Turbo

The VNT mechanism — the turbo fault most people miss

Most modern diesel turbochargers are variable-geometry — VNT or VGT. Inside the turbine housing sits a ring of adjustable vanes that change the angle of the exhaust gas onto the turbine wheel.

The aim is the best of both worlds: boost already at low revs, without the turbo overboosting at high revs. It is that mechanism which lets modern diesels pull from 1,500 rpm.

Why it seizes

The vanes sit in exhaust gas at several hundred degrees. Soot and oil mist from the EGR system gradually build up in the mechanism, and on vehicles used mainly for short trips the deposits harden until the vanes move stiffly — and finally not at all.

It is the most common turbo fault on modern diesels. And it has nothing to do with the bearings.

The symptoms

• The engine goes into limp mode under load
• Lack of pulling power, especially uphill or under load
• Fault code on boost pressure regulation or the VNT actuator
• The fault comes and goes — worse in cold weather

The tricky part is that the actuator often tests fine when disconnected and tried on its own. It is not the actuator that has failed. It is the mechanism it has to pull that is stuck.

What a proper repair involves

Reconditioning a VNT turbocharger means more than new bearings. The mechanism has to be stripped, scale and deposits removed, and the vanes checked for free movement through their full travel — not just at each end.

And afterwards the actuator has to be calibrated. Electronic actuators have a learned end stop that must be set with diagnostic equipment after fitting. Set it wrong and the turbocharger delivers either too little or too much boost from day one.