If the air has escaped: the damaged compressor
If the impeller is damaged, the air that has been drawn in cannot be sufficiently compressed and the necessary boost pressure is not achieved. The most common cause of this are foreign objects, which damage the delicate impeller blades.
If you hear whistling, it’s a case of a leaky charge air path
If the charge air path is not leak tight, the built-up pressure escapes into the environment. You should therefore examine seals and charge air coolers very closely. Caution: old sealing rings in particular are prone to leakage when they are reinstalled!
A clogged air filter will also prevent a turbocharger from performing to its maximum potential: if the turbocharger cannot breathe freely, it cannot build up the full boost pressure!
If it has stopped running: the damaged turbine
The turbine wheel is driven by the outgoing exhaust gases and transfers this energy to the impeller via a shaft. If a problem occurs here, the turbocharger no longer achieves the necessary speed and the boost pressure falls. The most common type of damage to the turbine wheel is caused by foreign objects. These are usually small parts that break off from welded manifolds, for example. However, tiny particles from the engine or charge air cooler can seriously damage the blades of the turbine wheel.
If there’s a blockage: clogging on the exhaust side
The exhaust side can also cause performance problems: the exhaust gases need to pass through the turbocharger without significant back pressure. A blockage here may have a number of causes. In diesel engines, the source of the problem is most commonly a clogged diesel particulate filter (DPF).
If the control system fails: defective controls
Boost pressure control is handled by several components. In particular, you should check electric valves, actuators, and vacuum hoses for damage and correct operation. If the wastegate and the variable turbine geometry (VTG) are no longer correctly controlled, this will be especially noticeable in the lower rpm range. Performance problems can also be caused by VTG guide vanes blocked by soot and deposits, or by jammed linkages. You will usually find such errors in the engine control unit’s fault memory!
If the settings are wrong: modified calibration
Every turbocharger is calibrated when it is assembled in the factory. Alterations to the variable turbine geometry (VTG), for example, quickly lead to deviating boost pressures. If these values deviate from the target values, the engine control unit enters fail-safe mode in order to protect the components and dramatically limits engine output. The same applies to the wastegate: if the flap opens the bypass too early, power is reduced in the lower rpm range. If it opens too late, this can result in the turbocharger’s maximum rpm being exceeded.