What do the smoke signals from your tailpipe mean?

The color can help you read the problem

Last updated: February 2014

During normal operation, the emissions from a car's tailpipe should be invisible. But what if they're not? Should you be worried if you notice what looks like smoke? What should you do? The answers to these questions differ depending on the type of emissions you see.

In addition to indicating a possible problem, smoke from the tailpipe means dirty emissions. Most states and some municipalities mandate periodic tailpipe-emissions testing. If your car is emitting smoke, chances are good it won't pass muster come test time. Virtually all modern service facilities employ diagnostic machines capable of pinpointing the cause of the emissions in a matter of minutes. The cost is typically around $50.

Here's how to "read" tailpipe smoke signals to help you determine whether they're the byproduct of a harmless atmospheric phenomenon or the first sign of a major mechanical failure.

Thin white vapor: No cause for concern

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. It may even be accompanied by a slow drip of water. This is a common sight when cars, even new ones, are first started in the morning, since condensation has had time to build up overnight.

Blue or gray smoke: See your mechanic

Thick blue or gray smoke that doesn't dissipate quickly is most likely the result of oil being burned in the engine's combustion chamber. It can be caused by something as minor as clogged oil passages or it may point to something more serious that could require extensive engine work.

If your engine is burning enough oil to produce visible exhaust smoke, the oil level will drop over time and require periodic top-offs. Worse, burning oil can foul the engine's spark plugs, causing maladies ranging from a rough idle and reduced fuel mileage to hard starting and sluggish acceleration.

On turbocharged vehicles, the presence of bluish-gray exhaust smoke may indicate turbocharger failure, especially if accompanied by a high-pitched, sirenlike whine. The turbo may need to be rebuilt or replaced. The oil lines to and from the turbo should also be replaced at this time.

Regardless of the cause of this type of smoke, you should have the vehicle checked out by a qualified mechanic as soon as possible.

Black smoke: See your mechanic

Black, sooty smoke is usually symptomatic of an engine that's burning too much fuel. Because engines run inefficiently when 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.

Should the smoking persist, a clogged or dirty air filter is a likely culprit. (On carbureted vehicles, the choke and choke linkage could have a buildup of gum and varnish.) If the filter checks out OK, a faulty sensor, a clogged fuel injector, or another intake-system component may be to blame. Because of the vast complexity of modern fuel-injected engines, your best bet may be to have the car checked out by a mechanic with specialized training in these types of repairs.

Thick white smoke: Call a tow truck

Unlike the wispy white vapor described above, billowing white smoke is almost always an indication of serious engine trouble, and warrants immediate attention. If you continue to drive the vehicle, the engine could overheat and suffer extensive damage. Smoke of this sort is usually caused by the engine burning coolant, and can be the result of a blown head gasket or a damaged cylinder head, or a cracked engine block, which requires a new engine or an engine rebuild. Even a small coolant leak can lower the engine's coolant level, resulting in overheating and, potentially, catastrophic engine damage. A coolant leak into the engine's oil system may not cause any tailpipe smoke but could cause the oil to become thin and milky looking, and cause the coolant to look like brownish sludge. This, too, requires immediate attention.

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