I have just rebuilt the heads of my 1999 V10 6.8L over the last 3 months (hence I am now in this situation as have forgotten where this silly little vacuum hose came from)
Have put everything back together and it is running, albeit with a very slight miss. But I have one vacuum line that is coming from the Vacuum Reservoir and goes around the front of the R/H head with the other lines in the loom and then goes off on its own for about 700mm and has a rubber female moulded end. It is white in colour under a black protective sheath. I don't know where it goes? At the moment I have plugged it with a bolt. I can't seem to find a schematics overlay of the motor anywhere. Would anyone know where it goes? All the other hoses are on correctly as far as I know, as they all fell into place. Any assistance appreciated or even a diagram would be great. Thank you in advance.

 

Can you post a picture? It might help narrow things down.

 

My 99 has 2 white vacuum lines going from the reservoir.
The one that pokes out of the loom by itself goes to the evap valve test port that is bolted onto the AC box (99 F250 truck).

And thanks for this, When I just now went out and looked at my 99 I found the black one has a bad rubber elbow and is leaking.

 

Thanks for all the help, have finally worked it out. It was the vacuum hose for the VMV / EVAP Service Port. So all is running well, and about to take it for a longer spin on the weekend, after I put the wheels back on.