A vehicle smokes when it has an oil leak, a clogged carburettor, a leak in the coolant system or is in danger of overheating. Smoke coming from the engine or the exhaust pipe is an indication there is a problem under the hood and the vehicle needs to be serviced immediately.
Colored smoke can indicated where seals and valves are breaking down within the engine. Blue or gray smoke indicates valves and seals may be worn and leaking oil. Blue smoke can indicate faulty piston rings that should be changed. Black smoke indicates carburettor trouble or a dirty air filter. A clogged choke or leaky fuel injectors can cause black smoke to billow from the exhaust. White smoke indicates cracked head gaskets, a cracked engine block or that transmission fluid is entering the manifold. In such cases, the vacuum modulator may need to be replaced.
Smoke coming from under the hood suggests a larger mechanical issue. The experts at How Stuff Works state, "If the smoke is coming from under your hood, that probably means you ignored white smoke coming out of the tailpipe, and now your engine is overheating." Regular maintenance and preventative measures help keep the engine in smoke-free, working order.