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

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

• A. Current atmospheric pressure
• B. Absolute manifold pressure
• C. Zero
• D. Maximum pressure

Answer: (A)

The manifold pressure gauge shows the absolute pressure inside the intake manifold. When the engine isn’t running, there’s no suction created by the pistons, so the manifold isn’t drawing air and is effectively open to the outside air. The pressure inside the manifold then equals the surrounding atmospheric pressure at that location. So the gauge reads the current atmospheric pressure, which changes with altitude and weather. It isn’t zero, since air is present in the intake, and it isn’t maximum pressure, since there’s no force-building pressure in the manifold without the engine running.

The manifold pressure gauge shows the absolute pressure inside the intake manifold. When the engine isn’t running, there’s no suction created by the pistons, so the manifold isn’t drawing air and is effectively open to the outside air. The pressure inside the manifold then equals the surrounding atmospheric pressure at that location. So the gauge reads the current atmospheric pressure, which changes with altitude and weather. It isn’t zero, since air is present in the intake, and it isn’t maximum pressure, since there’s no force-building pressure in the manifold without the engine running.