Manifold Absolute Pressure Sensor (MAP)

 MAP sensors (manifold absolute pressure sensors) measure the pressure (or lack thereof) inside the intake manifold. When the engine is not running, the pressure inside the intake manifold is the same as the outside perssure (barometric pressure). When the engine starts, vacuum is created inside the manifold by the pumping action of the pistons on the intake stroke and the restriction created by the throttle plate(s). At full open throttle with the engine running, intake vacuum drops to almost zero and pressure inside the intake manifold once again nearly equals the outside barometric pressure due to the throttle plates opening eliminating the resriction of air flow into the engine.

     Barometric pressure typically varies from 28 to 31 inches of Mercury (Hg) depending on your location and climate conditions. Higher elevations have lower air pressure than areas next to the ocean or someplace like Death Valley, California, which is actually below sea level. In pounds per square inch, the atmosphere exerts 14.7 PSI at sea level on average.

     The vacuum inside an engine's intake manifold, can range from zero up to 22 inches Hg or more depending on operating conditions. Vacuum at idle is always high and typically ranges from 16 to 20 inches Hg in most vehicles. The highest level of vacuum occurs when decelerating with the throttle closed. The pistons are trying to suck in air but the closed throttle chokes off the air supply creating a high vacuum inside the intake manifold (typically four to five inches Hg higher than at idle). When the throttle is suddenly opened, as when accelerating hard, the engine sucks in a big gulp of air and vacuum plummets to zero. Vacuum then slowly climbs back up as the throttle closes increasing restriction of air to the engine.

     When the ignition key is first turned on, the powertrain control module (PCM) looks at the MAP sensor reading before the engine starts to determine the atmospheric (barometric) pressure. So in effect, the MAP sensor can serve double duty as a BARO sensor. The PCM then uses this information to adjust the air/fuel mixture to compensate for changes in air pressure due to elevation and/or weather.