less economical in service. Some older engines need to be de-rated, i.e. the fuel-pump delivery may have to be set below the manufacturer’s specification if the emission of smoke is to be avoided. As an engine wears with normal use, the compression will drop requiring the pump to be de-rated accordingly.

Injector servicing

It is essential that injectors be regularly serviced by a reputable diesel specialist, according to the manufacturer’s recommendations and checked for the following:

  • nozzle leakage or dribbling
  • correct atomisation
  • correct opening pressure
  • back leakage
  • correct injector for the vehicle.

Prolonged injector malfunction can cause major engine damage.

Crankcase breathers

As with petrol-engine vehicles, worn or broken rings or pistons allow gases from the cylinders to pass into the crankcase and out in to the atmosphere via the crankcase breather. Excessive smoke from a diesel vehicle’s crankcase breather indicates serious engine problems.


Lugging the engine, which is labouring the engine in too high a gear, will also cause excessive smoke emissions. Drivers should be made aware of the minimum engine speed that must be used to avoid smoke emissions. Overloading a vehicle or operating the engine ‘on the governor’ (that is, at maximum RPM) for long periods when prevailing conditions require less may cause smoke emissions.

Testing vehicles

The commonly used free-acceleration test (vehicle stationary) to test for exhaust smoke is inadequate as it does not simulate the conditions that exist when the vehicle is under load. Because it can be difficult for the driver to observe exhaust smoke from inside the cab, the vehicle should be driven on the road under varying conditions of load and speed and observations made from behind, preferably from a following vehicle. The vehicle should be at normal operating temperature during the test, unless a faulty choke is suspected, when the vehicle should be tested from a cold start. The vehicle should be road-tested at regular intervals and after servicing.