Thin, white smoke coming from an exhaust pipe usually indicates normal condensation buildup in the exhaust, but thick, white smoke can indicate a problem like the engine burning coolant, a cracked engine block, a blown head gasket or damaged cylinder head. Thick, black smoke is generally a sign that the engine is burning excess fuel, and is typically not an indicator of as serious a problem as thick, white smoke.
While a small leak with the coolant may not be a major cause for concern, if it is ignored long enough it can result in some significant repair bills and a good bit of damage to the engine. A leak in the coolant can also mix with other fluids in the vehicle, creating additional issues.
A cracked engine block, blown head gasket or damaged cylinder head are all serious issues, and can result in costly repairs.
If the exhaust smoke is thicker and comes out in large clouds, it generally means that there is something burning within the car. A blown head gasket could be a possible reason for coolant burning in the car; a cracked engine block is also a reason for coolant burning in the vehicle.
Gray smoke that is coming out of a tail pipe is usually a major cause for concern. Gray smoke often signals that there is a problem with the oil system of the car. The car may be burning more oil than necessary and may be running hotter than it should.
Black smoke coming out of the back of a tail pipe signals that too much fuel is being used and burned up. If the vehicle is burning too much fuel, it should be fixed to ensure that money is not being wasted on fuel that is simply burning up and not being used to power the vehicle. Diesel vehicles are the most common vehicles to have black smoke come from the exhaust pipe.