Exhaust smoke colours — what do they mean?

The exhaust of a healthy, warmed-up engine is practically invisible. If clear smoke comes out of the tailpipe, its colour says a great deal about the cause — and lets you quickly judge how serious the situation is.

White vapour — usually nothing wrong

Light white vapour in winter or after a cold start is condensed water from the exhaust system. It disappears once the engine warms up and has no smell. It’s a normal physical phenomenon.

Thick white smoke — coolant

Thick white smoke that doesn’t disappear once the engine warms up and has a sweetish smell means coolant is being burned. The most common cause is a damaged head gasket. It comes with a drop in the coolant level and often a milky film under the oil filler cap.

Bluish smoke — the engine is burning oil

Bluish, greyish smoke, especially when accelerating or after engine braking, means oil is being burned. The cause may be worn piston rings, valve seals or a damaged turbocharger.

Black smoke — too rich a mixture

Black smoke is unburnt fuel. In diesels it can be the effect of hard acceleration, but constant black smoke points to too rich a mixture: a clogged air filter, a faulty mass airflow sensor or the injectors.

A quick cheat sheet

• White vapour, disappears once warm — normal
• Thick white, sweetish smell — coolant, urgent diagnostics
• Bluish — the engine is burning oil
• Black — too rich a mixture, filter or injection
• Grey, acrid — possible combustion problems

The colour of the smoke won’t replace diagnostics, but it lets you tell a trifle from a situation in which it’s better not to drive on.

If the car is burning oil, keep checking the level and don’t delay the change. We’ll check the oil’s condition and help assess the cause during a call-out visit.