Okay question:

Do our Titans have a fuel return line? If so, where?

Or, do we have schrader valves on the fuel rail(s)?

I've looked through the manual, but can't seem to locate.

Fuel return line = Nope
From Fuel pressure regulator to the Fuel damper then straight to the Fuel injectors.
The Fuel Damper keeps equal pressure for the Injectors.
There is an EVAP service port on the line; just to the right of the engine cover.

There is a diagram on the Engine Control System page 23 of the Complete nissan service manual.

Whata exactly is the functionn of the evap line... what happens if you open or close that(valve)?

Whata exactly is the functionn of the evap line... what happens if you open or close that(valve)?

The EVAP line is like well I guess you can say its like a return line back to the fuel tank. The purpose of it is to return unused fuel/vapor from the engine through the evap canister(filter) and back into the gas tank. More or less it's just a fuel vapor filter. Disconnecting or closing it would be very harmful. Probably illegal

It's really not an open/close valve. Just a Pressure test valve