Reading Manifold Absolute Pressure in Live Data
The Manifold Absolute Pressure's function is to send the instantaneous pressure of the intake manifold to the car's electronic control unit (ECU). The data produced by the manifold absolute pressure sensor is utilized by the car's ECU to determine the absolute amount of fuel that the engine may use while leaving no surplus oxygen or unburned fuel.
This allows the vehicle's internal combustion engine to function properly with no harmful fuel while also ensuring the vehicle's steady functioning with the use of precise stoichiometric calculations to ensure appropriate combustion. It also assists the engine in determining ignition timing by providing total mass air flow information at all times.
The ECU uses the Manifold Absolute Pressure (MAP) sensor to estimate the engine load. When the engine is not running, the pressure inside the manifold is the same as outside, but when it starts, pressure will drop as the pistons pump constantly. This Vacuum will suck the air in. The harder the engine runs, more air is required, and the lower the vacuum pressure. This will lead to the manifold pressure increasing close to the outside environment pressure again.
When you floor your car, intake manifold pressure will increase. The maximum value is around 26 to 30 inches of Mercury (Hg). The pressure at idle often ranges from 3 to 5 Hg. Depending on where you live, the manifold pressure when the engine does not work can vary from 28 to 31 Hg. This is also referred to as barometric pressure.
When you turn the ignition on, a good MAP sensor should read barometric air pressure. To verify this value, you can compare the value on your scan tool with the actual barometric pressure to see if they match or not.
This is from a Chrysler that was experiencing intermittent surging & stalling. This vehicle had no DTCs, but the issue could be duplicated repeatedly.
The TPS is in green while the vehicle lightly accelerates & decelerates. Notice a visible drop in the RPM PID in yellow when the vehicle is acting up. The MAP sensor, in red, drops out to impossible values. Notice the RPM surges directly following the incorrect MAP sensor signal.
Due to the signal dropouts, the electrical connection is tested at the MAP sensor. Once it was confirmed that all connections and wiring were good, the MAP sensor was replaced which resolved the no-code surging issue.
Pressure Sensor Issues/Diagnosis
Pressure sensors can cause driving or transmission shifting issues. A speed density vehicle will rely heavily on the MAP sensor while a MAF equipped vehicle relies on it less. You can observe pressure sensor issues using live data with an OBD2 scan tool.
One of the easiest checks to perform is with the key on, engine off (KOEO). Observe all the pressure sensors (MAP, BARO, Boost, etc.) and make sure they match your current actual barometric pressure. If any of them are off, continue testing the sensor in question and its associated wiring.