I have a 1992 F350 diesel dually with the 7.3 liter engine. In the past six years I had to replace the starter twice. Diesel engines can show many different problems associated with slow starting or no starting. When it takes a lot of cranking to get the engine started, it may be the starter going bad. Diesel engines develop heat inside the cylinders by fast compression of the air inside the cylinders. If the engine is not spinning fast enough by the starter, the cylinders do not get hot enough to fire the fuel/air mixture, even if the glow plugs are working OK. When I replaced the starter, the engine started immediately rather than struggling to start.