Here's a link to the "IPR Diagnostics"
Here's some more description of the IPR taken from an International HEUI brochure that used to be around a long time ago. I changed the acronyms to the ones we use here. Sorry for the lack of the pictures that go with the description below!
The IPR is an electronically controlled pilot operated pressure control valve. The basic components of the IPR are the: Body, Spool Valve, Spool Spring, Poppet, Push Pin, Armature, Solenoid, and Edge Filter (later ones don’t have the edge filter).
The IPR controls HPOP outlet pressure in a range between 450 and 3,000 psi. An electrical signal to the solenoid creates a magnetic field which applies a variable force on the poppet to control pressure.
OPERATION – Engine Off
With the engine off, the valve spool is held to the right by the return spring and the drain ports are closed.
OPERATION – Engine Start Up
Approximately 1,500 psi of oil pressure is required to start a relatively warm engine. If the engine is cold (coolant temperature below 32F), 3,000 psi of oil pressure is commanded by the PCM.
Oil flow through the IPR is as follows: Pump outlet pressure (red) enters the end of the body and a small amount of oil flows into the spool chamber through the pilot stage filter screen and control orifice in the end of the spool. The electronic signal causes the solenoid to generate a magnetic field which pushes the armature to the right. The armature exerts a force on the push pin and poppet holding the poppet closed allowing spool chamber pressure to build. The combination of spool spring force and spool chamber pressure hold the spool to the right closing the drain ports. All oil is directed to the pressure rail until the desired pressure is reached.
OPERATION – Engine Running
Once the engine starts, the PCM sends a signal to the IPR to give the rail pressure desired. The injection control pressure sensor (ICP) monitors actual gallery pressure. The PCM compares the actual rail pressure to the desired rail pressure and adjusts the signal to the IPR to obtain the desired rail pressure.
The actual oil flow through the IPR during engine operation is as follows: Pump outlet pressure (red) enters the end of the body and a small amount of oil flows into the spool chamber (orange) through the pilot stage filter and the control orifice in the spool. The pressure in the spool chamber is controlled by adjusting the position on the poppet and allowing it to bleed off some of the oil in the spool chamber. The position on the poppet is controlled by the strength of the magnetic field produced from the electrical signal from the PCM. The spool responds to pressure changes in the spool chamber (left side of the spool) by changing positions to maintain a force balance between the right and left side of the spool. The spool position determines how much area of the drain ports are open. The drain port open area directly affects how much oil is bled off from the pump outlet and directly affects rail pressure. The process of responding to pressure changes on either side of the spool occurs so rapidly that the spool is held in a partially open position and pump outlet pressure is closely controlled. The IPR allows infinitely variable control of pump outlet pressure between 450 psi and 3,000 psi.
Hope this all helps! Cheers!