The turbocharger is not boosting properly -- damaged vanes, worn bearings, oil seal leak, or wastegate stuck. Result: significant power loss, black or blue smoke, and poor engine performance under load. Common on high-hour machines.

What you'll see

Significant power loss under load. The engine may idle OK but cannot produce full power when the compressor demands it. Black smoke if the turbo is not generating enough boost (insufficient air for combustion). Blue or grey smoke if the turbo oil seals are leaking (oil being burned in the combustion chamber). You may notice a change in the turbo sound -- normally a smooth high-pitched whine, a failing turbo may screech, rattle, or be excessively loud. Some engines will derate or shut down via the ECU if boost pressure is too low.
Before you assume this is the problem

Not all diesel compressor engines have turbochargers -- smaller units often use naturally aspirated engines. Check whether your engine is turbocharged before investigating this cause. If the engine is naturally aspirated, skip this cause. If the engine has black smoke, check the air filter first. See: Black Smoke from Clogged Air Filter.

See all causes of excessive smoke / poor engine power →

How to diagnose

  1. Check for play in the turbo shaft

    With the engine off, remove the intake pipe from the turbo compressor inlet. Reach in and feel the compressor wheel. Try to move it side-to-side (radial play) and in-and-out (axial play). A small amount of radial play is normal (0.05-0.10mm), but if you can feel obvious wobble or if the wheel touches the housing, the bearings are worn.
    Result: Minimal play, wheel spins freely = bearings OK. Obvious wobble or contact = worn bearings.

  2. Check for oil in the intake or exhaust side

    With intake and exhaust pipes removed, look inside both sides of the turbo. Oil residue on the compressor (intake) side indicates the compressor seal is leaking. Oil on the turbine (exhaust) side indicates the turbine seal is leaking. Both cause oil consumption and smoke.
    Result: Dry = seals OK. Oil present = seal failure.

  3. Listen to the turbo under load

    With the engine running and under load, listen to the turbo sound. A healthy turbo produces a smooth, high-pitched whine that rises with engine speed. Screeching, grinding, or rattling sounds indicate bearing failure or wheel contact. A loud whooshing or hissing may indicate a boost leak in the intake piping after the turbo.
    Result: Smooth whine = OK. Unusual sounds = bearing or seal failure.

How to fix it

  1. Replace or rebuild the turbocharger

    A turbo with worn bearings or leaking seals needs rebuild or replacement. Exchange turbos (rebuilt units) are available from turbo specialists and are more cost-effective than new. When installing, always pre-fill the oil supply to the turbo before starting the engine for the first time -- dry startup destroys turbo bearings in seconds.

  2. Check the oil supply and drain

    Turbo bearings are lubricated by engine oil. A restricted oil supply line or blocked oil drain line will cause premature turbo failure. When replacing a turbo, always clean or replace the oil supply line and check that the drain line is clear. Also change the engine oil and filter.

Common mistakes

Never shut down a diesel engine immediately after running at full load. The turbo is still spinning at high speed and the sudden loss of oil pressure (engine stopped = oil pump stopped) causes the bearings to run dry. Always let the engine idle for 1-2 minutes before shutdown to allow the turbo to slow down and cool. This is especially important on portable compressors that operators tend to just switch off when done.

Parts & tools

Rebuilt or new turbocharger. New oil supply line. New oil and filter. This is a specialist job -- turbo work requires clean conditions and proper torquing.

Review safety precautions before starting →

Safety

The turbocharger spins at 50,000-150,000 RPM and gets extremely hot. Never touch it while running or recently stopped. Allow at least 30 minutes of cooling before inspection. Turbo oil seals can leak and oil can contact the hot exhaust manifold -- fire risk.

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