Manually flushing a hydraulic steering system should be done with the engine running.

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Flushing a hydraulic steering system manually should be done with the engine off. The power steering pump is driven by the engine, so if the engine is running during a manual flush, the pump will actively circulate fluid under pressure. That makes the process hard to control, can push dirty fluid into places you don’t want, and poses a safety risk from high-pressure fluid spray or unintended pump loading. With the engine off, you can isolate the system, drain and replace fluid more safely and observe the system for leaks or air pockets. After the flush is complete, you can start the engine and turn the wheel to help bleed any remaining air, then recheck fluid level.