Interesting snippet on this thread
viewtopic.php?f=4&t=88541can anyone advise if it is correct?
Had occasion to go to ford dealers as they had an offer on fuel and oil filters, Got the lad to show me a new fuel filter top, with the +- knob on it and like mine it turns from the middle to the plus but not to the minus. As I was putting a new filter on mine I took the housing off and tired running clean diesel through it in my parts washer. You only have to look at it to see that it controls the return from the pump in some way. Anyway as I had suspected it basically sets the return to the tank of excess fuel from the injection pump, and normal is in the centre position and when you turn it to + it allows a greater flow back to the tank of the excess fuel.
So as far as I can see the only use for it is to allow the system, particularly the pump to clear air out after being worked on. So if you had say changed the lift pump and you turned it to maximum the fuel through the pumps would quickly clear any air out in the first couple of minuted running, then return it to the middle.
If doing the filter, if you fill it before fitting you would not need this anyway as it would clear itself after a couple of miutes anyway and not really help you.