Ethylene glycol reduces the cooling efficiency of water because it has a lower heat capacity as compared to water. A substance with low heat capacity would mean that the material only needs a small amount of energy to increase its temperature. So, for ethylene glycol, the heat transfer would be faster decreasing the cooling efficiency.
According to most car mechanics, plain water is the best coolant to use in an engine provided the engine is not being exposed to freezing temperatures. if the car is subject to freezing temperatures then a mixture of water and ethylene glycol (antifreeze) is recommended but it does not cool as efficiently as plain water. why would ethylene glycol reduce the cooling efficiency of water?