Mechanic working on exhaust (© Stockbyte/Getty Images)

Mechanic working on exhaust

When everything's working normally, your tailpipe emissions should be almost invisible. But should you be worried if you notice what looks like smoke? Here's how to read tailpipe exhaust to help you determine what to do.

Thin White Vapor

Don't be concerned
A thin cloud of white vapor that quickly dissipates after leaving the tailpipe is probably the result of normal condensation buildup inside the exhaust system. The vapor is usually accompanied by a slow drip of water from the tailpipe. That is a common sight when cars, even new ones, are first started in the morning, because condensation has built up overnight. If a car has been sitting for several days, a thicker cloud of white vapor might be emitted.

Blue or Gray Smoke

See your mechanic
Thick blue or gray smoke that doesn't dissipate quickly is probably the result of oil being burned in the engine's combustion chamber. It might be something serious that will require extensive engine work.

If your engine is burning enough oil to produce visible blue exhaust, then the vehicle's oil level will drop over time and require periodic, perhaps frequent, top-offs. Worse, burning oil can foul the engine's spark plugs, causing such problems as a rough idle, reduced fuel mileage, hard starting, and sluggish acceleration. It might cause the catalytic converter to fail.

On turbocharged vehicles the presence of bluish-gray exhaust smoke might indicate turbocharger failure. The turbocharger might need to be rebuilt or replaced. The oil lines to and from the turbo should be replaced at the same time.

Regardless of what is causing that type of emissions, you should have your vehicle checked out by a qualified mechanic as soon as possible.

Black Smoke

Have it checked out
Black, sooty smoke is usually symptomatic of an engine that's burning too much fuel. Because engines run inefficiently when they're cold, they use extra fuel at start-up to ensure a smooth idle and stumble-free acceleration. If the smoke clears up as the engine warms to operating temperature, it's probably nothing to worry about.

If the black smoke continues, a faulty sensor or a clogged fuel injector, fuel-pressure regulator, or other intake-system component might be to blame.

Because of the vast complexity of modern fuel-injected engines, your best bet might be to have the car checked out by a mechanic with specialized training in those types of repairs. Black smoke is usually accompanied by the "check engine" light.

Thick White Smoke

Call a tow truck
Unlike the wispy white vapor described earlier, billowing white smoke is almost always an indication of serious engine troubles that warrant immediate attention. If you continue to drive a vehicle in that condition, the engine could overheat and suffer extensive damage.

Smoke of that sort is usually caused by the engine burning coolant and can be the result of a blown head gasket, a damaged cylinder head, or a cracked engine block, the last of which requires an engine replacement. Even a small coolant leak can lower the engine's coolant level, which results in overheating and can lead to catastrophic damage to the engine. A coolant leak into the engine's oil system might not cause tailpipe smoke but could cause the oil to become thin and milky-looking and the coolant to look like brownish sludge. Such a leak also requires immediate attention.