well, you ansered one of my questions; the line is for excess fuel that was not injected into the engine. now that being the case, it is needed, in saying that, shut down time,[on diesel fuel, or bio in my case] will take longer. i guess the best way to have a two fuel system is to have a small filter in place with return line port on it, have a two port valve ahead of the filter. then you can select what type fuel to be used. down side is that, purging time will increse. guess there's no way around it. any other ideas welcome .