In recent years, hydroxy has been widely studied and used for Internal Combustion Engines. Engines using hydroxy gas will improve economic and technical indicators, further reducing HC and CO emissions. However, the limitation of this solution is that NOx emissions tend to increase. This is a toxic exhaust component that has many negative effects on human health as well as the environment. This study presents results when installing a hydroxy gas generator on a four-cylinder diesel engine with code 490 QZL, which combines with a solution using the EGR system. The results show that, compared to the original engine, the amount of NOx gas decreased from 42.8 to 55.1% at the surveyed speed range from 1400 to 2800 rpm. In particular, at the speed of 2600 rpm, the largest decrease in NOx value was found. In addition, HC and CO also had the largest reductions of 18.2% and 17.3%, respectively, at 2400 rpm. However, engine power decreases, and fuel consumption increases. At 2400 rpm, the gas recirculation rate is 12.7%, the largest decrease in power is 1.8%, while the largest increase in fuel consumption is 4.9%.