I am having an issue with my 1991 F250, 7.3 IDI. After it sits overnight an I try to start it, it will fire right up then stall after a second or 2. After that it takes 10-15 seconds of cranking to get it started. I can shut it right off and it will start immediately. Return line leak?

 

It has been awhile since I worked on a friend of mines but my guess is it has to do with the glow plugs. There is a "unit" on the back in the center that controls them and it is known to go bad in time. Also the ONLY glow plugs to use are Bosch/Motorcraft the rest are junk. Get a multitester and strt checking the glow plugs.
The Unit at the back is a timer I believe. If you get all the glow plugs working right I don't think you will have any problems starting it when cold,that is why you have no problems once you get it running. I doubt it has anything to do with fuel as the fuel delivery is mechanical thus does not give many problems. DON'T use starting fluid to start it.It can damage the engine.
My friend used Autolite glow plugs and they just didn't last,when we changed the unit /timer in the back and put Motorcraft glow pugs in his problems stopped.
Good luck