A petrol engine can burn oil without producing visible evidence of blue smoke, especially if it has good compression and burns the oil cleanly. This occurs because efficient combustion may prevent noticeable smoke emissions, even when oil is being consumed internally. However, it is important to note that it is not acceptable for any new engine, or an engine in good internal condition, to burn large quantities of lubricating oil.
Excessive oil consumption is often an indication of underlying issues, even if the engine appears to be running smoothly.
Blue smoke is often noticeable during a cold start, and this can indicate reduced oil control within the engine. One common cause is the presence of carbon fouling deposits around the piston rings, which can prevent them from sealing effectively. Over time, these deposits may build up due to incomplete combustion or poor-quality oil, allowing oil to bypass the rings and enter the combustion chamber, where it burns and produces the characteristic blue smoke.
Another possible cause is cylinder glaze, a condition where the cylinder walls become polished or coated with a smooth glaze due to prolonged low-load running or improper break-in procedures. This reduces the ability of the piston rings to grip the cylinder walls effectively, resulting in increased oil consumption and, consequently, blue smoke.
It is important to note that blue smoke should not be evident at any stage of engine operation, whether during start-up, acceleration, deceleration, or at idle. The presence of blue smoke is a clear indicator of excessive oil being burned, which not only compromises engine performance but may also lead to longer-term issues, such as:
- Increased carbon deposits: Further contributing to fouling and reduced efficiency.
- Damage to the catalytic converter: Excess oil burning can contaminate the catalyst, reducing its effectiveness and potentially requiring costly replacement.
- Spark plug fouling: Oil deposits on the spark plugs can lead to misfiring, reduced fuel economy, and increased emissions.
To address blue smoke at cold start, it is important to diagnose and correct the underlying issue. Potential steps include:
- Inspecting piston rings: Checking for wear, carbon fouling, or damage that may affect their sealing ability.
- Checking for cylinder glaze: If cylinder glaze is present, light honing or reconditioning of the cylinder walls may be required.
- Ensuring proper oil grade and quality: Using the manufacturer-recommended oil grade can reduce deposit formation and improve oil control.
- Examining valve stem seals: If oil is leaking into the combustion chamber through the valve guides, replacing the seals may be necessary.
- Monitoring maintenance practices: Ensuring proper engine break-in procedures, avoiding prolonged idling, and maintaining the PCV system to minimise deposits and oil consumption.
Eliminating blue smoke requires a systematic approach to identify and rectify the underlying causes. For a well-maintained engine, blue smoke should not occur at any point in its operation.