Bosch Relief Valve and Pump Pressure Test Diagram

Fuel pressure gauge connected between the transfer pump outlet and secondary fuel filters: 1, fuel filter housing; 2, transfer pump; 3, tester

This test is a check to ensure that the injection pump relief valve is, in fact, opening at the correct pressure and that the transfer pump is performing correctly. If the relief valve is stuck open or is opening at too low pressure, the fuel delivery pressure within the injection pump housing will be too low to sustain sufficient flow to the plunger and barrel of the individual pumping assemblies. On the other hand, if the relief valve is stuck closed or opens at too high a pressure setting, the fuel within the injection pump housing, which is also used for cooling and lubricating purposes, will run hot. This can result in a loss of horse power due the expansion of the fuel, since a less dense fuel charge will be delivered to the injectors and combustion chamber. In addition fuel that is too hot can cause internal pump pluger damage due to its instability to properly cool and lubricate the component parts. Note that only 25 to 30% of the fuel delivered to the injection pump housing is actually used for combustion purposes. The remainder cools and lubricates the injection pump components.