What does a manifold pressure gauge indicate when the engine is not operating?

The manifold pressure gauge is designed to measure the pressure within the intake manifold of an aircraft engine. When the engine is not operating, there is no combustion occurring, and consequently, the negative pressure created by the engine's intake during operation is absent. In this state, the gauge will indicate atmospheric pressure, which at sea level is approximately 14.7 PSI.

This measurement represents the ambient pressure of the surrounding atmosphere, which is crucial for pilots and mechanics to understand the operating conditions around the engine. In contrast, when the engine is running, the manifold pressure would typically drop below this atmospheric baseline due to the engine creating a vacuum as it draws in air for combustion. Understanding this distinction helps ensure accurate monitoring of engine performance during operation.