Turbocharged Diesel Engine Lacks Power Due To Stuck VGT Mechanism

What Happened?

After three months of operation my Hyundai Accent CRDi (Diesel) Sedan taxicab lost power: it could hardly move forward in first gear. I made a visual inspection on the engine and found out that the VGT Actuator Lever was stuck (see Fig. 1). Normally the actuator stays at retracted position when the engine is idling, and extends downward as the engine is revved up. In this case the actuator remained in retracted position even when the engine was revved up.

We removed the actuator and then swayed the VGT lever very gently until we got a smooth full play of the lever. We made sure to do it very gently to avoid damage to the internal components. Then we installed the actuator back on, and run the engine. Brrrmmmm!!! Wow! My car was fired up and sounded like it’s ready to go.

We tested the car around and we’re very happy with the result. It’s been in operation for thirteen months now without any problem of that sort.

There Must Be A Reason

Although my taxicab was now working, I just felt that there was still unfinished business. A couple of questions were left unanswered. Why did the VGT mechanism get stuck, and why would the engine lose power when the VGT mechanism gets stuck? There must be a reason. In search for the answers I’ve done a little investigation and research which I will share with you.

Why Was the VGT Mechanism Stuck?

The drivers assigned to this cab reported that they used to leave the engine running at idle for long periods of time, while they were parked and while they were dining. I suspect edthe vanes inside the turbo charger (see Fig. 2 and Fig. 3) were clogged up with soot which might have accumulated after many hours of idling. Whatever it is, the important thing to note is that:

  • Rocking the VGT lever solved the problem
  • After the drivers were instructed to avoid idling for long hours, my taxicab has been in operation for thirteen months now without this problem of stuck VGT mechanism returning.

I believe it was the long idling of the engine that caused the VGT mechanism to stick.

Why Would the Engine Lose Power When The VGT Mechanism Is Stuck?

Quick Answer:

The reason why the engine of my Hyundai Accent (Diesel) lost power when the VGT mechanism got stuck is that the ECU (Engine Control Unit or Electronic Control Unit) limits the power output of the engine to avoid engine damage. The ECU does this by disabling the ignition advance and introducing fuel cut. Under these conditions, no matter how hard you push the accelerator pedal, the engine just won’t have enough power.


To fully understand the above statements, we need to discuss a little bit about Variable Geometry Turbochargers (VGT). VGTs are used in engines to push just enough air into the combustion chamber to increase the engine's power output. A VGT has two main sections: the pump and the driver (see Fig. 1). The driver drives the pump, and the pump pushes air into the combustion chamber. The driver, usually a turbine assembly, is operated by the exhaust gas that comes out of the engine through the intake manifold.

The amount of air needed to be pushed into the combustion chamber (or boost) varies at different engine rpm. Obviously more air is needed at higher rpm. The amount of boost is controlled by the VGT mechanism, consisting of the movable vanes, the actuator lever, and the actuator (Fig. 2 and Fig 3.). The actuator may be a vacuum type or servo type. In both cases, the actuator is controlled by the Electronic Control Unit (ECU) or Engine Control Module (ECM).

Fig. 2 shows the position of the vanes at low engine rpm. The partially closed vanes create narrow paths for the exhaust gases to pass through. This accelerates the rather weak exhaust gases and directs them almost squarely into the turbine blades, thus, creating enough torque to drive the pump even at low engine rpm.

Fig. 3 shows the position of the vanes at high rpm. The vanes are open resulting in lesser or no acceleration of the exhaust gases. Moreover, the exhaust gases are directed to the turbine blades at an oblique angle. These two combined reduce the effective thrust of the strong exhaust gases on the turbine blades, thereby preventing overboost.

If the vanes are stuck at partially closed position as in Fig. 2 and the engine is revved up to high rpm, the strong exhaust gases will be accelerated even further and will be directed squarely to the turbine blades, overdriving the turbocharger. When this happens two things can possibly happen:

  • Too much air will be pumped into the combustion chamber (overboost) which will cause overheating and eventually damage the engine.
  • The turbocharger will be damaged due to excessively high rotational speed (rotational runaway).

If the vanes are stuck at open position as in Fig. 3, the turbocharger will fail to provide the needed boost (underboost) at lower engine rpm and the engine will stumble.

The undesirable effects of overboost and underboost are prevented by the sophisticated closed-loop control of the ECU (Fig. 4). The ECU senses the engine rpm through the crankshaft sensor and then adjusts the boost accordingly via the VGT actuator. The amount of boost is sensed by the ECU through the boost pressure sensor.

When the ECU senses an underboost condition that lasts for a certain period of time, a trouble code is flagged and the Malfunction Indicator Lamp (MIL) may appear ON on the dashboard.

When the ECU senses an overboost, the ECU limits the power output of the engine by setting the ignition timing to 0 degrees and by limiting the amount of fuel that is feed into the combustion chamber. This aims to prevent damage to the engine. A trouble code will be flagged and the MIL will be turned on.

Now you know why my Hyundai Accent CRDi taxicab lost power when the VGT mechanism was stuck.

"It may not be necessary nor practical to do it ourselves, but it's always nice to know how things are done correctly."

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