* Unburnt Fuel (typically bluish-white or light grey smoke): This is more common. When slowing down, especially engine braking (letting the engine slow the car), the throttle is closed. However, if the fuel mixture is too rich (too much fuel relative to air), or there's a problem with the fuel delivery system (injectors, pressure regulator etc.), some fuel might not be fully burned in the combustion chamber. This unburnt fuel exits the exhaust as smoke. This is especially noticeable at lower RPMs and under engine braking conditions.
* Oil (typically bluish-grey or blue smoke): This indicates a more serious issue, usually involving worn valve stem seals, piston rings, or a leaking head gasket. As the engine slows, the crankcase pressure changes, potentially allowing oil to be drawn into the combustion chamber and burned. The smoke will often have a more pronounced bluish tint and may smell strongly of burnt oil.
Things to consider to help diagnose:
* Color and Smell of the Smoke: As mentioned above, color and smell are crucial clues.
* Amount of Smoke: A little puff might be normal, especially after a cold start or hard driving. Significant plumes of smoke are a problem.
* When it Happens: Only when slowing down and engine braking? Does it happen during acceleration too? This helps isolate the issue.
* Recent Maintenance: Have you recently changed the oil? Have there been any modifications to the engine?
Next Steps:
1. Check your oil level: Low oil level alongside blue smoke strongly suggests an oil leak into the combustion chamber.
2. Inspect the exhaust system: Look for obvious leaks or signs of oil buildup.
3. Consult a mechanic: Diagnosing this accurately requires a professional inspection. They can perform compression tests, leak-down tests, and other diagnostics to pinpoint the exact cause.
Ignoring this could lead to further engine damage. It's best to address the issue promptly.
