What does the colour of my exhaust smoke mean?

White, blue and black exhaust smoke each point at a different system. How to tell harmless cold-morning vapour from coolant, oil or fuel, and which needs a garage this week.

Difficulty: beginner
Time: 10 min
Read: 4 min
Safety: caution

Short answer

Thin white vapour on a cold morning that disappears within a minute or two is condensation and is entirely normal. Thick white smoke that persists, smells sweet and comes with coolant disappearing points at coolant entering the engine. Blue smoke means oil is burning; black smoke means too much fuel. Only the first of those is nothing.

Petrol and diesel burn to carbon dioxide and water, so a certain amount of visible water vapour is a sign the engine is working correctly rather than badly. The trick is separating that from the three things that are not supposed to be in the exhaust: coolant, engine oil and unburnt fuel. Each produces a distinctive colour, and each has a distinctive pattern of when it appears — at start-up, on acceleration, on the overrun, or all the time. That pattern is often more diagnostic than the colour itself.

Safety

Never run an engine in a closed garage or enclosed space to look at the exhaust. Exhaust gas contains carbon monoxide, which has no smell and causes collapse without warning. Do this outdoors, and never sit in a running car in a garage with the door shut.

What the colour of exhaust smoke points at

Step by step

Check whether it disappears as the engine warms.Condensation vapour is at its worst on a cold, damp morning, thins after a minute or two, and vanishes once the exhaust is hot. If your "smoke" follows that pattern it is water and needs nothing.
Smell it, from a sensible distance.Coolant burning gives a distinctly sweet smell. Unburnt fuel smells of fuel. Burning oil smells acrid and slightly like a hot barbecue. Do not put your face near the tailpipe and do not do this in a garage.
Note when it appears.Blue smoke only on start-up after standing points at valve stem seals letting oil seep down overnight. Blue smoke on acceleration points more at piston rings. Blue smoke when you lift off after a long pull is often turbo seals. All three are different repairs.
For white smoke, check the coolant level, cold.Persistent white smoke plus coolant that keeps disappearing with no puddle underneath is the classic head gasket picture. Also look under the oil filler cap for a creamy emulsion, though a small amount there can just be condensation on a short-journey car.
For black smoke, think fuel and air.Black smoke is unburnt fuel — too much fuel or not enough air. On a diesel a puff under hard acceleration on an older car is common; constant black smoke means a fault, often a blocked air filter, a faulty sensor or injector problems. On a modern petrol car black smoke is unusual and always worth investigating.
Get the fault codes read for anything persistent.Smoke is a symptom of a system, and the engine management usually knows more than you can see. A code reader narrows blue, white or black smoke down to a circuit.

Common mistakes

Panicking at cold-morning vapour — It is the single most common false alarm here, and it is at its most dramatic on exactly the frosty mornings when people first notice it.
Adding oil to keep up with blue smoke and doing nothing else — It keeps the engine alive but the underlying wear continues, the catalytic converter is being poisoned by oil, and the car will fail its emissions test.

If it doesn't work

Thin white on cold mornings, gone in a minute

Cause: Water vapour condensing in a cold exhaust — Fix: Nothing. This is normal combustion, and more visible in cold damp weather and on short journeys.

Thick white, sweet smell, coolant level dropping

Cause: Coolant entering the combustion chamber, commonly a head gasket — Fix: Stop driving it and get it diagnosed. Continuing risks hydraulic damage and a much larger bill.

Blue puff on start-up only

Cause: Valve stem seals allowing oil past while parked — Fix: Common on higher-mileage engines. Not usually urgent, but monitor oil consumption and mention it at the next service.

Blue smoke and rising oil consumption

Cause: Worn rings or bores, or a failing turbo — Fix: Needs proper diagnosis. Keep records of how much oil you are adding and how often — that figure is what the garage will ask for first.

Black smoke on a diesel, with a warning light

Cause: Blocked filter, faulty sensor, or injector fault — Fix: Get the codes read. Sustained black smoke wastes fuel, contaminates the DPF and is an MOT emissions failure.

Grey-blue smoke and a burning smell but nothing from the tailpipe

Cause: Oil leaking onto the hot exhaust from outside the engine — Fix: That is a leak, not a smoke problem. Find where it is dripping from — it is a fire risk on a hot manifold.

Questions people ask

My diesel smokes when I first pull away and then stops. Is that a problem?

A brief puff of dark smoke from an older diesel under load is common and not necessarily a fault. Anything from a modern diesel with a particulate filter fitted is worth investigating, because the filter should be catching it.

Can smoke fail an MOT?

Yes. The MOT includes a visible smoke and emissions test, and excessive smoke of any colour is a failure. It is also one of the fault types that gets worse quickly if ignored.

Is white smoke always a head gasket?

No. It can also be a cracked head, a failed inlet manifold gasket on some engines, or on a diesel, unburnt fuel appearing as a whitish haze when an injector is faulty. The distinguishing feature of coolant is the sweet smell and the disappearing coolant.