If a Hydraulic-Assist system has assist with the engine running, but not with the engine off, the most likely cause is __________.

In a hydraulic-assist system, the operation of the system relies on the hydraulic fluid pressure, which is typically maintained by a power steering pump that is driven by the engine. When the engine is running, the pump provides the necessary hydraulic pressure to assist with steering. If the engine is off, the power steering pump is not operational, and generally, the hydraulic assistance is lost.

If steering assistance is available only when the engine is running and absent when it is off, this typically points to a problem with a component that is responsible for storing hydraulic pressure. The accumulator in the system serves the purpose of maintaining hydraulic pressure for a short duration even when the engine is off. A defective accumulator may fail to maintain that pressure, resulting in the inability to provide hydraulic assistance when needed.

This indicates that despite the engine running, the ability to provide consistent steering assistance may be compromised due to the accumulator's failure, which is directly tied to the operation of the assist system. Therefore, when the engine is off, the lack of assist is likely attributed to this failure. The other options, while potentially problematic in their own right, do not specifically address the scenario where assist is available with the engine running but not when it is off.