The exhaust pipe is used to dispel excess gases from the engine, and typically, there should not be smoke of any color coming from the pipe. When something is going wrong under the hood, it may result in smoke pouring out of the tailpipe. Blue, white and black smoke are all common sights when there is a leak of some type of fluid.
Blue smoke is the most common type you will encounter, and this is caused by an oil leak somewhere in the engine. White smoke is often steam, which could indicate there are low coolant levels, possibly caused by a leak. If you see black smoke, you may have a fuel leak or a rich fuel mixture. When this occurs, the combustion process is not burning off all the fuel.
Sometimes you may notice gray smoke coming from your tailpipe. This does not point to a separate problem, but in fact the gray smoke is simply a variant of either blue or black smoke. To determine whether your car is leaking oil or fuel, you can check a few different indicators.
Start with the oil level. If it's low, this could be the source of the problem. Add more fluid and check back in a few days. A significantly reduced level in a short period of time means your system likely has a leak somewhere. When oil leaks out into the engine, it often finds its way into high-temperature areas, which burn the fluid and create blue, or gray, smoke. Leaks can also occur if seals are broken, so check the condition of the o-rings, pistons and valves.
Once you have ruled out oil as a source of the gray smoke, you're left with determining the source of a fuel leak problem. The fuel filter, fuel injectors and even the engine computer can all lead to an overabundance of fuel in the combustion chamber. Low fuel pressure can lead to gasoline leaking through the injectors into the chamber as well.
As soon as you determine the root of the problem, you should fix whatever is going wrong. No matter what color smoke is coming from your car, it can lead to major issues if left unaddressed.