If a coolant recovery container is not part of the pressurized cooling system, what may be done while the engine is at operating temperature?
Correct answer: A – Remove that container’s cap and add coolant
A cap on a coolant container that is not part of the pressurized system may be removed so coolant can be added while the engine is at operating temperature. The radiator cap and every other pressurized part must remain closed until the system has cooled. Opening a hot pressurized cooling system can release steam and hot coolant.
Source: Maine Commercial Driver License Manual — § 2.14.1, “Vehicle Checks—Engine Coolant”



