lewisweller1982

BINGO SOLVED IT>
 
 I'm back and after a lot of head scratching and no fault codes PIWIS or Durametrics and no help from the "German car specialist garage" who said it was normal to have a Cayenne turbo drive like a total pig I found the answer!!!
 
The car would hesitate and felt sluggish at low rpm, and when you accelerated and let off the braking was terrible (when I slipped to neutral the braking was good so not a brake vacuum servo leak), the engine is fighting the brakes which in fact got me thinking on the correct thought pattern to find the problem.  
The only reason the engine would fight the car is if the boost built up wasn't able to divert around the turbo, the Diverter valve obviously does this function and after some reading up the Bosch ones can be suspect. so I took the plunge and bought two new from Porsche Bosch 710 P revision.  
Whilst fixing the steering rack leak I whipped off the passenger wheel arch cover and swapped the DV, but when I checked it was already 710 P revision and actually did function when test manually, so I then started to think about the vacuum piping to the DV. I blow down the vacuum pipe and heard a leak in fact 3!! 
the vacuum pipe to the driver side was melted and leaking at the nylon "T" located under the Throttle Body inlet area, chopped that bit off and reconnected, tried again and still could hear leaking, traced it the rigid vacuum pipe which runs along the left bank injectors area to the Change over valve.
As you can see from the pictures its melted in two places.
What throw me off is I didn't understand the function of the DV and the Change over valve exactly, yes the DV will open when throttle closes to prevent turbo stall and surging which damaged the turbo and gives turbo "lag", but also the change over valve is boost pressure when accelerating and vacuum when off throttle. Most technical data I read didn't disclose this detail.
 
So my understanding now is:
On throttle the changeover valve is positive pressure open to the DV to assist in holding the boost preventing the DV brass spring from compressing then lift off the change over valve will switch to crankcase vacuum to pull the DV open to by pass.
This explained why my CTT would have crap low rpm acceleration and boost with hesitation and kangaroo de-acceleration and engine fighting the brakes to stop as the boost had nowhere to escape to.
 
Maybe someone can explain this sequence more accurately or correct me on this info, I would welcome it.