HEUI Injector Explained – How Hydraulic Electronic Unit Injection Works

In this video, we'll explore how one of the most sophisticated diesel fuel injectors works—the Hydraulic Electronic Unit Injection (HEUI) injector. Unlike conventional diesel injection systems, HEUI does not use a fuel high-pressure pump. Instead, it relies on a high-pressure engine oil pump, while the fuel pressure is hydraulically intensified inside each injector. HEUI injectors were widely used on Caterpillar 3126, C7, and C9 engines, as well as on Ford Power Stroke 7.3L and 6.0L diesel engines. In this video, we'll examine the complete operating principle—from the oil and fuel supply systems to pilot injection, the main injection event, and hydraulic pressure intensification inside the injector.