Home mechanic (basic tools) ยท Time: 15 to 30 min (estimate) (estimate)
Smoke colour is a quick clue. Black smoke is unburned fuel (too rich). Blue or grey smoke is burning oil. White vapour that soon clears is water condensing in a cold pipe; thick white smoke that stays and smells sweet is coolant burning. Colour alone does not prove a fault: confirm it with the plug, the oil level and a compression test. Note that some manuals, including the K-Light, call oil smoke "white"; on this site we call it blue or grey.
Safety
- Do this outside. Exhaust contains carbon monoxide, which can kill in a closed room.
- The exhaust is hot. Hold the card a short distance from the outlet, not on it.
- Do not run a smoking engine hard to "clear it".
You need
- A white card, a torch and a plug spanner.
Steps
- Start from cold. Watch the exhaust during the first minute. Thin white vapour that drips water and clears when the pipe is warm is normal condensation.
- Warm the engine fully. Watch at idle, then while a helper opens the throttle smartly and lets it close.
- Note when the smoke appears: - on opening the throttle and under load: points to piston rings and cylinder wear, or rich mixture - after idling and when the throttle is closed (for example coming down a hill): points to valve stem seals or worn valve guides - all the time: points to rich mixture, overfilled oil or a two-stroke oil fault
- Hold the white card near the outlet for a few seconds at idle. Dry black soot means rich mixture. Oily wet spots mean oil. Clean water drops on a warm engine mean coolant.
- Check the oil level. Above the upper mark can cause smoke; the K-Light smoke chart starts with this check.
- Remove and read the spark plug: black dry soot (rich), black wet and oily (oil), very clean and white (coolant or lean). See Read a spark plug.
- Two-stroke engines always make some light blue smoke; it gets heavy if the oil mix or oil pump setting is too rich.
What it can mean
| Colour | First check | Go to |
|---|---|---|
| Black | Air filter, choke, carburettor or injector | Black smoke from the exhaust |
| Blue or grey | Oil level, compression | Blue or grey smoke from the exhaust |
| White that stays | Coolant level, oil colour | White smoke or steam from the exhaust |
| From the breather or filler | Blow-by | Smoke or oil mist from the crankcase breather |
Check your work
- You wrote down colour, when it appears, oil level and plug colour. Those four facts together point to the cause.
If it did not fix the problem
- Oil smoke with low compression: Test engine compression.
- Black smoke with good compression: Clean or replace the air filter.