My car is serviced regularly, with engine oil, filter and spark plugs replaced as and when needed. But recently, I noticed a drop in performance and fuel efficiency, as well as black smoke from the exhaust. What could be the problem?

A smokey exhaust is a sign of engine trouble. Black smoke means there is too much fuel in the mixture that is being combusted.

With older cars that run on carburettors, it means a tuning of the carburettors is necessary. Adjusting a carburettor is not complicated, but requires some know-how that many young mechanics do not possess.

Some cars with early versions of fuel injection also require adjustments to the air-fuel mixture to eliminate black smoke emissions. If allowed to prolong, the condition is likely to cause fouling of the spark plugs and carbon build-up in the combustion chambers.

Most modern cars have electronically controlled fuel injection, which cannot be manually adjusted.

One element that is frequently forgotten during service is the air filter. As a result, many cars continue running with impaired or clogged intake filters. This can cause the engine to run on a fuel-rich mixture, resulting in black smoke.

After more than 80,000km, a component in the intake tract called the mass air-flow sensor (which measures the quantity of air entering the system) gets covered with fine dust that escapes the air filter. When this happens, the electronic controller receives false air-flow information, leading to incorrect fuel management. This can also lead to black smoke.

Cleaning the air-flow sensor is not difficult. There are aerosol sprays you can buy for this. Look for "MAF Sensor Cleaning Fluid".

A mechanic would usually remove the throttle body that houses the sensor and clean it by spraying over the sensor. As it is a delicate electrical component, this procedure requires extreme care.


The difference a clean air-flow sensor makes to performance and fuel efficiency can be stark.

Shreejit Changaroth

A version of this article appeared in the print edition of The Straits Times on January 18, 2020, with the headline 'TorqueShop'. Subscribe