What is a Variable Nozzle Turbine (VNT)?
The introduction of the VNT (Variable Nozzle Turbine) in 1989 has made it easier for car manufacturers to take advantage of the torque and fuel economy of direct injection diesel engines. VNT allows turbochargers to boost engines at lower speeds whilst matching the performance of larger turbos at higher speeds. They also reduce NOx emissions from diesel-powered vehicles.
The Variable Vane Assembly in a turbo controls the turbo shaft speed and turbo boost pressure, which is required for the RPM and vehicle’s throttle position. The vanes direct the exhaust airflow through the turbo exhaust wheel to increase turbocharger shaft speed and then open to allow air to pass around the exhaust wheel to vary the shaft speed, as necessary.
What can go wrong?
Due to the efficiency and accurate adjustment required, the clearances in the Variable Vane Nozzle Ring are very small.
Compared to a warm engine, when an engine is started cold the vehicle's ECU injects more fuel, resulting in increased emissions and carbon. Carbon deposits cling to the turbo exhaust housing, including the nozzle ring. If a driver typically only uses their car for short trips or cold starts, the turbo will rarely get hot enough to burn off the carbon deposits. As these deposits build, the variable ‘vanes’ (the flaps inside a turbocharger that are operated by the actuator and regulate exhaust gases in the turbine housing) begin to jam and become locked in position. As a result, an insufficient amount of air will enter the engine causing issues with over fuelling and even more carbon buildup. Long motorway runs where the engine gets hot, a good run every week and keeping the engine revs high can help to prevent the issue, but in many cases the engine management light may turn on and the car could drop into ‘limp mode’. In other words, the turbo has ‘sticky vanes’. Sticking VNT is a common problem with VNT type turbochargers. When the Nozzle Assembly sticks, the driver may experience a lack of power (boost faults), vehicle smoking and potentially complete turbo failure.
How we can help
As an alternative to replacement, Recoturbo offers a cleaning service to overcome this common issue. Simply send us your turbocharger and we will strip and assess the unit into its individual component form, including the Variable Vane Nozzle Ring. Following a clean and polish, the unit will be reassembled and calibrated before being returned. In some instances, excessive carbon buildup or a sticking VNT can cause the failure of the turbo actuator. When this occurs, we can also replace or possibly repair your turbo actuator.
Our VNT clean and repair service starts at only £100. If you require help with your turbocharger, our technicians are here to help. Call us on 01302 896 460 for help and advice.