White exhaust smoke: normal or head gasket?

A little white smoke on a cold start, which clears within moments, is normal: it is water vapour (CarVerif). Thick white smoke that persists once the engine is warm, especially if the coolant level drops, means the engine is burning coolant: head gasket, oil cooler or EGR cooler.

Normal or not?

• Normal: light, on start-up in cold weather, it clears as soon as the exhaust warms up.
• Not normal: thick, persistent, with a falling coolant level. Check the expansion tank with the engine cold.

Causes of persistent white smoke

The head gasket

A leaking head gasket lets combustion gas or coolant pass from one circuit to the other: pressure in the cooling system, white smoke, falling coolant level (Fiches-auto, Ford Kuga 2). A real drop in coolant with white smoke calls for a check of the head gasket, the cooler, the injectors and the whole circuit (Fiches-auto, Ford Focus 4).

Some engines are known for it. Ford acknowledged that some 2.3 EcoBoost engines in 2016 and 2017 Focus RS models were using coolant, producing white smoke, sometimes at very low mileage (Caradisiac). L'argus also describes a sealing defect between the cylinders and the cooling circuit on the Toyota Corolla 2.0 hybrid, with coolant loss, white smoke and overheating if ignored (CarVerif).

The EGR cooler

On diesels, the small radiator that cools the recirculated gas can leak: coolant goes into the intake and burns. An Audi Q5 2.0 TDI owner reports heavy white smoke and coolant loss, fixed by replacing that cooler (Fiches-auto).

Diesel injectors or starting

On a diesel, persistent white smoke with laboured cold starts can come from the injection or the glow plugs (CarVerif, Skoda Octavia 1.9 TDI). The injectors of the 2.0 D-4D in E120 Toyota Corollas can produce white or black smoke (GetCarwise).

White smoke from under the bonnet

If the smoke comes from under the bonnet rather than the exhaust, it is often coolant escaping: stop and let it cool before opening anything (Caradisiac). See our guide to the temperature warning light and overheating.