I'm an auto mechanic who want to give a hint to my fellow friends about a problem I had with a Jetta 4 TDI . The problem might be the same for Golf TDI's or Audi's. The problem with the Jetta was that the car would loose power sometimes for no reason, if you switch off the engine and re- start the engine the problem is gone. Well after a while the problem was persistent . What I did was to replace the rubber pipe from the turbo to the waste gate bypass regulator that sort out the problem. But as you know after a few months the problem came back ,this time it was not the rubber pipe , it was something else , I needed to sit down and re think , before you can just look at a engine and see the problem all most immediately but with today’s modern cars you cant do that. I had to plug in the Diagnose equipment and see what faults are stored  in the memory of the car's computer . The report I got was code P1557 means that there is a deviation of pressure in the turbo boost – not within specs of the designed of the engine management. I have to look at the turbo waste gate , what I found was that the waste gate was stacked . This caused the turbo to over boost and the engine ECU will detect it and force the engine in limb mode to prevent damaged.  See this photos of a clean waste gate end a dirty waste gate.
I am dirty
I am clean
So either I clean the wast gate or replace the whole turbo charger , this car got 158000 km on the clock it is better to replace the turbo.
More info on the Waste gate

About the N75 Waste gate Bypass Regulator Valve. The N75 "race valve" caused me also to look into this matter. I haven't time to search the whole past, so some readers might find this just another review. Nonetheless, here's my take for anyone else who might be interested. I'm motivated to do this by people who think they can go the Audi engineers one better by screwing with the unit's calibration screw. The following will explain why this is a dangerous undertaking.

Figure 1 shows how the Waste Gate controls boost, and Figure 2 shows how the ECU controls the Waste Gate by means of the Waste gate Bypass Regulator (N75)
In your imagination, connect the outlet of high pressure from the Impeller (blue) in Figure 1 to the bottom inlet (green) in Figure 2, and the right side outlet (gray) in Figure 2 to the Waste Gate actuator (Pressure Unit) in Figure 1..
Notice the Restrictor in Figure 2. It allows only a small amount of air to enter from the green port below. As a, result the pressure above the Restrictor (that goes to the Waste Gate actuator) can be regulated by bleeding off controlled amounts of air through the left side purple port of N75. The bleed-off rate is controlled by how far the electric coil pulls up its central plunger, allowing air to scape under the blue cap at its bottom. That pull-up distance is precisely controlled by a pulse width modulated signal from the ECU. To achieve the intended results, N75 is factory calibrated by a screw above the plunger that sets the proper spring pressure of a spring between the plunger and the blue cap. It's easy to see that one might get a little more boost by turning the calibration screw up a bit, thus increasing the bleed-off rate and reducing the pressure to the Waste Gate actuator. But wait,if the ECU finds the boost higher than expects, it will modify its signal to N75 to correct the
situation within limits. If those limits are exceeded, surging will set in, particularly under part
load. The moral of the story: Don't screw with the screw. Trying to fix any drivability problems this way, only hides some other fault.
Problem No 2 : If you draw a diagnose report on a Polo tdi 1900, or 1400 cc engine and the codes you get from the report is as follows :”18074 P1666    Injector 1 - current circuit Open circuit” and “16685 P0301   Cyl.1 Misfiring recognised  [SP]   Open circuit”. Before buying Injectors , which are very expensive in SA , test/check the wiring from the injectors to the all the connectors , most of the time it is a wiring problem.
Here's how this appears to work:
A "No-Flow State" exists during idle, when there is no boost and the Waste Gate is held shut by the spring in its actuator (Pressure Unit). When you step on the accelerator, boost builds up, and so does the pressure in the green bottom.
port of Figure 2. This also pressurizes the gray outlet of N75 leading to the actuator (Pressure Unit) in Figure 1. Once the air pressure in that actuator overcomes the spring pressure, the Waste Gate opens and allows some of the exhaust gases to bypass the turbine wheel. However, thisaction alone would open the Waste gate at lower boost pressures than are desirable. Here's where N75 comes in
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