I have a 1990 F250 with the 7.3 IDI with at least 300,000 miles on it. The truck has spent several years with little usage and no maintenance. When I got it, it would run, but ran very rough. After some internet research, I learned it was probably due to air in the fuel system. I bled the fuel filter and it helped tremendously, but did not solve all of it. I then started to fix some stuff. I replaced the fuel filter with a one piece WIX filter with water sensor port. I also replaced the return system cups, orings and fuel lines. I replaced the fuel line from the filter housing to the injector pump. I also replaced the fuel line on the back side of the motor that connects the fuel return system to the return line going back to towards the tank. Today I replaced the one-way valve that connects the filter housing to the fuel return system. Today I also replaced the fuel filter vacuum sensor because I noticed it was leaking. After bleeding the fuel filter again after replacing these, it seems to be running much better although I believe the engine is still missing bad on one cylinder. (This is probably another issue for another day. I think I have an injector laying out. I got the truck from my dad, who purchased it used and put about 200,000 miles on it. He has never serviced the injectors, so I am guessing they are pretty worn.) I know that there is a flexible line between the old mechanical fuel pump (lift pump????) and the fuel filter. Dad says this was originally metal, but when he replaced the fuel pump at some time in the past it got kinked, so he cut out the bad part and replaced it with a flexible hose. I am going to replace this on Saturday. My question is, are there any other areas I need to replace or look at that could let air in the system?