The Vehicle Engine Cooling System Simulation (VECSS) computer code has been developed at the Michigan Technological University to simulate the thermal response of the cooling system of an on-highway heavy duty diesel powered truck under steady and transient operation. This code includes an engine cycle analysis program along with various components for the four main fluid circuits for cooling air, cooling water, cooling oil, and intake air, all evaluated simultaneously. The code predicts the operation of the response of the cooling circuit, oil circuit, and the engine compartment air flow when the VECSS is operated using driving cycle data of vehicle speed, engine speed, and fuel flow rate for a given ambient temperature, pressure and relative humidity.The one-dimensional, transient, compressible ram air/engine compartment cooling air-flow model was incorporated into the VECSS to enhance the accuracy and the reliability of the cooling air flow rate, temperature, and pressure variation predictions for the ram air/engine compartment air with improved predictions of the performance characteristics of the fan, radiator, charge air cooler (CAC), condenser, and the effect of system installation variables. A one-dimensional, unsteady engine heat transfer model was added to calculate the engine liner, head, and piston surface temperatures. The heat transfer model was validated with Detroit Diesel Corporation's (DDC) steady state engine data. The single pass cross flow radiator mathematical model which accurately predicts the coolant and air outlet temperatures given the inlet flow and temperature conditions was also added. The model was validated with Behr McCord's steady state heat rejection data and agreement between the data and the model were within 2%. Each of the basic components of the cooling circuit was analyzed individually and modeled and the model predicted results were compared with the corresponding steady-state experimental data. The oil circuit was modeled to represent the configuration for the DDC Series 60 engine. New flow and power characteristics for the oil pump were used and models for the oil filter, regulator, and oil thermostat were developed. The oil cooler mathematical model was validated with experimental data from DDC. For the first time, transient pipe lines were introduced in the VECSS program from the turbocharger to the CAC and from the CAC to the engine air intake manifold. A lag due to inertia in the turbocharger compressor rotor speed was also introduced.