I have been trying to piece together information on the VP44 so I could understand how it works. I am not sure this info will help but here goes.
The ECM looks at all of the sensors used on the engine. These sensors include the engine coolant temperature (ECT), engine RPM, manifold air pressure (MAP), intake air temperature (IAT) accelerator position sensor (APPS) and vehicle speed sensor (VSS). The ECM then looks up the correct duty cycle for the injection duration and timing. This info is sent to the fuel pump control module (FPCM) located on top of the VP44 using the controller area network bus (CANBUS). It is up to the FPCM to then send signals to the timing solenoid and high-pressure fuel solenoid. There are only two wires connecting the FPCM and the solenoids and the wire to the high-pressure solenoid is the one that gets tapped with a major fueling box. The high-pressure fuel solenoid, for stock programming, will have a maximum duty cycle of about 40%. This is how long it stays open when injecting fuel. Adding an aftermarket box can increase the duty cycle to about 65% injecting more fuel. All of this injected fuel is at a high pressure, 17,000 psi or more. The internal VP44 fuel pump pressure is somewhere around 300 psi and will vary with RPM as the internal VP lift pump is connected to the engine driven input shaft.
One limitation of the VP is on how much fuel is available to be injected. There is an internal hydraulic accumulator the feeds the high pressure pump. If the high-pressure fuel solenoid stays open longer than about 65%, the accumulator runs out of fuel and the high pressure falls.
One of the major problems with the VP is it dying using a fueling box. This is caused by interference in the high-pressure pump that causes it to bind up. This problem also causes the timing to not be achieved as commanded by the FPCM, setting the P0216 DTC.
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