Visiting Pops now. HE does not drive since late July, and did not drive much for long stretches this year. He still has his car, which I use when I visit. An old Altima (mid to late aughts). At any rate, the last time I gassed it was in early August. I drove it for a week or so, short distances, in early September. Now I'm visiting, and the thing runs OK the first drive in the AM (about a mile), but then is pretty rough when idling on the return trip. Idiot check engine light flashes. Is this likely crud settling out of the gas? Could it be vapors in the tank? Water? Ideas?