If the engine is difficult to start in cold weather , you should check fuel injectors , fuel pump and fuel priming pump and compression. Also you need to check preheat unit but never do preheat when use either! To check compression you wil need compression tester - the compression must be between 300 and 500 PSI with no more than 50 PSI difference between the cylinders. You can understand the reason for the low compression as add a LITTLE oil in the cylinder , turn engine several revolutions, and check it again.Compression grow significantly if the reason is in piston ,piston rings and cylinder liners.If the compression does not grow significant reason is in the valves. Remove injectors ,check compression and bring injectors to the diesel shop to check it. Be careful with copper sealing washers in the injector bore - they are specially made for the job . They must seal against high pressures and be of correct thickness for best engine performance.Do not use washers of different type.