Blue or blue-grey smoke indicates engine oil burning in the combustion chamber. Worn piston rings or cylinder liners allow oil to pass into the combustion area. This is a sign of an engine that needs an overhaul. High-hour problem.

What you'll see

Blue or blue-grey smoke from the exhaust, especially noticeable on startup and under load. The smoke has a distinctive oil-burning smell (different from the raw diesel smell of white smoke or the sooty smell of black smoke). Engine oil consumption increases -- you need to top up oil more frequently. The exhaust may leave an oily residue. Engine power is reduced because compression is lost past the worn rings. This is a gradual condition that worsens over thousands of hours.
Before you assume this is the problem

Blue smoke from the turbocharger (leaking oil seal) looks similar but is a different problem and much cheaper to fix. Check the turbo first. See: Turbocharger Problem. Also: a small puff of blue smoke on startup only (after sitting overnight) is often valve stem seals rather than piston rings -- a less severe condition.

See all causes of excessive smoke / poor engine power →

How to diagnose

  1. Monitor oil consumption

    Track how much engine oil you add between oil changes. Normal consumption varies by engine size, but if you are adding oil every 50-100 hours, there is a problem. Compare to the engine manual specification. Excessive oil consumption combined with blue smoke confirms oil is burning in the cylinders.
    Result: Excessive oil consumption = confirms ring/liner wear.

  2. Perform a compression test

    Remove the glow plugs or injectors and use a compression tester on each cylinder. Compare readings between cylinders and against the engine specification. Low compression on one or more cylinders indicates worn rings, valve problems, or head gasket failure. A wet test (add a tablespoon of oil to the cylinder and retest) will confirm ring wear: if compression increases significantly with oil added, the rings are worn.
    Result: Low compression that improves with oil = worn rings. Low compression unchanged with oil = valve or head gasket issue.

How to fix it

  1. Engine overhaul

    Worn piston rings and cylinder liners require an engine top-end or full overhaul. This includes new pistons, rings, and potentially re-sleeving or boring the cylinders. This is a major job that should be performed by a diesel mechanic or engine rebuild specialist. Consider the economics: if the engine and compressor are high-hour, it may be more cost-effective to replace the complete unit or source a rebuilt engine.

  2. Consider the cause of premature wear

    If the engine wore out prematurely: Was the correct oil used and changed on schedule? Was the air filter maintained? Did the engine overheat repeatedly? Were the valve clearances adjusted? Poor maintenance dramatically shortens engine life. Address the maintenance practices on the replacement or rebuilt engine.

Common mistakes

Do not just keep adding oil and ignoring blue smoke. The oil that enters the combustion chamber also contaminates the compressor oil separator and the compressed air. It increases oil carry-over in the compressed air and can clog the separator element. Also: do not use thicker oil to mask the symptoms. Using 20W50 instead of 15W40 may reduce smoke slightly but does not fix the wear and can cause starting problems in cold weather.

Parts & tools

Compression tester. This is a diesel mechanic or engine rebuilder job requiring specialized tools, machine shop work, and calibration equipment.

Review safety precautions before starting →

Safety

Oil-burning exhaust is a fire hazard in certain environments (timber, mining, fuel storage). If the machine must operate in hazardous areas, address this issue before continuing operation.