Q: MAP sensor reading a higher voltage than it should.

The diagram in the Service Manual depicts the relationship between the voltage output of the sensor and absolute pressure, not gauge pressure as measured with a vacuum pump. To obtain the value of absolute pressure to use in that diagram subtract the negative gauge pressure of about 3 PSI (150 mm as you noted) from atmospheric pressure. That resulting value of absolute pressure corresponds to about 3 volts in the diagram and consequently the voltage output that you are getting is about what it should be. Your MAP sensor is fine. P0505 is typically caused by an idle air control (IAC) valve that is contaminated with carbon, the IAC passages are clogged or restricted with dirt, debris or carbon build up, or the engine has a vacuum leak in the intake system after the throttle body that the ECM cannot overcome with IAC mediated adjustments. To have your circumstance diagnosed and resolved, the recommended service is a check engine light diagnostic. If you request that diagnostic, the responding certified mechanic will promptly resolve this right at your location. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.