What the vanes do
A variable-geometry turbo has a ring of movable vanes around the turbine wheel. Garrett, which calls its version a variable-nozzle turbine (VNT), describes what they do. Closed down at low engine speed, they squeeze the exhaust through the turbine for more turbine power and more boost. Opened up at higher engine speed, they let the exhaust flow freely, so the turbo does not overspeed and boost is held where the engine needs it. BorgWarner calls the same idea variable turbine geometry (VTG).
Each vane has its own small arm, and a single unison ring moves all the arms together. The ring is turned by an internal control arm, linked through a short shaft and an external crank to the actuator. Garrett notes that on a modern engine the vane position is "constantly and rapidly changing" in response to the engine control unit, for emissions as well as for the driver.
It is mainly a diesel technology, but not only. BorgWarner says a turbo it supplied in 2006 for the Porsche 911 Turbo 3.6-litre engine was "the first series application of the gasoline VTG", and Garrett says its first high-volume petrol variable-nozzle turbo was launched with Volkswagen in 2017.
Why the vanes stick
- Carbon. MAHLE describes carbon build-up jamming the vanes in their position for low engine speeds. Soot is a known enough problem that Detroit Diesel, a maker of heavy-duty engines, patented a routine in which the engine cycles the vanes to clean off accumulated soot.
- Oil. MAHLE lists coked guide vanes as one effect of a turbo leaking oil, so a blocked breather or restricted drain can end up as a vane fault. Our guide to a turbo leaking oil covers those causes.
- Debris. Pieces of a broken valve or piston ring can bend the vanes, which MAHLE says causes a significant loss of power.
Symptoms
- Boost fault codes. The vanes are there to control boost, so when they cannot follow their commands, actual boost drifts from the boost the engine asked for. Underhood Service explains that this mismatch is what sets P0299 (underboost) or P0234 (overboost), and that codes for the turbo actuators are often stored alongside.
- Lack of power and limp mode, the usual symptoms of an underboost or overboost code.
- Overspeed. MAHLE warns that vanes jammed in the low-speed position overspeed the turbo as the revs rise.
- Weak low-speed boost. It follows from Garrett's description that vanes stuck open leave the engine short of boost at low engine speed.
Testing
- Read the codes and watch the live data, including boost pressure, as Underhood Service describes for any boost fault, and deal with actuator codes first.
- Watch the mechanism at start-up. Garrett says that at key-on and start-up it is normal for its variable-nozzle turbos to show movement in the actuator, vane arm and vane mechanism.
- Do not force an electronic actuator. Garrett says the gears in its electronic actuators are "self-blocking", and moving the operating arm or rod by hand or with a tool may break them and make the turbo unusable.
- If the actuator itself has failed, check that the vanes move freely before another is fitted, for the reason below.
Fixing sticking vanes
A new actuator on its own does not free seized vanes, because it still has to drive the same jammed mechanism. Calibration matters as well. GCG Turbochargers Australia, a Garrett distributor, explains that Garrett calibrates each electronic actuator to its own turbo on a flow bench as the final assembly step, that these actuators "should never be interchanged from one turbo to another, even when from the same turbo part number", and that "just loosening the three actuator retaining bolts will take the calibration outside of specification".
A lasting repair therefore means dismantling the turbo, cleaning or renewing the vane mechanism, rebuilding the core where it is worn, and setting the actuator up to suit that turbo. Our turbo rebuilds page covers the rebuild, and turbo diagnostics covers tracing the fault.
Prevention
MAHLE's general remedies apply: use the oil the vehicle maker specifies, keep to the service intervals, check the crankcase ventilation and deal with oil leaks before oil reaches the turbine.