Engine thermal management systems (TMS) are gaining importance in engine development and calibration to achieve low fuel consumption and meet future emissions standards. To benefit from their full potential, a thorough understanding of the effects on engine behavior is necessary. Steady state tests were performed on a 2.0L direct injection diesel engine at different load points. A design of experiments (DoE) approach was used to conduct exhaust gas recirculation (EGR) and injection timing swings at different coolant temperatures.The effect of the standard engine controller and calibration was observed during these tests. The injection timing strategy included a significant dependency on coolant temperature, retarding injection by about 3° crank angle between coolant temperatures of 70°C and 86°C. In contrast, EGR strategy was essentially independent of coolant temperature, though physical interactions were present due in part to the EGR cooler.Of the three mechanisms investigated, EGR had the largest effect on Nitrous oxides (NOx) and fuel consumption and also had the largest potential for NOx reduction over the stable engine operating range. Despite this, a reduction in coolant temperature from 86°C to 50°C reduced NOx by 4.5% at high load condition. However, coolant temperature adjustments offer similar or better trade-offs than the other calibration parameters. For example, coolant temperature offers 10% improvement NOx per percentage increase in fuel consumption, compared to 4% for injection timing and 13% for EGR.Higher coolant temperatures reduce ignition delay and premix burn during combustion, but this effect is more pronounced at lower engine loads. Analysis of mean effective pressures (MEP) showed friction MEP (FMEP) and pumping MEP (PMEP) reduce with higher operating temperatures, which yielded lower gross indicated MEP (IMEP). FMEP was slightly higher at higher load, with a 1.5% to 5.4% increase depending on operating temperature. This increase was expected to be due to the increased loading between parts within the engine, but would be offset by potentially higher local oil temperatures.