The relief valve in the actual Fuel pump is a Safety Relief valve to protect the pump if there is a restriction between the pump itself and the 233-6742 VALVE GP-CHECK FUEL such as a Blocked/restricted Secondary filter or fuel line obstruction.
What Cat realized in late 2009 was the Safety relief valve in the pump was set too low and it was constantly going off resulting in low Fuel pressure and little work for the 233-6742 VALVE GP-CHECK to do.
Ideally the 233-6742 VALVE GP-CHECK should be controlling the fuel rail Pressure and the Pump Safety relief Valve should never Crack off at all unless there is a blockage and or restriction causing the Pump Safety Relief Valve to modulate..
A typical way to test the Pump Safety Relief Valve in the Pump is to block the return fuel line at the Secondary filter where it goes back to Tank or where the hose returns off the back of the Cylinder head.
Idle the engine and record the pressure.
WARNING!!! DO NOT INCREASE ENGINE RPM ABOVE IDLE whilst you have the return line closed off for testing.
Make sure the gauge is on the Filter inlet side so as to notice the Pump Safety Relief Valve open and close. You can see the Needle Jumping as the valve Modulates (pops open and close).
This test result will then support the argument as to whether the 233-6742 VALVE GP-CHECK or the Pump or the Pump Safety Relief Valve is faulty.
It is also a good Idea to check both sides of the Secondary filter for pressure differential to ensure no restrictions across the system between pump and Cylinder head Rail pressure.
Having said that its of the utmost important to check the Fuel pressure on both sides of the Secondary filter PRIOR to testing the Pump Safety Relief Valve.
A partially Blocked Secondary Fuel filter will record a Higher than normal Fuel pressure due to the inability of the Pump Safety relief valve to bypass enough flow to manage the pressure. (when recorded on the inlet side of the Secondary Filter Head). So another words sometimes HIGH fuel pressure at this test point can mean a Secondary filter Restriction causing low fuel Rail pressure hence the importance of checking fuel both sides of the Secondary Filter Head.
This HIGH reading also gives the impression all is well when in fact its quite the opposite!