Thick smoke and the gauge climbing? Stop first
If thick smoke keeps pouring out, the temperature gauge is rising, a red light comes on, or you can smell burning, find somewhere safe to pull over and switch off. Smoke from under the bonnet gets the same answer. Get everyone out on the side away from the traffic and stand well clear. Call roadside assistance if you have it. If anyone is hurt or in danger, call 000.
What does the colour of the smoke mean?
| What it usually points to | How urgent | |
|---|---|---|
| Thin white, cold start | Water vapour and condensation. The NSW EPA says it normally disappears once the vehicle warms up. | Not a fault. Nothing to do. |
| Thick white, stays when warm | Coolant getting into the engine and burning, often through the head gasket. Other causes are possible. | Stop driving and have it checked. |
| Blue or grey | Oil being burnt. The NSW EPA says this normally means engine wear or damage, such as worn rings or pistons. A failed turbo seal can do it too. | This week, and check the oil level. |
| Black, petrol engine | Too rich a fuel mix. RAC WA lists poor tuning, a dirty air filter or a faulty sensor. | Soon. It wastes fuel and can foul the engine. |
| Black, diesel engine | Fuel not burning completely. Faulty injectors, a clogged air filter or overfuelling are common causes. | Soon. The NSW EPA warns a long-running injector fault can cause major engine damage. |
Why is my diesel blowing white smoke?
A cold diesel can puff white for a moment after starting, and on a cold morning that's often condensation. The test is what happens once it's warm. If the white smoke fades, there's usually nothing to chase.
If it hangs around, it's one of two things: fuel that isn't burning, or coolant that shouldn't be there. Fuel-side causes include injectors, glow plugs (the heaters that help a diesel start cold) and low compression. Coolant means the head gasket or something near it. Smell helps a little: raw diesel fumes point one way, a sweet smell points the other. The mechanic needs to test for it, because the two can look alike at the tailpipe.
EPA Victoria also notes some diesels make a little smoke under load as part of normal running, such as going uphill. A brief puff on a hill isn't the same as a steady plume.
What to do before you call anyone
Time it
Note how long the smoke lasts and when it shows up: only on a cold start, when you accelerate after coasting, under load, or all the time.
Get someone to film it
Colour is hard to judge in a mirror. Ask someone to film the tailpipe with the engine warm, then send that to the mechanic. Do it parked, and never while you're driving.
Check the levels on a cold engine
Park on level ground and wait for it to cool right down. Check the oil on the dipstick and look at the coolant reservoir from the outside. A falling level of either is the most useful clue you can hand over.
Check the oil and tell the mechanic
Milky or creamy oil on the dipstick is a reason not to start the car again. Say so when the mechanic calls. It changes whether the car should be driven in at all.
What will the mechanic check?
For blue smoke, they'll look at oil use, the turbo if there is one, the PCV valve (a small valve that vents the engine's crankcase) and then compression. For black smoke, they'll start with the air filter and read the engine computer for fuel and sensor faults. On a diesel they'll test the injectors before condemning anything dear.
For white smoke, the first job is proving whether coolant is getting in, usually with a pressure test on the cooling system and a check for exhaust gas in the coolant. Ask to see what they tested. Smoke is a clue, not a diagnosis, and a part shouldn't be replaced on colour alone.
This is engine work, so it suits a mechanic who does engine diagnosis regularly, and one who knows diesels if yours is a diesel. Get a written estimate before work starts. Consumer Affairs Victoria gives the same advice.
What you can do, and what's the mechanic's job
You can
- Note the colour, the timing and the smell.
- Check oil and coolant levels on a cold engine, with the reservoir cap left on.
- Keep the air filter and servicing up to date.
Leave to the mechanic
- Pressure and compression testing.
- Injector, turbo and fuel system testing.
- Deciding whether the car is safe to drive.
Where we'd start
Thin white smoke that's gone in a few minutes: leave it. Blue or black smoke with a car that drives normally: we'd check the oil today and send the job this week. Thick white smoke, a falling coolant level or a gauge that's moved: we'd stop driving it, and say so in the note on the card, so the mechanic can tell you whether to tow it or come to it.