1988 F250 7.3 starting troubles

My very first truck! I've replaced glow plugs, fuelfilter,but it still wont start on its own used starter fluid got it running once last weekdrives and runs fine but when i tried to do it again it just keeps turning over so i quit trying for fear of draining batteries asked diesel instructer he said the glow plug wait light should stay on 15 secs to get a proper start and he said to replace some module that 7.3L have this trouble with this part what should i do maybe its the fuel lines possibly water not sure any advice would be great