Questions and Answers

Cooling System Test Question

Q. Vince, Great site! You've got a great gift for communicating automotive technology. I took one of your tests, Who Wants to Be an Auto Mechanic?, and was doing pretty well until I came to this question. But I'm a bit perplexed. Is this question/answer correct? An open thermostat will cause UNDER cooling? 'Under cooling' would seem to mean insufficient cooling, but a thermostat stuck open will actually produce OVER cooling, won't it?

Won't the effect be that the water in the block keeps moving to the radiator, and the desired operating temperature may not be achieved? If so, that open thermostat would create OVER cooling, no?

Jim

A. I catch a lot of people with this one. "undercooling" does not necessarily mean overheating. It means that the coolant is moving too fast through the cooling system to pick up all the heat it is capable of carrying.

"Under cooling" is what happens when the coolant flows through the engine too quickly. Since one of the jobs of the thermostat is to not only control engine temperature, it also controls the speed of the coolant through the system. If the coolant flows too fast, it does not have time to pick up engine heat and results in what's defined as "undercooling". Since "undercooling" usually does result in "overheating", it's common to think of the two as the same.

This can be demonstrated with your car's heater. Put a thermometer in the center vent and turn the heat on HIGH and fan speed on LOW. After five minutes or so, note the temperature. Then put the fan speed on HIGH. You will see that the temperature has dropped because the air is moving too fast to pick up all the heat it is capable of carrying. Not enough to call the heater bad, but enough to make a difference.

"Undercooling" just means that the cooling system is not working as efficiently as it was designed to.

A few years ago I took a course on A/C and Cooling Systems sponsored by Everco. I took the same stance as you when the instructor stated this in class and he had to show me before I accepted what he was saying as fact. So the answer is correct, albeit a somewhat obscure term.