Blue exhaust smoke: causes, seriousness and affected cars

Blue exhaust smoke means the engine is burning oil, while black smoke points to over-fuelling (GetCarwise). Most often, the oil gets past a worn turbo, piston rings that no longer seal or hardened valve stem seals. On a used car, it is a reason to walk away until the cause is known.

What does blue smoke mean?

Engine oil reaches places it should not: the combustion chamber or the intake, where it burns with the fuel. The oil level drops between services; if nobody checks it, the engine can eventually run short of oil and fail (GetCarwise).

The most common causes

A worn turbo

This is the most cited cause on turbocharged engines. When the turbo bearings or seals leak, oil enters the intake: blue smoke, whistling, loss of power, reluctance to rev (CarVerif, Audi Q5 2.0 TDI; CarVerif, Opel Antara 2.0 CDTI). On the Nissan X-Trail 2.0 dCi, turbo failures are reported between 50,000 and 100,000 km with these symptoms (CarVerif).

Clogged piston rings

On the 2008-2012 Audi and Volkswagen 1.8 and 2.0 TFSI (EA888), clogged oil control rings let oil through: blue smoke on start-up and under acceleration, frequent low-oil warnings, and more than a litre used every 1,000 miles (GetCarwise).

Worn valve stem seals

Hardened valve stem seals let oil drip into the cylinders when the engine is off: blue smoke shows mostly on start-up, and the oil level drops quickly (Euro Premium Parts, BMW 6 Series). A Mercedes GLK owner describes "heavy blue smoke on start-up" on the 1.8 petrol (Fiches-auto).

Crankcase ventilation

A faulty breather (PCV valve) can draw oil vapour into the turbo and intercooler: blue smoke, oil loss and reduced performance (GetCarwise, Volvo V70).

Is it serious?

Yes, as soon as the smoke is regular. The engine is using oil and can fail if the level drops too low (GetCarwise). On some diesels, a turbo that sucks in oil can also make the engine run away, burning its own oil until it breaks: this is documented on the 1.9 dCi 120 of the Renault Mégane II (Mégane II guide).

Check the oil level straight away, then have a garage find the cause: a turbo, piston rings or valve stem seals differ in both cost and urgency.