An important function of all hoses, lines and fittings is to carry fuel with maximum absence of air. When the fuel tank cap is in place and the injection pump and fuel pump are drawing fuel through the lines a low vacuum of 0-1 lb. mercury is created. This occurs because the fuel which the engine uses must be replaced by air.
During this vacuum condition, the slightest leak, which may not leak fuel externally, could draw air into the system and depending on the volume of air, a wide variety of engine malfunctions are possible. These may show up as M.P.G. complaints, smoke complaints, performance complaints and hard starting or not starting conditions.
For example, suppose the inlet fitting was slightly loose at the engine fuel filter. This would probably have an external leak and be a complaint of fuel leak or smell of diesel fuel accompanied by a โ starts but then dies and canโt re-startโ complaint. It is possible that when the engine is shut down the fuel could syphon out of the lines and fuel pump and back into the tank. It is then replaced by air which entered at the loose fitting. The fuel system is now empty and as a result the engine must be cranked until the lines are full again.