The growth in demand for engine coolants made from propylene glycol has created interest in engineering information on their performance. Although the properties of propylene glycol coolants and ethylene glycol coolants are similar, testing was required to determine the effects of this change for current designs at the limits of the expected range of operating conditions.The performance of propylene glycol coolants at high ambient temperatures and under high load conditions was compared to ethylene glycol coolants both on the engine dynamometer and in vehicles operated on specific sections of highway in the high desert areas of California, Nevada and Arizona and a desert proving ground.The dynamometer work was done on a high production volume 3.8 liter V-6 engine. Data gathered included temperature measurements at critical cooling system and engine metal locations at a range of coolant compositions and coolant return temperatures simulating different ambient temperatures.The vehicle work utilized a fleet of five matched pairs of production vehicles of different type and marque. Data collected included critical temperature in the cooling system as well as engine metal which could be obtained in this type of testing. The pairing was used to eliminate vehicle differences by using an AB/BA comparison.The results will be reported, comparing the heat transfer performance of propylene glycol engine coolants to ethylene glycol coolants in high temperature service. Conclusions will be drawn about the significance of the small difference of heat transfer coefficient and boiling point between propylene glycol coolant and ethylene glycol coolant in the context of engine operation.