Dual fuel engines employing pilot diesel injection to ignite premixed natural gas provide an opportunity for liquid petroleum fuel replacement and for reduced emissions of oxides of nitrogen (NOx) and particulate matter (PM). A Navistar T444E turbocharged V8 engine was converted to operate in dual fuel mode by metering the compressed natural gas (CNG) with an IMPCO Technologies, Inc. regulator and electronic valve while retaining the stock Navistar Hydraulically-Actuated Electronically-Controlled Unit Injection (HEUI) system for diesel pilot injection. A dedicated controller was designed and constructed to allow manual control of diesel fuel injection pulsewidth (FIPW), diesel injection advance (ADV), hydraulic injection control pressure (ICP) and natural gas mass flow. The controller employed two Microchip, Inc. PIC-based microcontrollers: one to perform initialization of a Silicon Systems, Inc. 67F867 engine interface peripheral, and the other to perform the runtime algorithms. The dual fuel engine was connected to a DC dynamometer and the exhaust gases were ducted to a full-scale dilution tunnel feeding gaseous emissions analyzers and a PM filter system. A series of tests was performed to find the optimum FIPW, ADV, and ICP settings for the diesel pilot injection. When injection advance was held constant, replacement of the diesel by CNG at a torque of 335 N-m and a speed of 1500 rpm yielded a modest drop in NOx and a rise in carbon monoxide (CO) and total hydrocarbon (HC) levels. When the pilot injection parameters were set at optimal levels, the dual fuel mode offered a 19% reduction in NOx and a 73% reduction in PM for the 580 N-m, 1500 rpm setpoint. At the 335 N-m, 1500 rpm setpoint, a reduction in NOx of 33% and a reduction in PM of 58% was realized. The emissions improvements were obtained while showing a marginal efficiency increase at both setpints.