Air in Fuel
Air trapped in diesel fuel systems is one of the main reasons for a hard-starting engine. Air can enter the fuel system at loose joints in the piping, leakage in seals, or through a spray nozzle that does not close properly. Letting the vehicle run out of fuel will also cause air to enter the system. Like water, air can interfere with the proper flow of fuel from the tank to the cylinder. A large amount of air in a system will prevent fuel pumps from picking up fuel and pushing it though the system. Bleeding the system can remove air. Consult the current service manual for the proper procedure.