Diesel engines are the versatile power source and is widely used in passenger car and commercial vehicle applications. Environmental temperature conditions, fuel quality, fuel injection strategies and lubricant have influence on cold start performance of the diesel engines. Strategies to overcome the cold start problem at very low ambient temperature include preheating of intake air, coolant, cylinder block. The present research work investigates the effect of coolant temperatures on passenger car diesel engine’s performance and exhaust emission characteristics during the cold start at cold ambient temperature conditions. The engine is soaked in the -7°C environment for 6 hours. The engine coolant is preheated to the desired coolant temperatures of 10 and 20°C by an external heater and the start ability tests were performed. The coolant temperature of 10°C in the -7 °C environment improved the fuel combustion and thereby reduced the cranking period by half; reduced the peak HC emissions and NOx emissions by 85% and 30% respectively. The cold ambient conditions increased the accumulation mode particles by 60% and decreased the nucleation particles by 70% compared to that of normal ambient temperature (+20°C). The coolant temperature of 10°C decreased the accumulation particles by ∼ 30% compared to normal ambient temperature. The number count of larger particulates of size 100-1000 nm doubled whereas particulates of diameter less than 100 nm reduced by 20% at -7°C environment compared to +20°C. Coolant temperature of 20°C in the low ambient temperature conditions decreased the total particulate mass to 1/7th of that at cold ambient conditions. The coolant heating strategy improved the cold start performance of the engine at cold ambient temperature conditions and thereby would reduce the overall driving cycle emissions as well.