This study investigates the effect of engine warm-up time on the emissions of hydrocarbons, carbon monoxide, and nitrogen oxides. Data were analyzed under eight different engine speeds and two load conditions (15% and 30%) over 15 minutes, both thermostat and non-thermostat conditions. The results indicated that at 1500 rpm and 15% load, installing a thermostat reduced warm-up time by up to 29%. Increasing engine speed by 500 rpm on average shortened this time by 27% at 15% load and 32% at 30% load. Furthermore, hydrocarbon emissions were very high during the first three minutes after start-up but stabilized within the range of 150–250 ppm after warm-up. Nitrogen oxides exhibited the most significant variation; doubling the engine speed from 1500 to 3000 rpm tripled their concentration, and doubling the load increased it by approximately 1.6 times. Moreover, under low-speed and load conditions, installing a thermostat reduced carbon dioxide emissions during and after the warm-up period. Ultimately, it can be concluded that shorter warm-up times through the use of a thermostat and optimal management of this period significantly reduce pollutants and have a positive impact on controlling emissions from the engine. These results emphasize the importance of utilizing optimized cooling systems to decrease pollution and improve the environmental performance of combustion engines.