An engine burns oil when oil gets past the piston rings or valve seals into the combustion chambers, or is drawn in through the crankcase ventilation or a turbocharger. The classic sign is blue-gray exhaust smoke, but small amounts can be burned with no visible smoke at all, showing up only as a falling oil level between changes. When the smoke appears is the clearest clue to where the oil is getting in.
Blue smoke: when it happens and why
| When | Likely cause |
|---|---|
| On startup, then clears | Worn valve stem seals letting oil seep down overnight |
| On deceleration or after idling | Valve seals or guides; high vacuum draws oil past them |
| Under acceleration and load | Worn or stuck piston rings, cylinder wear |
| Turbo engines, under boost | Turbo oil seals; see turbo failure symptoms |
| Constantly, with oil in the intake | A faulty PCV valve or system; see PCV valve |
Burning oil but no smoke
A small amount of oil burned steadily may never show as smoke, while the catalytic converter cleans up much of what is left. Track consumption: top up to the full mark, record the mileage and measure how much you add. Also rule out leaks; see oil leaks.
How much oil consumption is normal?
Every engine uses some oil, and makers publish their own allowances, which vary widely. A noticeable increase, fouled spark plugs or a need to top up between every change is worth investigating; see reading spark plugs.
Engines known for oil consumption
Some engines with cylinder deactivation and some turbocharged engines are widely reported to use oil; see 5.3 Vortec problems and EJ257 problems.
How to diagnose it
- Measure consumption over a set distance.
- Check the PCV valve and hoses.
- Inspect spark plugs for oily deposits on specific cylinders.
- Run compression and leak-down tests; a wet test that raises compression points to rings; see compression test.
- On turbo engines, check for oil in the intercooler piping.
Is burning oil harmful?
Beyond the cost of topping up, burning oil fouls spark plugs, can damage the catalytic converter and, if the level is not watched, can let the engine run low on oil. Check the level regularly while you diagnose it.
Fixes
- A new PCV valve is inexpensive and sometimes cures heavy consumption.
- Valve stem seals can often be replaced with the heads on the engine.
- Additives and thicker oil may slow consumption but do not repair worn parts.
- Worn rings and cylinders mean a rebuild or a replacement engine; see piston rings, engine replacement cost and our engines.
More guides: synthetic vs conventional oil · valve cover gasket leak.
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