The exhaust system is designed to remove noxious gases from the car's engine that are left over after the combustion process. Sometimes, something can upset the balance and the resulting waste can be a great indicator of problems under the hood that might be otherwise unnoticeable until something goes wrong. Exhaust smoke should be faintly opaque if visible at all, and it should dissipate into the air almost as soon as it leaves the tail pipe. There are three colors that the exhaust smoke can be when something goes wrong - black, blue and white.
If you look into your rear-view mirror and plumes of black smoke block your view of the road behind you, you should probably pull over as soon as possible. Black smoke is caused by excessive fuel in the cylinder area that cannot be burned off all the way. When this happens, the fuel can also work its way into the engine oil and contaminate it. You may also notice that your vehicle's fuel economy is not as high when black smoke is coming from the tail pipe.
Fuel leaks can occur due to a damaged fuel line, a leaky fuel injector or a faulty engine computer. A clogged air filter or a broken fuel pressure regulator may also be responsible for upsetting the balance of the fuel-to-air ratio.
Fuel is not the only liquid that can leak and cause the exhaust to change colors. Blue smoke typically means that the oil is leaking. When it leaks into the cylinder, it gets burned up along with the fuel, burns in the hot engine and the result is blue smoke. This problem tends to occur more often in older vehicles than newer ones, and the likely cause of a leaky O-ring or a damaged gasket. When you notice blue smoke, it is important to address the issue as soon as possible. An oil leak can eventually damage the spark plug, which could lead to an engine misfire. Essentially, if you let the oil continue leaking, it will just cause more problems as time goes by.
White smoke is also caused by leaking fluids, except this time, water and antifreeze are the suspects. It is simply steam created when the engine tries to burn of the fluid that leaks into the cylinder. Antifreeze is supposed to be approximately a 50/50 mix of water and coolant, which is why it results in steam instead of a thick smoke like that caused by oil or fuel.
When you notice excessive white smoke coming from the tail pipe, the first thing you should check is the coolant level. If there isn't enough in the reservoir, add more. The solution can be purchased premixed or you will need to mix the antifreeze with equal parts water before adding it to the car. It's also important to examine the car's oil. If the antifreeze has managed to get into the oil, it will have a creamy, chocolate appearance. The car should not be driven until the issue is fixed, as coolant in the oil can cause severe internal damage. Chances are that antifreeze leaking in the engine cylinder or in the oil was able to get there due to a blown head gasket.
Whatever color the smoke is, if it is apparent, there's likely something leaking under the hood. This issue should be addressed as soon as possible to avoid further problems.