I have just fixed a problem on my 2.5 petrol t4. if was idling very badly. Hunting up and down the revs and general being a pain at low revs. Had it checked by a vw specialist and diagnostics were all ok but it was still bad. Guess what! looking at yesterday i noticed a vacume pipe connected to a pin on the rhs of the top of the engine. The pipe was very fraid and on closer inspection was leaking. I put a new pipe on today and it is now running fine. sweet as a nut. The moral of the story is this. check all the pipes b4 you spend a lot of money on experts.