All About Auto

When black smoke comes out from exhaust of your car while you drive it means that the air fuel/ratio is too rich and when the air/fuel ratio is too rich, more gas is being burned from the car engine combustion chambers.

What causes the air fuel/ratio to be rich?

As you may not aware of, one of the common cause is a clogged air filter, a clogged air filter will restrict the amount of air coming inside the engine combustion chamber. Therefore, when a black smoke comes out the exhaust of your car the first thing you need to check is the air filter, check whether the filter is clogged. When doing this just remove the air filter and visually check if there an accumulation of dirt on the filter. If dirt is present then it is more likely the cause of air/fuel ratio being rich. Fixing this one is easy you just have to replace the filter.

But for instance, there is no dirt present on the filter, then a possible cause of the problem has something to do with your car fuel injection system, the fuel injector sensor may be bad, the job of the sensors is to measure various engine factors that use to calculate how much fuel is injected into the engine cylinder, if the sensors gone badly the air/fuel ratio will be wrong causing the black smoke out the exhaust. The sensor that commonly causes black smoke from exhaust when faulty are the mass airflow sensor, throttle position sensor and engine coolant temperature sensor.

It is difficult to identify which of the three is the problem therefore you need to bring the car to a reputable car shop then ask the mechanic to check your car with engine analyzer, replace the sensor at fault to to fix the problem.

Car with carburetor engine
 
For car with carburetor the most commonly cause of black smoke problem is a stuck choke plate. To find out if it is indeed a stuck choke plate problem, locate the carburetor and remove the air filter unit to be able to look down the carburetors barrel, examine the location of the choke plate inside the carburetor barrel, if the choke plate is nearly close or close then your car suffers from stuck choke plate problem and this is the cause why there is black smoke coming out the exhaust. To correct the problem apply choke spray to the choke linkage if this does not correct the problem you need a new choke. 

However after examining you found that the choke plate not stuck then the only remaining possible cause that makes your car to emit black smoke from exhaust is a vacuum leak, if this is the case then you'll need to bring your car to a reputable car shop and ask the mechanic to test your car for a vacuum leak. Take this problem at once to avoid much bigger problem that may arise from having a black smoke from exhaust of car.