Suggestion. Try talking to a diesel injection specialist. You have these guys
https://www.welhamgroup.co.uk in Leicester, and possibly others too. I only know a little about the VP44 system, but understand that your problem may not necessarily be to do with the EDC but could be to do with a sticking valve on the mechanical side.
VP44s were fitted in millions so they are pretty common units but sadly subject to a great deal of arm chair logic. If anything went wrong with mine, I would probably admit defeat and give it to an expert to sort out as they are inherently complex - bit like taking an auto gearbox to bits and re-building it.
This might also help:
How Does the VP44 Fuel Pump Work?
The VP44 fuel pump is an electronic rotary pump that delivers fuel at extremely high pressures, and additionally controls the fuel flow rate and timing for the engine. The added responsibility of controlling the flow rate and timing means additional electronics, and also means that the VP44 has, in addition to physical part failure due to the constant high pressure and wear inside the housing, the opportunity for electrical failure through the on-board PSG, or ECU. These two failures account for the vast majority of VP44 fuel pump failures.
Internal Housing Wear Failure.
As stated earlier, the VP44 not only provides pressure, it controls engine timing and fuel delivery. As the timing is constantly changing, the internal component that moves to create this timing movement, the timer piston, is under constant motion, and under constant pressure. With time, the friction of this movement creates wear and grooves in the pump housing. Since the timer piston works under a pressure principle, it needs to maintain pressure on one side to cause movement. Once wear and grooves are created in the pump housing, fuel leaks from either side, balancing pressure, and the piston ceases movement. Once this starts to happen, the pump loses timing advance, the engine loses power, and you consume more and more fuel. The repair for this condition is replacement of the pump housing and internal parts.
PSG, or ECU Failure.
Each VP44 fuel pump has a separate computer connected to two internal sensors. The main cause of failure is with the wires connecting the two internal sensors to the PSG. The pump itself creates considerable heat, and after enough heat cycles, the wires and connectors become brittle, split, crack and fail. Additionally, there can be failures of the PSG and sensors themselves, though this is less common. The solution here is to replace the entire PSG.
