Reciprocation Engine Induction and Cooling Systems Oral Practice Test

The expansion cap mainly keeps the cooling system under the correct pressure while letting coolant expand as the engine heats up. A spring‑loaded valve inside the cap opens at a set pressure to vent excess coolant into the overflow reservoir, preventing dangerous buildup and raising the coolant’s boiling point so the engine doesn’t overheat. When the system cools, coolant is drawn back from the overflow into the radiator, helping maintain the proper coolant level. This cap isn’t responsible for filtering air, lubricating the water pump, or adjusting ignition timing, which are separate functions outside the cooling system.