Does anyone know what the MAPressure is supposed to be in our trucks? Mine's reading at 8 psi.

 

The MAP sensor is used as an input to the Powertrain
Control Module (PCM). It contains a silicon
based sensing unit to provide data on the manifold
vacuum that draws the air/fuel mixture into the combustion
chamber. The PCM requires this information
to determine injector pulse width and spark advance.
When manifold absolute pressure (MAP) equals
Barometric pressure, the pulse width will be at maximum.
A 5 volt reference is supplied from the PCM and
returns a voltage signal to the PCM that reflects
manifold pressure. The zero pressure reading is 0.5V
and full scale is 4.5V. For a pressure swing of 0–15
psi, the voltage changes 4.0V. To operate the sensor,
it is supplied a regulated 4.8 to 5.1 volts. Ground is
provided through the low-noise, sensor return circuit
at the PCM.
The MAP sensor input is the number one contributor
to fuel injector pulse width. The most important
function of the MAP sensor is to determine barometric
pressure. The PCM needs to know if the vehicle is
at sea level or at a higher altitude, because the air
density changes with altitude. It will also help to correct
for varying barometric pressure. Barometric
pressure and altitude have a direct inverse correlation;
as altitude goes up, barometric goes down. At
key-on, the PCM powers up and looks at MAP voltage,
and based upon the voltage it sees, it knows the
current barometric pressure (relative to altitude).
Once the engine starts, the PCM looks at the voltage
again, continuously every 12 milliseconds, and compares
the current voltage to what it was at key-on.
The difference between current voltage and what it
was at key-on, is manifold vacuum.
During key-on (engine not running) the sensor
reads (updates) barometric pressure. A normal range
can be obtained by monitoring a known good sensor.
As the altitude increases, the air becomes thinner
(less oxygen). If a vehicle is started and driven to a
very different altitude than where it was at key-on,
the barometric pressure needs to be updated. Any
time the PCM sees Wide Open Throttle (WOT), based
upon Throttle Position Sensor (TPS) angle and RPM,
it will update barometric pressure in the MAP memory
cell. With periodic updates, the PCM can make
its calculations more effectively.
The PCM uses the MAP sensor input to aid in calculating
the following:
² Manifold pressure
² Barometric pressure
² Engine load
² Injector pulse-width
² Spark-advance programs
² Shift-point strategies (certain automatic transmissions
only)
² Idle speed
² Decel fuel shutoff
The MAP sensor signal is provided from a single
piezoresistive element located in the center of a diaphragm.
The element and diaphragm are both made
of silicone. As manifold pressure changes, the diaphragm
moves causing the element to deflect, which
stresses the silicone. When silicone is exposed to
stress, its resistance changes. As manifold vacuum
increases, the MAP sensor input voltage decreases
proportionally. The sensor also contains electronics
that condition the signal and provide temperature
compensation.
The PCM recognizes a decrease in manifold pressure
by monitoring a decrease in voltage from the
reading stored in the barometric pressure memory
cell. The MAP sensor is a linear sensor; meaning as
pressure changes, voltage changes proportionately.
The range of voltage output from the sensor is usually
between 4.6 volts at sea level to as low as 0.3
volts at 26 in. of Hg. Barometric pressure is the pressure
exerted by the atmosphere upon an object. At
sea level on a standard day, no storm, barometric
pressure is approximately 29.92 in Hg. For every 100
feet of altitude, barometric pressure drops .10 in. Hg.
If a storm goes through it can change barometric
pressure from what should be present for that altitude.
You should know what the average pressure
and corresponding barometric pressure is for your
area.

 

thats nice but map sensore read voltage and vac there is no psi to the map. should be around 5 volts key on eng off 0 vac key on eng running should drop in voltage to about
1.50 to 2.5 depend on where you are vac shoud be eng vac at idle

 

Well i guess i mean the manifold pressure then.

 

Like previously stated , the pressure will vary. If you are at 8 , that is basically in the middle range of the tolerance 0 to 15. But it depends on many things like in Aims post. I'd say your ok there. Did you see that reading on the scanner you talked about? You would be better off checking the vacuum with a gauge. It can tell you a lot about the condition of things.