I have a 1990 ford f150 6cyl in which i have changed the fuel filter, both fuel pumps(have duel tanks), and i am still having a fuel pumping problem. The pumps pump for a long time before shutting off. The truck will not start until they shut off. Have checked the fuel lines fuel is pumping throught the fuel filter and also into the injectors and fuel is also returning back to its proper tank. Any ideas on what to replace now?