Commercial Driver License (CDL) Class B Practice Test

When is it safe to remove the radiator cap?

1. When the engine is running

2. Only when the engine is cooled

3. Immediately after turning off the engine

4. When the temperature gauge is normal

The correct answer is: Only when the engine is cooled

The safe time to remove the radiator cap is when the engine is cooled. This is crucial because the cooling system is pressurized when the engine is operating, leading to the risk of hot coolant spraying out if the cap is removed while the engine is running or immediately after it has been turned off. Hot coolant and steam can cause serious burns or injuries. Waiting until the engine has thoroughly cooled allows for the pressure to dissipate and makes it safe to remove the cap without risk of injury from hot liquids or steam. In contrast, removing the cap when the engine is running or immediately after shutting it off poses significant safety hazards due to the high temperatures and pressures present at those times. Similarly, while a normal temperature gauge reading may suggest that the engine is not overheating, it does not account for residual heat and pressure that can remain in the cooling system. Therefore, always allowing the engine to cool down makes it safe to proceed.