When the smoke appears tells you a great deal
| When the smoke appears | Most likely cause |
|---|---|
| On a cold start, then disappears | Hardened valve stem seals |
| After a long idle, when you accelerate again | Valve stem seals or turbo |
| Continuously, especially under load, with oil consumption | Worn piston rings or pistons |
| Under acceleration, with oil in the intake hoses | Turbo, or excessive crankcase pressure |
The causes in detail
- The turbo. When its bearings or seals are worn, the oil lubricating it passes into the intake or exhaust. Bear in mind that most turbo oil leaks have an external cause, such as a blocked oil return, a clogged breather or a choked air filter. Fitting a new turbo without fixing that underlying cause will simply result in the new unit leaking as well.
- Valve stem seals. As they age, they harden and allow oil from the cylinder head to run down the valve stems. The smoke is typically most noticeable on start-up and when picking up speed.
- Piston rings or pistons. When worn, they allow oil to be drawn up into the cylinders. The smoke becomes constant and the oil level drops noticeably between services.
Blue, white or black: don't get them confused
Thick, persistent white smoke accompanied by a drop in coolant level points to a head gasket failure. Black smoke, particularly on a diesel, indicates excess fuel or insufficient air (injection, EGR valve, an air leak on a hose) — it is almost never the engine itself. A thin wisp of white smoke when cold that clears within a few minutes is simply condensation.
Can you keep driving?
For a few days, provided you check the oil level every time you fill up. The main risk is running low on oil, which can destroy the engine. Blue smoke also damages the catalytic converter and the diesel particulate filter.
On a diesel, if the engine runs away on its own (the revs climb without you pressing the accelerator), pull over immediately and switch off the ignition: the engine is burning its own oil uncontrollably.Repair or replace?
A faulty turbo should be replaced, but only after the root cause has been addressed: an oil change, new filters and a check of the oil feed and return lines. Valve stem seals are replaced by working on the cylinder head. Worn piston rings mean a tired engine: an internal rebuild will often cost as much as an exchange unit.
A reconditioned engine on an exchange basis is supplied as a bare engine: you keep your ancillaries (turbo, injection system, alternator), which makes it considerably cheaper than a complete engine. It is tested and covered by a warranty of up to 24 months.
