What would you expect the manifold pressure gauge to read on an inoperative engine?

In the case of an inoperative engine, the manifold pressure gauge would read ambient atmospheric pressure. This is because the manifold pressure gauge reflects the pressure within the engine's intake manifold. When the engine is not running, there is no intake air being drawn in, and therefore, no vacuum is created within the manifold. The pressure inside the manifold will equal the ambient pressure surrounding the aircraft.

When an engine is operational, the manifold pressure is typically lower than ambient pressure, indicating a vacuum condition due to the process of drawing air into the engine for combustion. Conversely, if the engine is stopped, the engine's intake cannot generate any higher or lower pressure than that of the ambient air surrounding the aircraft, resulting in the gauge indicating atmospheric pressure.