What does a manifold pressure gauge indicate when a reciprocating engine is not running?

A manifold pressure gauge measures the pressure within the engine's intake manifold. When a reciprocating engine is not operating, the vacuum created during the intake cycle is absent, and the gauge reflects the ambient atmospheric pressure. This is because the gauge is typically calibrated to read zero when the engine is at rest. Therefore, when the engine is not running, the only pressure it can indicate is the existing atmospheric pressure, which serves as the baseline for manifold pressure readings. This feature is essential for pilots and engineers as it helps them understand ambient conditions and prepare for engine start-up.