Exhaust smoke color: white, blue or black explained

What it means

In every cylinder, an air-fuel mixture is compressed and ignited, and the spent gases are pushed out through the exhaust. When the smoke changes color, something other than that mixture is being burned, or the mixture itself is wrong: oil shows as blue, too much fuel as black, and coolant as white smoke that does not clear once the engine is warm.

That is why smoke color is one of the fastest diagnostic clues a technician has.

Common causes

• Oil being burned in the combustion chamber, seen as blue smoke
• Worn valve-stem seals or piston rings, high crankcase pressure, or a faulty breather or PCV system, all of which burn oil
• On a turbo engine, oil past the turbo's sealing rings or a restricted oil drain
• The engine running rich, seen as black smoke
• A stuck-open or leaking fuel injector, one cause of rich running
• Coolant entering the combustion chamber, seen as white smoke that persists after warmup
• On a diesel, glow plugs, with white smoke or rough running for the first minutes of a cold start

What to do now

1. Note the color, and when it appears: on a cold start, after warmup, or under acceleration.
2. Check the oil level, and whether it has dropped since the last check.
3. If the smoke is white, let the engine cool fully before checking the coolant level.
4. With black smoke and a fuel smell or a visible leak, stop. A leak near a hot engine is a fire risk.
5. Have the source diagnosed before anyone replaces a part, especially a turbocharger.

What the color tells you

• Blue or blue-grey smoke: oil being burned, from the engine or the turbo
• Black smoke: the engine running rich, burning more fuel than it should
• Black smoke with a fuel smell or a visible leak: stop, because a leak near a hot engine is a fire risk
• White smoke that persists after warmup: possibly coolant in the combustion chamber, so get it checked urgently
• White smoke together with overheating: deserves a same-day look
• Diesel, white smoke only for the first minutes of a cold start: often the glow plugs

What people often get wrong

That blue-grey smoke means the turbo has failed. It means oil is burning. Valve-stem seals, piston rings and crankcase pressure can all do the same thing, so the cause should be identified before anyone replaces a turbocharger.

When the car must go to a workshop

White smoke that persists should be checked urgently, and white smoke with overheating deserves a same-day look. For blue or black smoke, book it in soon, and ask for the cause, not just the part.

Read next

• Car burning oil: how to tell and what it means
• Car losing coolant: where is it going?
• Turbo losing power: what to check before the turbo
• Used car cold start: why you should see the engine start cold

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.