I am going to check the glow plugs on a 06 cat 303.5. Have had a prob with it not firing of quickly. Worse on first start. Where are they located? Im thinking on back side of engine, only getting access by taking out the seat?
Double check your cranking voltage at the starter. Ours had a bad ground causing low voltage when cranking. The slower cranking made the engine start slower, especially when cold. I added another ground cable from the started to the frame and all was good to go. Another guy on here had the same issue last week and fixed it with a 2nd ground also.