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First, you need to know that the waste gate actuator (WGA) is activated by pressure, available only when on boost, to reduce the boost that the turbo creates. So the WGA cannot be active at idle and an engine at idle cannot ever develop any boost as the exhaust gas flows are very low can cannot get the turbo spinning enough to do anything.
With a cold engine, grab the WG lever and move it to determine that it is free and not stuck open.
Understand that a break or shutoff in the APC/BCP hoses will remove regulation from the turbo and unregulated high (dangerous) boost levels will result.
Remove the hose to the WGA with an warmed up engine. Drive with a +very gentle throttle+ in a quiet traffic area and see what boost you can make. If you can get boost to the start of the red on the indicator, get of the throttle, shutdown and reconned the hose. DO NOT try to see how high the boost can go into the red or you may have severe damage.
If you got boost above, then you system is boost capable and the problem lies with the boost controller. If you could not make boost, then there is a large leak in the system, which may be external (hose torn of popped off) or the bypass valve (BPV) is creating an internal leak.
If you got boost, then the boost pressure control valve (BPC) may be at fault. Check that the hose to the BPC from the turbo compressor housing hose barb will flow. The WGA is not at fault, because if its interal diaphram was torn, you would get high boost, not low. The BPC valves can be plugged up so that its 'bleed' orifice does not flow. Some can be restored to normal operation by blowing out every whichway with shop air and perhaps some WD40 to assist cleaning. Some BPC valves will need to be replaced.
If you could not make boost, the turbo could be trashed, but more often than not, it is something else. Examine the turbo hoses and joints. Check for damage to smaller hoses near the throttle body. Check that the BPV diaphram will hold pressure (apply suction by mouth), check the BPV VAC line from end to end. Check hose clamps. Feel for damage on area that you cannot see. Also check the intercooler (IC) for damage.
For good on boost operation, you need good fuel, good plugs (the correct ones, in good shape and properly gapped), and clean fuel injectors.
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