In recent years, stricter regulations on emissions and higher demands for more fuel efficient vehicles have led to a greater focus on increasing the efficiency of the internal combustion engine. Nowadays, there is increasing interest in the recovery of waste heat from different engine sources such as the coolant and exhaust gases using, for example, a Rankine cycle. In diesel engines 15% to 30% of the energy from the fuel can be lost to the coolant and hence, does not contribute to producing work on the piston. This paper looks at reducing the heat losses to the coolant by increasing coolant temperatures within a single cylinder Scania D13 engine and studying the effects of this on the energy balance within the engine as well as the combustion characteristics. To do this, a GT Power model was first validated against experimental data from the engine. Using a Water-PEG mixture as coolant, the coolant temperature was then varied from 60°C to 200°C for both the liner and the cylinderhead. This sweep was done for multiple combinations of engine loads and speeds as well as for different air-fuel ratios. It was found that at the higher air-fuel ratios, an increase in coolant temperature led to an increase in indicated efficiency as well as an increase in exhaust gas temperature and enthalpy. However at lower air-fuel ratios there is a decrease in indicated efficiency with higher coolant temperatures. It was also seen that ignition delay at higher temperatures was shorter with the combustion duration being longer. The change in combustion phasing was found to be dependent on engine load. While the higher coolant temperature simplifies heat recovery from the coolant itself, the consequently higher exhaust gas temperatures observed means that the heat losses are moved more towards the exhaust where energy recovery is easier.