The turbo is a vacuum modulated device that is used to control the boost pressure by modulating the vacuum on actuator. More the vacuum is on the actuator, higher is the boost which is controlled by the ECU. There are various kinds of actuators in the market. These help to tune your car and if you have any issues with the actuators you can repair and replace them. The VNT actuator is found in the VW of mk4-mk6 and Audi TDI to move the vanes in the VNT turbo. The sticky VNT turbo actuator uses the limp mode by not moving the vanes as expected by computer of the car.
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| VNT Actuator |
What are the specifications of the actuator?
The vnt actuator appears similar in look and location with wastegates. This is the reason why people get confused between the two. The term wastegate is also used many times instead of the actuator.
- The actuator actually uses a lever or rod to move vanes in the turbo. The difference between the two is that the actuator is found on mk4 Garrett turbo which uses a vacuum to pull the rod inside, while the wastegate is found on older mk4 TDI and it issues pressure to work.
- The new TDI uses vacuum instead of vacuum to do the work.
- Adjusting the VNT rod does not help in changing the movement. If there is some kind of problem with the movement, it is important to check the vacuum line and other maintenance items and replace them. Adjusting the rod if all the other items are working fine is just useless on fuel response and can cause risky hike in boost. When you are only modifying the turbo or setting up the turbo, you can adjust the VNT rod.
Causes of Limp mode
- Mis-adjusted actuator: The actuator is the item on the turbo where the vacuum hose is present. There is a rod coming out of the can or ball shaped device which has a computer coding written on every rod length. The turbo responds the way the coding is given. If the rod lengths are set at wrongly, the turbo behaves differently than expected from ECU and as a result, the boost control might fail. The manufacturers make the mistake of setting the actuators rod lengths.
- Boost leaks : The part of intake track that lies between the intake manifold and compressor outlet can have holes which are termed as boost leaks. If there is a hole in the intake track, the boost pressure will leak. This in-turn will cause the MAP sensors to send signals to ECU about boost will not be performing the turbo’s behavior. One can suspect the boost leaks on hearing the hissing sounds or black oily film near the intake track.
- Vacuum leaks : The ECU adjusts the volume of the vacuum that reaches the turbo using the vacuum pump. There could be leaks or cracks in the lines that connects the brake booster with the vacuum pump. There are many other vacuum lines along with Ts that split the unregulated vacuum off. If any of the lines or Ts is leaked then there could be a problem.
- Jammed VNT vanes : The shoot develops on the vane in the exhaust housing if the car is not properly driven. This can cause VNT vanes to jam letting either the car to overboost or underboost. It is important to cycle the actuator or clean the turbo to solve the problem.
- Faulty actuator : Any problem with the actuator can cause some serious issues. Whether leaks in the actuator, or insufficient spring strength, you will be needing replacement.
Apart from this, the problems can also be caused by wrong driving or with time as the turbo might be damaged physically. It needs replacement for normal performance. If you still want to know more than go through this link and get some more idea.
