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How variable nozzle technology operates


The VNT (Variable Nozzle Turbine) turbocharger controls the flow of exhaust gas reaching the turbine inducer to suit the different power requirements of an automotive engine. This enables higher turbine speeds with low power demands without causing over speed during high power demands.

The variable vanes are controlled by the ECU via vacuum, pressure or electronic actuators and continually adjusts the effective nozzle area to maintain the required boost levels with the following advantages:

Rapid response to changing power demands over a wide operating range.

Reduced exhaust back pressures.

Improved emission control.

The operation of a VNT system is affected by many factors and components of the ECU system and often it is the failure of these that effects turbocharger operation which can show as:

ECU error codes.

Low Boost or Under Boost.

High Boost or Over Boost.

VNT Problem-Solving.

Always ask for as much fault detail as possible as this will help identify possible causes/areas of malfunction.

  • What was the engine speed and load?
  • Did the problem occur during acceleration or deceleration?
  • What was the engine temperature and how long had it been running?
  • What ECU error codes were reported?
  • What other vehicle problems were noticed?

Checking VNT operation.

Checking VNT operation consists of mechanical inspection and operational checks of the actuator signal/pressure versus the boost pressure.

  • Check all wiring, wiring harnesses and connectors of electronic actuators.
  • Check the actuator connecting arm for slight movement at each end which will indicate the connections to be free of corrosion and damage.
  • Remove the connecting arm from the VNT lever assembly and check the lever for free movement.
  • Does the boost pressure respond to actuator pressures/signals for increased and decreased power demands?
  • What is the maximum vacuum pressure during a power demand and does it reduce as the power increases?
  • What actuator pressures/signals are measured during periods of low, medium and high engine power demands?


The above checks will determine correct turbocharger operation in relation to the actuator signals/pressure and can also help in identifying ECU system failures.