Pressure cap vents coolant into the radiator overflow tank when coolant pressure exceeds what psi?

• A. 5 PSI
• B. 10 PSI
• C. 15 PSI
• D. 20 PSI

Answer: (C)

A key idea is that the radiator cap acts as a pressure-relief valve in the cooling system. It is set to open at a specific pressure to prevent overpressure. For most passenger vehicles, that setting is fifteen pounds per square inch. When the coolant heats up and expands, pressure rises until it reaches that rating; at that point, the cap opens and excess coolant is pushed into the overflow tank. This keeps the system from forcing coolant out elsewhere and provides a reserve for expansion. As the engine cools and pressure drops, coolant is drawn back from the overflow into the radiator, helping maintain the proper level. Pressures such as five or ten psi would vent too early and won’t maintain the necessary boiling-point elevation, while twenty psi is higher than what most standard systems use.