The HEUI fuel injection pump is in reality the HPOP (high-pressure oil pump) on a Ford 7.3 or 6.0L Powerstroke diesel. The Ford 6.0L (International) Powerstroke diesel uses electro-hydraulic fuel injectors located under the valve covers.
The high-pressure oil pump (located at the rear of the engine under the turbo charger) exerts extremely high oil pressure (up to 4000 psi) to build fuel pressure. A lack of sufficient pressure due to leaks or worn parts in the pump will cause a no-start situation resulting from insufficient fuel pressure.
A diesel engine ignites its fuel by the heat generated through highly compressing the fuel rather than relying on a spark for ignition. These engines offer good fuel economy and excellent longevity, lasting much longer than a conventional fuel vehicle.
They also produce superior torque for hauling or towing.
The flip side to this coin is that they are not maintenance free. As a matter of fact, they require very specific service intervals and have proved to be very unforgiving should the service be ignored.
Such high oil pressure results in more rapid wear causing leaks and failures over time. Generally, these failures are isolated to O rings or the pump itself. The pump can experience an internal or gasket failure, both of which will reduce oil pressure. The second major cause are the O rings on the oil pressure rails supplying pressure to the fuel injectors located under the valve cover.
O ring failures are not curable with the replacement of the O rings. The threaded section of the shaft forward of the O ring wears from the high pressure. Replace these parts with new or rebuilt parts only.
To extend the useful life of the oil pump, seals and O rings the oil and filter must be changed religiously between 3000 to 5000 miles. Diesels must not use normal automotive oil due to the high pressure. The oil must be API rated CI-4 or higher. The viscosity must be SAE 15W-40. These oils will always be in a separate section for diesels.
To avoid premature fuel-related O ring failures always avoid fuel with a high sulfur content. It kills the O rings causing further leaks.