White, blue or black exhaust smoke: what it tells you about your engine

What you see

At start-up, when accelerating or all the time, the exhaust is puffing out coloured smoke. The colour, the timing and the smell tell you almost everything.

White smoke

Light, cold engine, clears quickly, no smell: water vapour. Normal, especially in winter.

Thick, persists with a warm engine, sweetish smell: coolant is burning in the cylinders. Causes: head gasket, cracked cylinder head, leaking EGR cooler (diesel). Check the level in the expansion tank, look for “mayonnaise” under the oil filler cap, and watch whether the engine overheats. Do not drive: overheating and engine failure are the likely result.

On a diesel, white-grey smoke only when cold, smelling of diesel: badly burnt fuel, often failed glow plugs or an injector. See Glow plug light flashing.

Blue smoke

Oil is burning. Causes, from most to least likely:

1. Valve stem seals (a blue puff at start-up after the car has been parked, then nothing).
2. Worn piston rings (blue smoke all the time, noticeable oil consumption).
3. Turbo (blue smoke when accelerating or lifting off, oil in the intake hoses).
4. Blocked crankcase breather / oil separator: oil gets sucked into the intake.
5. Overfilled engine after a generous oil change.

Measure oil consumption over 1,000 km. With DiagAuto AI, check whether the fuel trims are abnormal and whether a catalytic converter code appears (P0420): oil fouls the catalytic converter.

Black smoke

Too much fuel, not enough air:

1. Clogged air filter.
2. MAF sensor under-reading the air → P0101; on a petrol engine, a P0172 code (running rich) confirms it.
3. EGR valve stuck open → P0401.
4. Leaking or badly adjusted injectors (diesel).
5. Turbo not building boost (boost leak, P0299).
6. Faulty intake pressure sensor.

On a diesel fitted with a DPF (diesel particulate filter), visible black smoke is abnormal: the DPF may be cracked (P2002).

What you can do right away

Note the colour, the timing (cold/warm, accelerating/lifting off) and the smell. Check the oil and coolant levels. Read the codes and display air flow, fuel trims and intake pressure (the boost pressure on a turbo engine) in the “Sensors” screen of DiagAuto AI. Describe all of this to the AI chat, which does not need a code to point you in the right direction.

When to go to the garage straight away

Thick white smoke with a falling coolant level; heavy blue smoke with a whistling turbo; black smoke with loss of power and a smell of fuel.