One should determine if engine temperature has anything to do with the problem, ambient temperature should also be considered. Close attention should be paid to the cranking R.P.M. as well as the glow plug system. Starting problems are often electrical related since cranking R.P.M. and the glow plug system are very critical to getting the fuel to burn. Batteries and the starter should also be a consideration if cranking R.P.M. seems slow. Once the electrical system is ruled out, air entering the system should be considered, especially if the engine cranks, dies and is then hard to restart.
Air can enter the system through any break in the fuel line or through a bleedback condition.
To determine if air is the problem, install a clear return hose on the return side of the injection pump. If air comes out it is getting in the inlet system someplace. The injection pump is very seldom the cause for air entering the system. In most cases the fuel lift pump or a faulty inlet line usually ends up being the problem. Engine problems can also be the cause of starting problems.