Contained within the hydraulic head (outlets) of the injection pump where the high-pressure fuel lines are connected to the injection pump are delivery valves (one per cylinder) (Figure 19-73), which are designed to
open at a fixed pressure and deliver fuel to the injectors in firing-order sequence.
These valves function to ensure that there will always be a predetermined fuel pressure in the fuel lines leading to the fuel injectors. Another major function of these individual delivery valves is to ensure that at the end of the injection period for that cylinder there is no possibility of secondary injection and also that any pressure waves during the injection period will not be transferred back into the injection pump.
If secondary injection were to occur, the engine would tend to misfire and run rough. The delivery valves ensure a crisp cutoff to the end of injection when the fuel pressure drops off in the line and also maintains fuel in the injection line so that there is no possibility of air being trapped inside the line.