White Smoke From the Exhaust: Causes and When It Is Serious

Exhaust smoke is a useful clue, and its color matters. White smoke has two very different explanations, from harmless to serious.

Harmless: condensation

A cold engine produces water vapor that condenses in the exhaust system. On a cold or humid morning you may see thin white vapor that fades quickly and disappears once the engine warms up. You may also notice a small puddle of water dripping from the tailpipe. That is normal.

Serious: coolant in the combustion chamber

If the smoke is thick, stays after the engine is warm and has a sweet smell, coolant is probably burning inside the engine. Typical causes:

• Blown head gasket. The seal between the engine block and the cylinder head fails and coolant leaks into the cylinders.
• Cracked cylinder head or engine block, often after overheating.
• Intake manifold gasket leak on some engines.

Other signs of a coolant problem

• The coolant level keeps dropping with no visible leak under the car.
• The engine overheats or the temperature gauge climbs.
• Milky, light brown residue under the oil filler cap or on the dipstick, which means coolant is mixing with oil.
• Bubbles in the coolant reservoir when the engine is running.
• Rough idle and misfires.

Other smoke colors for comparison

• Blue or gray smoke: oil is burning (worn piston rings, valve seals or a faulty turbocharger).
• Black smoke: too much fuel in the mixture (dirty air filter, leaking injector, faulty sensors).

What to do

1. Watch the smoke after the engine reaches normal temperature. If it is gone, it was condensation.
2. If it stays, check the coolant level when the engine is cold. Never open the radiator cap on a hot engine.
3. Do not keep driving with an overheating engine. Running it hot can warp the cylinder head and turn a gasket job into an engine replacement.
4. Ask a garage for a cooling system pressure test and a combustion leak (block) test. These tests show quickly whether the head gasket is leaking.

Where to start

Look for a nearby mechanic with strong engine repair reviews, describe exactly when the smoke appears and whether you have lost coolant. A clear description saves diagnosis time.