Elias had already checked the mechanicals. The compression was perfect. The fuel lines were clean. He knew the problem was electrical, buried deep in the wiring harness or the ECU itself.
Controls the volume of fuel entering the high-pressure pump. It uses a Pulse Width Modulation (PWM) signal to maintain target rail pressure. 3. Critical Engine Sensors isuzu 4hl1 ecu pinout link
Are you dealing with a electrical system chassis? Elias had already checked the mechanicals
Here's a brief explanation of each pin's function: isuzu 4hl1 ecu pinout link