The Manifold Absolute Pressure (MAP) sensor is a key sensor because it senses engine load.
The sensor generates a signal that is proportional to the amount of vacuum in the intake manifold.
The engine computer then uses this information to adjust ignition timing and fuel enrichment.
5psi of MAP (Live Data PID) = 14.7psi (atmospheric pressure) – 9.7psi (manifold vacuum, gauge readings)
PWT Management
When the engine is working hard, intake vacuum drops as the throttle opens wide. The engine sucks in more air, which requires more fuel to keep the air/fuel ratio in balance. 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. 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. When conditions change and the vehicle is cruising along under light load, coasting or decelerating, less power is needed from the engine. The throttle is not open very wide or may be closed causing intake vacuum to increase. 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.
About Vacuum
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. 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.
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. Some vehicles use a separate “baro” sensor for this purpose, while others use a combination sensor that measures both called a BMAP sensor.
Turbo
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. But the MAP sensor doesn’t care because it just monitors the absolute pressure inside the intake manifold.
MAP vs Boost Pressure Sensor
Map sensor reads absolute air pressure, i.e.. atmospheric is 14.7 psi.
Boost gauge reads pressure relative to atmospheric. e.g. 10 psi boost is 10 psi above atmospheric, which equals 14.7 + 10 = 24.7 psi of absolute pressure.
Airflow estimation
On engines with a “speed-density” electronic fuel injection system, airflow is estimated rather than measured directly with an airflow sensor. The computer looks at the MAP sensor signal along with engine rpm, throttle position, coolant temperature and ambient air temperature to estimate how much air is entering the engine. The computer may also take into account the oxygen sensor rich/lean signal and the position of the EGR valve, too, before making the required air/fuel mixture corrections to keep everything in balance. This approach to fuel management isn’t as precise as systems that use a vane or mass airflow sensor to measure actual airflow, but it is not as complex or as costly either.
Less sensitive to vacuum leaks. Any air that leaks into an engine on the back side an airflow sensor is “un-metered” air and really messes up the fine balance that’s needed to maintain an accurate air/fuel mixture. In a speed-density system, the MAP sensor will detect the slight drop in vacuum caused by the air leak and the computer will compensate by adding more fuel.
On many GM engines that have a mass airflow sensor (MAF), a MAP sensor is also used as a backup in case the airflow signal is lost, and to monitor the operation of the EGR valve. No change in the MAP sensor signal when the EGR valve is commanded to open would indicate a problem with the EGR system and set a fault code.
Analog MAP Sensors
The MAP sensor consists of two chambers separated by a flexible diaphragm. One chamber is the “reference air” (which may be sealed or vented to the outside air), and the other is the vacuum chamber which is connected to the intake manifold on the engine by a rubber hose or direct connection. The MAP sensor may be mounted on the firewall, inner fender or intake manifold.
A pressure sensitive electronic circuit inside the MAP sensor monitors the movement of the diaphragm and generates a voltage signal that changes in proportion to pressure. This produces an analog voltage signal that typically ranges from 1 to 5 volts.
Analog MAP sensors have a three-wire connector: ground, a 5-volt reference signal from the computer and the return signal. The output voltage usually increases when the throttle is opened and vacuum drops. A MAP sensor that reads 1 or 2 volts at idle may read 4.5 volts to 5 volts at wide open throttle. Output generally changes about 0.7 to 1.0 volts for every 5 inches Hg of change in vacuum.
Digital MAP Sensors
Ford
Ford BP/MAP sensors (barometric pressure/manifold absolute pressure) also measure load but produce a digital frequency signal rather than an analog voltage signal. This type of sensor has additional circuitry that creates a 5 volt “square wave” (on-off) voltage signal. The signal increases in frequency as vacuum drops.
At idle or when decelerating, vacuum is high and the BP/MAP sensor output may drop to 100 Hz (Hertz, or cycles per second) or less. At wide open throttle when there is almost no vacuum in the intake manifold, the sensor’s output may jump to 150 Hz or higher. At zero vacuum (atmospheric pressure), a Ford BP/MAP sensor should read 159 Hz.
Anything that interferes with the MAP sensor’s ability to monitor the pressure differential may upset the fuel mixture and ignition timing. This includes a problem with the MAP sensor itself, grounds or opens in the sensor wiring circuit, and/or vacuum leaks in the intake manifold (airflow sensor systems) or hose that connects the sensor to the engine.
Typical drivability symptoms that may be MAP related include:
Surging
Rough idle
A rich fuel condition, which may cause spark plug fouling.
Detonation due to too much spark advance and a lean fuel ratio.
Loss of power and/or fuel economy due to retarded timing and an excessively rich fuel ratio.
A vacuum leak will reduce intake vacuum and cause the MAP sensor to indicate a higher than normal load on the engine. The computer will try to compensate by richening the fuel mixture and retarding timing — which hurts fuel economy, performance and emissions.
MAP Sensor Testing
Waveform Analysis
First, make sure engine manifold vacuum is within specifications at idle. A low intake vacuum reading or excessive backpressure in the exhaust system can trick the MAP sensor into indicating there is a load on the engine. This may result in a rich fuel condition. A good MAP sensor should read barometric air pressure when the key is turned on before the engine starts.
Unusually low vacuum may be due to:
vacuum leak,
retarded ignition timing, an
exhaust restriction (clogged converter), or an
EGR leak (EGR valve not closing at idle).
Unusually high vacuum may be due to:
A restriction in the air intake (such as a plugged air filter). This would result in a load low indication from the MAP sensor and possibly a lean fuel condition.
Vacuum Pump Test
Check the sensor’s vacuum hose for kinks or leaks.
Then use a hand-held vacuum pump to check the sensor itself for leaks.
The sensor should hold vacuum.
Any leakage calls for replacement.
Throttle Test
Scan tools that display OBD II data will also display a “calculated load value” that can be used to determine if the MAP sensor is working or not. The load value is computed using inputs from the MAP sensor, TPS sensor, airflow sensor and engine speed. The value should be low at idle, and high when the engine is under load. No change in the value, or a higher than normal reading at idle might indicate a problem with the MAP sensor, TPS sensor or airflow sensor.
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