I had this on my 95 300tdi. I traced it to the fuel tank sender unit!!
Where the olive and gland nut connect to the tank. the tank is a plastic pipe sticking out which the olive and gland nut connect to. The plastic pipe is a push fit into the sender unit then glued in. this can fail and either leak or let in air causing a slow start after stood for a while. Just saying - Worth a check.

blue beasty said:

Back to the filter housing on a 300 I think

Click to expand...

Bit dark and wet outside so not going to look but pretty sure it goes back to the FIP

Watch this 300tdi Injection pump removal refit - using Paddock PM029 timing pin kit - YouTube at about the 50 second point is where the leak back connects to