© 2022 Elsevier Ltd. All rights reserved.In this study, copper oxide nanoparticles (CuONPs) were synthesized by sol-gel method and characterized by SEM. The stability and the heat transfer performance of the synthesized nanoparticles was evaluated by adding to the radiator coolant at different concentrations. The average diameter of the CuONPs produced was 80 nm and they were in a spherical structure. Surfactants were applied in the stabilization of CuONP-doped nanofluid using SDS and PEG 6000. In stabilization tests, CuONPs at 0.1, 0.2, 0.4 and 0.6 wt% concentrations remained stable for 2 days at pH: 7.0. The effect of CuONPs contribution to the coolant at different rates on heat transfer in a diesel vehicle was tested at concentrations 0.4 and 0.6 wt% and at 4 different engine powers (5.2, 7.3, 9.4 and 11.5 kW). The highest efficiency was detected in CuONPs contribution of 0.6% and engine power of 7.3 kW. With the use of 0.6% CuONPs, the heat transfer increased by about 40%. The effect of nanoparticle additive at the same concentrations at 11.5 kW engine power was monitored by thermal camera. The results show that CuONPs synthesized by the sol-gel method are beneficial for heat transfer when added to the radiator coolant in diesel engines.