HEUI = Hydraulically Electronically Unit Injector
HEUI is an electronic fuel injection system for medium and heavy-duty diesel engines jointly developed by C-aterpillar and Navistar International.
Its key feature is that it doesn't directly inject high-pressure diesel fuel; instead, it uses high-pressure engine oil as the "power source" to pressurize the diesel fuel for injection.
Popular HEUI diesel injectors:
NAVISTAR and FORD G2.8/G2.9.
CAT C7, C9, C-9, C3126, 3412E, 3412A, ISUZU,
All mentioned HEUI injector could be mounted and tested on a professianl diesel injector test bench.