I have an ASE G1 test question for you: Technician A says a cooling system can overheat it the thermostat is stuck open. Technician B says a cooling system can overheat if the radiator cap can’t seal. Who is correct? A or B? or maybe both?

Technician A is wrong, if a thermostat is stuck open, the coolant is allowed to flow and will not cause the engine to overheat. If the thermostat was stuck shut, then the engine would overheat. So, technician B is correct. If a cooling system is not under pressure, the boiling point is lower. Let’s do some math! Each one pound of increased pressure raises the boiling point by 3 degrees Fahrenheit. If the boiling point of water is 212 degrees Fahrenheit at sea level, and the cooling system has a 20 PSI cap, this means the water inside the cooling system now has a boiling point 272 degrees Fahrenheit!!!!!

If the cooling system has a leak and can’t hold pressure, the 212 degrees temperature is right between the typical operating range of 185 and 225 degrees.

Also, the correct system pressure is required for proper water pump seal lubrication. Too little pressure and the seals might not seal properly against the shaft. Too much pressure and excessive amounts of coolant might be forced past the seals and contaminate the bearings of the pump. This could lead to failure.

So when you take the ASE G1 or ASE A1, make sure you read and understand the full question before giving an answer. Also, work through the two scenarios in your head from both technicians A and B.

This video is sponsored by Carter.