In the present study, the convective heat transfer enhancement of carbon nanotube (CNT)-water nanofluid has been studied experimentally inside an automobile radiator. Heat removal rate of the coolant flowing through the automobile radiators is of great importance for the optimization of fuel consumption. In this study, four different concentrations of nanofluids in the range of 0.15–1 vol. % were prepared with the addition of CNT nanoparticles into water. The CNT nanocoolants are synthesized by functionalization (FCNT) and surface treatment (SCNT) method. The effects of various parameters, namely synthesis method, variation in pH values and nanoparticle concentration on the Nusselt number are examined through the experimental investigation. Results demonstrate that both nanocoolants exhibit enormous change Nusselt number compared with water. The results of functionalized CNT nanocoolant with 5.5 pH exhibits better performance compared to the nanocoolant with pH value of 6.5 and 9. The surface treated CNT nanocoolant exhibits the deterioration in heat transfer performance. In addition, Nusselt number found to increase with the increase in the nanoparticle concentration and nanofluid velocity.