89 f150 5.0 I had 3/4 tank of fuel in front and rear tanks I use front tank then rear I noticed on front tank driving it would stall I would switch to rear and it would start running again I would switch right back to front tank and it would run ok then maybe next day it would do it again. Then I noticed front tank fuel gauge did not seem to be going down I checked rear tank and was almost empty. even though gauge read correct it was drawing from rear tank and set on front. I no the low presure pumps in tank is what switches the mech dual function switch and if one goes bad it wont switch and the high pressure pump will draw from other tank. But I can turn my key with selector on either tank and can hear pump kick on. Could it be in the selector....

 

Normal senario for a failing/failed selector valve. I believe Ford still uses +12vdc to move the valve from the front tank to the rear and then spring return to the front. The return spring is probably getting tired and the valve is hanging up.

 

This is how it works I could hear both pumps coming on so I replace selector valve $120 from WA. state god price $250 local. It did not solve problem. I check fuel pressure in front pump it was running but had very low pressure. The pressure from the in tank pumps is what switchs the valve in the selector so if there is not enough pressure coming from the pump the valve does not switch and the high volume pump will draw fuel from the othert tank without u knowing it $60 pump and fixed.....