Thick smoke and the temperature climbing? Stop
If heavy white smoke comes with the gauge rising, a sweet smell or a red light, pull over somewhere safe and switch off. That combination points to coolant getting into the engine, and driving on hot does more harm. Smoke from under the bonnet rather than the exhaust is a different emergency: get everyone out and well away from the car, and call 111.
What does the colour of the smoke mean?
| Usually means | How urgent | |
|---|---|---|
| Thin white on a cold start, clears when warm | Water vapour (condensation) | Normal |
| Thick white, sweet smell, doesn't clear | Coolant burning, often a head gasket | Stop and have it checked |
| Blue or grey-blue | Oil burning: worn rings or valve seals, or turbo seals | Soon; watch the oil level |
| Black or sooty | Too much fuel or too little air | Soon; fuel use goes up |
| A puff of blue at start-up only | Oil seeping past valve seals overnight | Mention it at the next service |
Why is my diesel smoking?
Diesels smoke more than petrol engines, and plenty of New Zealand utes and SUVs run on diesel. Wikipedia's diesel exhaust entry says black smoke is carbon that hasn't burned, from too much fuel or too little air, and that an engine smokes more from cold until it has warmed through.
On a diesel, black smoke under load often comes back to the air filter, the injectors, or a turbo that isn't delivering enough air. White smoke that hangs about on a warm diesel can be unburned fuel or coolant. A mechanic who does diesel servicing is the right person for both.
Blue smoke and the turbo
On a turbocharged engine, blue smoke isn't always the engine itself. Wikipedia's turbocharger entry says failed turbo seals let oil into the exhaust, often giving blue-grey smoke. A mechanic can tell the two apart by checking the turbo for play and oil in the pipes, alongside compression tests on the engine.
Will a smoky exhaust fail a WoF?
It can. The MTA's description of the WoF says the exhaust must not be smoky, as well as having no leaks and being no louder than the original system. A car that smokes heavily when the inspector checks it is likely to fail on the exhaust.
Vapour on a cold start isn't the same thing. If you're not sure where the line sits for your car, ask the inspector, and get the cause sorted before the WoF rather than after.
What to do before you ring anyone
Let it warm up
Watch for a few minutes. Smoke that clears as the engine warms is probably vapour.
Note the colour and when
Start-up, accelerating, idling or all the time. A short video on your phone helps.
Check the levels cold
Oil on the dipstick and coolant in the overflow bottle. Coolant dropping alongside white smoke is a strong clue.
Notice the smell
Sweet points to coolant, a burning-oil smell to oil, raw fuel to fuel.
What will the mechanic check?
For white smoke: a cooling-system pressure test and a check for combustion gas in the coolant. For blue: compression and leak-down tests, the engine's breather system and, if fitted, the turbo seals. For black: the air filter, the sensors, the injectors and any stored codes.
Get the diagnosis written down before the repair is quoted, because the fix for each colour is very different.
What you can do, and what's the mechanic's job
You can
- Check oil and coolant levels on a cold engine.
- Film the exhaust at start-up and again when warm.
- Keep an eye on fuel use if it seems to have gone up.
Leave to the mechanic
- Pressure, compression and leak-down testing.
- Turbo, injector and sensor diagnosis.
- Any engine strip-down or head gasket work.
Where we'd start
We'd film the smoke, check the oil and coolant cold, and send it as a diagnosis job with the colour in the note. Thick white smoke with the coolant dropping, we'd stop driving and ask for a mechanic who can come to the car.