White smoke from the exhaust
Thin white vapour that disappears within a metre on a cold morning is condensation and is normal. Dense white smoke that hangs in the air, smells sweet and continues after the engine is warm is coolant being burnt in the cylinders - head gasket, cracked head or, on some engines, a leaking EGR cooler. On a diesel, white-grey smoke that stings the eyes and smells of raw fuel is unburnt diesel from injection or glow plug problems rather than coolant.
Urgency
Stop driving
Possible causes, most likely first
- Head gasket failure
Sweet-smelling white smoke, coolant level dropping with no external leak, and bubbles in the expansion tank at idle. Confirm with a combustion gas test on the coolant (block tester turns from blue to yellow) and a cylinder leak-down test. - Cracked cylinder head or block
Same signs as a head gasket but often appearing after a severe overheat; may be worse when cold until the metal expands. Pressure test the cooling system at 1.2-1.5 bar with the engine cold and look for a cylinder that fills with coolant overnight. - Leaking EGR cooler (diesel)
White smoke under load and coolant loss without any oil contamination, common on modern diesels with water-cooled EGR. Remove the EGR pipe and look for wet coolant inside, or pressure test the cooler separately. - Unburnt diesel from a faulty injector or glow plugs
White-grey smoke that stings the eyes, mostly at cold start and idle, clearing as the engine warms. Check glow plug operation and current draw, then compare injector correction values on a scan tool for one that stands out. - Incorrect injection timing (diesel)
Continuous white smoke with poor power and a knocking idle after timing belt or injection pump work. Verify pump or camshaft timing against marks; a scan tool showing timing deviation confirms it. - Turbo coolant seal failure (water-cooled turbos)
White smoke that appears only after the turbo has been working hard, plus slow coolant loss. Inspect the turbo inlet and outlet for coolant residue and check the coolant lines to the turbo housing.
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What you can check yourself
- Smell the smoke: sweet means coolant, raw fuel means unburnt diesel, no smell and disappears quickly means condensation.
- Check the coolant level in the expansion tank when cold and mark it; recheck after a week of normal driving.
- With the engine idling and warm, open the expansion tank cap carefully and look for a steady stream of bubbles.
- Check the oil cap and dipstick for a creamy, mayonnaise-like residue indicating coolant in the oil.