When an engine fails to operate, stalls, misfires, or knocks, there may be air in the high-pressure pumps and in the fuel lines. Unlike liquid fuel, which is incompressible, when air is present in the system, compression and expansion of air will occur and the injector valves will either fail to open or will not open at the proper time. You can determine the presence of air in a fuel system by bleeding a small amount of fuel from the top of the fuel filter or by slightly loosening an air bleeder screw or plug. If the fuel appears quite cloudy, it is likely that there are small bubbles of air in the fuel.

When working with fuel systems, you should remember that if air is entering a fuel line, the pressure within the fuel line must be lower than atmospheric pressure. The smallest of holes in the transfer pump suction piping will allow enough air to flow into the system to air bind the high-pressure pumps. Carefully inspect all fittings in the suction piping. A loose fitting or a damaged thread condition will allow air to enter the system. On installations where flanged connections are used, be sure to check the condition of the gaskets. Inspect tubing (especially copper) and flexible hose assemblies carefully for cracks that may result from constant vibration or rubbing. The use of tubing and flexible hose assemblies on diesel engine fuel systems is common. You will find that flexible hose assemblies are used more on the supply or low-pressure side of the injection equipment while tubing is more commonly used on the high-pressure side of the

injection equipment. The use of tubing and flexible hose assemblies is also the means by which all pressure gauges of a diesel may be located on a central gauge board away from the system the gauges are monitoring. (Additional information concerning the fabrication and fitting of flexible hose assemblies and tubing can be found in Tools and Their Uses, NAVEDTRA 10085-B1 , and in Fluid Power, NAVEDTRA 16193-B, chapter 5.) If an engine is allowed to run out of fuel, you can expect trouble from air that enters the fuel system. If there is a considerable amount of air in the filter, a quick method of purging the system of air is to remove the filling plugs on top of the filter and pour in clean fuel until all air is displaced. You can then remove any air remaining in the system by using the hand priming pump.

For example, in the fuel system illustrated in view A of figure 9-11, you should open the system between the pump and the filter. Operate the hand priming pump until all air is removed and only clear fuel flows from the line. Then close the line. Repeat the same procedure at other points in the system, such as between strainers and the filters, between the filters and the high-pressure pumps, and at the overflow line connection (excess fuel return line) on the high-pressure pump housing. In small, high-speed diesel engines, you may need to prime only at the overflow connection. Since priming high-pressure lines is time-consuming, attempt to start the engine before purging these lines. However, do not crank the engine for more than the specified interval of time. If the engine still fails to start, you should prime the high-pressure lines. Since the procedure necessary to prime high-pressure lines will vary considerably with different installations, follow the NAVSEA technical manual instructions for the proper procedure.