sounds like the 6 way valve is not working. my truck was sucking off passenger side and returning on driver side switched out 6 port valve and fixed it. ground wire on sending unit maybe. when you drop tank pull sending unit and plug it in under truck and move it by hand and check fuel gauge is...
has anybody mentioned a faulty sending unit? i know its new but i bought a new delco oil sending unit worked for 2 weeks than nothing. replaced with another delco and been fine.