On engines with a "speed-density" electronic fuel injection system, airflow is estimated rather than measured directly with an airflow sensor.
But the MAP sensor doesn't care because it just monitors the absolute pressure inside the intake manifold. On turbocharged and supercharged engines, the situation is a little more complicated because under boost there may actually be positive pressure in the intake manifold. Some vehicles use a separate "baro" sensor for this purpose, while others use a combination sensor that measures both called a BMAP 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. So in effect, the MAP sensor can serve double duty as a BARO sensor. 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. Vacuum then slowly climbs back up as the throttle closes. When the throttle is suddenly opened, as when accelerating hard, the engine sucks in a big gulp of air and vacuum plummets to zero. 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). The highest level of vacuum occurs when decelerating with the throttle closed. Vacuum at idle is always high and typically ranges from 16 to 20 inches Hg in most vehicles. The vacuum inside an engine's intake manifold, by comparison, can range from zero up to 22 inches Hg or more depending on operating conditions. In pounds per square inch, the atmosphere exerts 14.7 PSI at sea level on average. Higher elevations have lower air pressure than areas next to the ocean or someplace like Death Valley, California, which is actually below sea level. 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.īarometric pressure typically varies from 28 to 31 inches of Mercury (Hg) depending on your location and climate conditions. When the engine starts, vacuum is created inside the manifold by the pumping action of the pistons and the restriction created by the throttle plates. When the engine is not running, the pressure inside the intake manifold is the same as the outside barometric pressure. MAP sensors are called manifold absolute pressure sensors rather than intake vacuum sensors because they measure the pressure (or lack thereof) inside the intake manifold. Typical MAP sensor outputs for an older GM application. The MAP sensor senses this and the computer responds by leaning out the fuel mixture to reduce fuel consumption and advances ignition timing to squeeze a little more fuel economy out of the engine. The throttle is not open very wide or may be closed causing intake vacuum to increase. When conditions change and the vehicle is cruising along under light load, coasting or decelerating, less power is needed from the engine. At the same time, the computer will retard (back off) ignition timing slightly to prevent detonation (spark knock) that can damage the engine and hurt performance. In fact, when the computer reads a heavy load signal from the MAP sensor, it usually makes the fuel mixture go slightly richer than normal so the engine can produce more power. The engine sucks in more air, which requires more fuel to keep the air/fuel ratio in balance. When the engine is working hard, intake vacuum drops as the throttle opens wide. The engine computer then uses this information to adjust ignition timing and fuel enrichment. The sensor generates a signal that is proportional to the amount of vacuum in the intake manifold. The Manifold Absolute Pressure (MAP) sensor is a key sensor because it senses engine load. Manifold Absolute Pressure MAP Sensors by Larry Carley copyright 2019 Auto Repair Library, Auto Parts, Accessories, Tools, Manuals & Books, Car BLOG, Links, Index