A while ago I was trying to trace a vacuum system pipe, to see what would happen if the EGR valve vacuum feed was disconnected and not properly sealed. I found there were no diagrams of the vacuum system as a whole, just odd bits of it here and there. The only thing left was to go out with a clipboard and trace the various little black pipes to see if it all made sense. The diagram I came up with is attached. It is of the system as fitted to my ASV (a TDI 110) engine. Should be pretty close for TDI 90's and 110's, less so for PD's and much less use for petrol engines. On the same page there is a block diagram of the gas flow through a TDI engine, and an attempt to diagram all the signals to and from the ECU; no warranty expressed or implied Please let me know of any errors or omissions and I'll correct the diagram.