For starters I have a 1991 5.8L I have had a drain in it I checked the codes nothing came up but an exhaust smog thing. So I then replaced the alt and the battery they both needed to be replaced. I also replaced the fuel pump relay and it still drains. It will drain fully within about 6-8 hours so what could be making my truck drain so fast????????? Also forgot to mention that when the trucks on the tail lights stay on without me having the lights on but im guessing thats just a bad groud.