Tried to search for these answers and gave up, so forgive any repeat questions. I have a 2016 GMC Sierra 3500. installed a DeeZee 40 gallon auxiliary fuel tank and DeeZee installation kit. I had the “roll over” valve (as they call it) horizontal at first and when I took stock tank gas cap off fuel came out. Shut off tank block valve, ran fuel below fill neck and turned adapter so roll over valve was vertical and on top of “T”. Opened valve to tank, drove it for about 10 minutes, got back home and took stock fuel cap off. Still overflowed. Is this normal? Also my fuel gauge went to “E” and check engine light came on. Had p0461 error code because gauge wasn’t moving. Is there a fix for that or keep tank valve closed until main tank is almost empty?