The Manifold Absolute Pressure (MAP) sensor is used by the
Powertrain Control Module (PCM) for engine load input. The PCM
uses this input, as well as others, to calculate the correct amount
of fuel to inject into the cylinders. The MAP sensor measures the
absolute pressure inside the intake manifold of the engine. At sea
level, atmospheric pressure is about 14.7 psi (pounds per square
inch). When the engine is off, the absolute pressure inside the
intake equals atmospheric pressure, so the MAP will indicate
about 14.7 psi. At a perfect vacuum, the MAP sensor will read 0
psi. When the engine is running, the downward motion of the
pistons create a vacuum inside the intake manifold (For the
purposes of engine control, when a technician says vacuum, what
they are really saying is pressure that is less than atmospheric
pressure). With a running engine, intake manifold vacuum usually
runs around 18 - 20 “Hg (inches of mercury). At 20 “Hg, the MAP
sensor will indicate about 5 psi. This is because the MAP sensor
measures “absolute” pressure, based on a perfect vacuum, rather
than atmospheric pressure. Make sure your shop replaces all the
vacuum hoses on the car...
