Problems with Variable Nozzle Turbine (VNT) Turbos

Since the late 1990s many diesel vehicles have a Variable Geometry Turbine (VGT) or Variable Nozzle Turbine (VNT) turbo fitted. These turbos minimise turbo lag, improve throttle response at low speed and provide much improved torque.

Most commonly, the design utilises a ring of moveable vanes around the turbine wheel to change the speed and direction of the exhaust gases acting on the turbine wheel. At low speeds the vanes move closer together which accelerates the gas flow onto the turbine wheel. At higher speeds the vanes open wider to prevent the turbo over boosting.


Despite these benefits, turbos can be prone to problems, which include:

  • VNT vane problems – often occur because of ‘short-run’ driving history so the vanes get minimal variation in pitch, resulting in carbon build up between them causing them to jam
  • Actuator problems – caused by the vanes clogging as above
  • Lubrication problems – caused by carbon build up in the oil, fouling the relatively small diameter turbo oil passages


Avoiding these problems requires a dedicated strategy. We recommend that you:

  • Buy a diesel vehicle only if it fits your usage requirements (refer to problems).
  • Seek advice from an experienced mechanic or diesel specialist about what specific problems your vehicle may encounter (especially if it is fitted with a particulate filter). This can save you lots of money on repairs over the life of the vehicle.
  • Use the correct oil specification for your engine. This generally means a fully synthetic oil has to be used (consider an upgrade if it’s semi-synthetic). Synthetic oil can last a longer time and cope better with the high temperatures generated by the turbocharger without carbonising in the turbo itself. Carbon buildup comes from the engine, not from the turbo. Oil is laden with soot and carbon held in suspension by additives that become less effective over time – quality is generally reflected by price.
  • Service your vehicle within (or earlier than) schedule and according to the manufacturer specifications.
  • Use quality OEM parts or equivalent (there is a difference).
  • Check for engine blowby (excessive pressure from the breather) and leaking injector base washers (common in some makes) which either restrict oil drain from the turbo or build up carbon in the oil, resulting in premature turbo failure.
  • Ensure you use good quality fuel and your diesel injection system is properly maintained to schedule using OEM or equivalent fuel filters. Clean fuel ensures the fuel system has a long service life and precise fuel atomisation is maintained to reduce carbon contamination in the oil.
  • Ensure all hoses and clamps in the intake system are regularly checked (every service) for splits and tightness. Ensure the intercooler and intake manifold are also checked for leaks, as loss of boost pressure can cause the turbo to overspeed trying to maintain boost pressure, resulting in turbo and possible engine failure. NB: The MAF sensor not operating to specification can also cause these failures.
  • It’s a good idea to fit a good quality catch can to collect oil vapour and carbon from the breather before passing back into the intake system to the turbocharger. This prevents carbon buildup in the manifold, protects the turbocharger and improves engine efficiency.
  • Ensure the EGR system is clean and operating correctly. EGR problems are probably the most common fault we find with diesel vehicles.
  • If an engine light or noise occurs from your engine STOP AND TAKE CARE OF IT STRAIGHT AWAY. Too often, small problems become big ones.



(NB: The red arrow in the image indicates the connection point when installed into a vehicle)