What do different colours of exhaust smoke mean?

The colour is a reliable guide to what is being burnt that should not be, and it narrows the diagnosis considerably.

Thin white vapour that disappears quickly is normal, especially on a cold morning. It is condensed water, a natural product of combustion, and it stops once the exhaust warms up.

Thick white smoke that persists is different and usually means coolant is entering the combustion chamber, typically through a failed head gasket or a cracked head. Supporting signs are a sweet smell, coolant level dropping with no visible leak, mayonnaise-like residue under the oil filler cap, and overheating. This is serious and driving on can turn an expensive repair into a replacement engine.

Blue or grey smoke means oil is being burnt. Common causes are worn piston rings, worn valve stem seals or a failed turbocharger seal. A clue to which: blue smoke mainly on startup points at valve stem seals, since oil seeps past them while parked, whereas smoke under acceleration points at rings or turbo.

Black smoke means the mixture is too rich — too much fuel for the available air. Causes include a clogged air filter, a faulty mass airflow sensor, leaking injectors or a failed fuel pressure regulator. It is more common and usually less catastrophic than white or blue, but it wastes fuel and will damage the catalytic converter over time.

Diesels produce some visible soot under hard acceleration, particularly older ones without a particulate filter, and a brief puff is not necessarily a fault.

A smell test helps: sweet suggests coolant, acrid and oily suggests oil, strongly of fuel suggests over-fuelling.