Measure boost at the intake manifold under full load and compare against specification. Low boost points to the turbocharger, a boost leak, or a restriction upstream. Excessive boost points to the wastegate or variable geometry control, or to an actuator that has stuck.
How to confirm it
- Confirm the specification and the measurement point
Establish specified boost at rated load from the service documentation, and where the manufacturer measures it. Boost measured at a different point in the intake gives a different number, and comparing it to specification at the wrong point produces false conclusions.
- Rule out air restriction first
Check the air cleaner element and restriction indicator, and inspect the intake path for collapse or blockage. An engine that cannot draw air cannot make boost, and a restricted filter is a far more common cause of low boost than a failed turbocharger.
- Pressurize the intake and check for leaks
With the engine off, pressurize the intake system and listen or use a leak detector for leaks from the turbocharger outlet to the manifold. A split charge air hose or a loose clamp loses boost with no obvious external sign.
- Measure boost under full load at operating temperature
Take the reading with the engine at rated load and rated speed, at operating temperature. Boost measured at idle or without load tells you almost nothing about whether the turbocharger is performing.
- Compare with specification and observe how boost builds
How boost builds is often more informative than the peak value. Boost that builds late or lags badly suggests sticking variable geometry vanes or an actuator problem; boost that never reaches specification suggests a worn turbocharger or a leak.
- Check wastegate or variable geometry actuator operation
On wastegate units, confirm the actuator moves through its travel and the linkage is free. On variable geometry units, confirm the vanes move through their full range and that the actuator responds to command. Sticking vanes are a very common fault and are frequently mistaken for a failed turbocharger.
- Check for exhaust restriction
A blocked or collapsed exhaust restricts the flow that drives the turbine and reduces boost. If everything else checks out, confirm exhaust back pressure is within specification.
What to do about it
Low boost with clean air filtration and no leaks: check actuator travel and vane movement first. If the actuator or vanes are the fault, the turbocharger itself may be serviceable, and this is a much smaller repair.
Low boost with a free actuator and a binding or damaged compressor: the turbocharger needs replacement or a cartridge. Confirm the correct turbocharger variant against the engine number, because several units are frequently used across one engine family.
Excessive boost: the wastegate or variable geometry control is not limiting boost correctly. On a diesel this raises combustion temperature and cylinder pressure and shortens engine life, so it should be corrected rather than accepted as extra performance.
Oil in the intake or exhaust: the turbocharger seals may have failed. This carries a runaway risk if the engine begins to run on its own lubricating oil, and the machine should be stopped rather than driven back to the workshop if it occurs.
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Request a quoteFrequently asked
How do I know if my turbocharger has failed?
Measure boost under load against specification, check shaft play and rotation, and inspect the compressor wheel for contact marks. Many faults attributed to turbochargers turn out to be sticking variable geometry vanes or a boost leak, both of which are cheaper.
Can I drive with low boost?
The machine will run with reduced power and higher exhaust temperature. Continuing to work it hard in that condition accelerates wear on the engine and can damage the turbocharger further. It is better to establish the cause than to compensate with lower expectations.
Why does boost matter for fuel economy and smoke?
Because the fuel the engine injects can only burn if there is air to burn it with. Low boost produces black smoke and wasted fuel along with the power loss, and both disappear when boost is restored.