I filled up today, and when I turned my car on to leave the needle only rose to half tank. This happened once before, except the needle didn't rise at all. Once I turn the car off, then start it back up, needle rises to full normally. Could this just be a lazy fuel pump float? This car sat for ~5 years prior to my purchasing it, maybe there is some old gas that gunked it up? Would something like Seafoam remedy this?