Manifold absolute pressure sensor
Manifold absolute pressure sensor refers to the manifold-pressure sensor used by the Pontiac Fiero ECM.
The manifold absolute pressure sensor tells the ECM how much pressure or vacuum is present in the intake manifold. The ECM uses this information to estimate engine load and to calculate fuel delivery, ignition timing, and drivability behavior.
Overview
The manifold absolute pressure sensor is often abbreviated as MAP sensor.
The MAP sensor is one of the main engine-load sensors used by the Fiero ECM. When intake manifold vacuum changes, the MAP sensor signal changes. The ECM uses this signal together with other inputs such as throttle position, coolant temperature, air temperature, oxygen sensor activity, and engine rpm.
A faulty MAP sensor, poor vacuum connection, damaged hose, wiring problem, or connector problem can cause major drivability issues.
Location
Image wanted: Photo or diagram showing the manifold absolute pressure sensor location. Ideally include both the 2.8L V6 and 2.5L four-cylinder locations if different.
The MAP sensor is mounted in the engine compartment and connected to intake manifold vacuum.
On the 2.8L V6, the MAP sensor is located near the upper intake / throttle body area and is connected to manifold vacuum by a short vacuum hose.
On the 2.5L four-cylinder, the MAP sensor is also located where it can receive an intake manifold vacuum signal, though the exact mounting location and appearance may vary by year and engine configuration.
The MAP sensor should not be confused with the Manifold air temperature sensor. The MAP sensor measures manifold pressure, while the MAT sensor measures incoming air temperature.
Sensor operation
The MAP sensor converts intake manifold pressure into an electrical signal for the ECM.
In general:
- High vacuum / low manifold pressure produces a lower load reading.
- Low vacuum / high manifold pressure produces a higher load reading.
- Opening the throttle usually lowers vacuum and increases the MAP reading.
- Closing the throttle usually increases vacuum and lowers the MAP reading.
The ECM uses this information to help determine how much air the engine is taking in.
Related trouble codes
MAP-sensor-related diagnostic trouble codes include:
- Code 33 - MAP sensor circuit, high voltage or unusually low vacuum / high pressure indication
- Code 34 - MAP sensor circuit, low voltage or unusually high vacuum / low pressure indication
Code 33
Code 33 indicates that the ECM is seeing a MAP sensor signal that represents unusually low vacuum or high manifold pressure.
Possible causes include:
- Disconnected MAP sensor vacuum hose
- Leaking MAP sensor vacuum hose
- Faulty MAP sensor
- MAP sensor wiring problem
- Poor connector contact
- Vacuum leak
- Engine running poorly enough to create low vacuum
- ECM input problem
Code 34
Code 34 indicates that the ECM is seeing a MAP sensor signal that represents unusually high vacuum or low manifold pressure.
Possible causes include:
- Faulty MAP sensor
- MAP sensor wiring problem
- Poor connector contact
- Open or shorted MAP sensor circuit
- Blocked MAP sensor vacuum hose
- Incorrect MAP sensor vacuum signal
- ECM input problem
Symptoms
Possible symptoms of a MAP sensor or MAP circuit problem include:
- Check engine light
- Hard starting
- Rough idle
- Hesitation
- Stalling
- Poor acceleration
- Poor fuel economy
- Rich running
- Lean running
- Black exhaust smoke
- Poor drivability
Diagnosis
Basic diagnostic checks include:
- Check for stored diagnostic trouble codes.
- Inspect the MAP sensor vacuum hose.
- Check for a disconnected, cracked, leaking, blocked, or incorrectly routed vacuum hose.
- Inspect the MAP sensor connector.
- Inspect MAP sensor wiring for damage.
- Check for corrosion or poor contact at the connector.
- Check for vacuum leaks.
- Check for other stored trouble codes.
- Compare MAP data through ALDL diagnostics if live data is available.
- Check ECM connector condition if the sensor, vacuum hose, and wiring appear correct.
Vacuum hose
The MAP sensor depends on an accurate manifold vacuum signal. A bad vacuum hose can make a good MAP sensor report incorrect information to the ECM.
Common hose problems include:
- Cracked rubber hose
- Disconnected hose
- Loose hose ends
- Collapsed hose
- Blocked hose
- Incorrect hose routing
Related topics:
ALDL data
Live ECM data through ALDL diagnostics can be useful when diagnosing MAP sensor problems.
If the MAP value is obviously wrong for the engine condition, the vacuum hose, sensor, wiring, connector, and ECM input should be checked.
MAP readings should be interpreted together with engine rpm, throttle position, coolant temperature, and other sensor data.
Relationship to fuel mixture
The MAP sensor is one of the ECM's most important fuel-calculation inputs.
A wrong MAP signal can cause the ECM to calculate engine load incorrectly, which may result in:
- Too much fuel
- Too little fuel
- Poor throttle response
- Rough running
- Stalling
- Poor fuel economy
Related inputs include:
Notes
A MAP sensor trouble code does not automatically mean the MAP sensor itself has failed.
Vacuum hose problems, wiring faults, poor connectors, vacuum leaks, engine mechanical problems, and ECM input problems can all cause MAP-related trouble codes.
Always inspect the vacuum connection before replacing the sensor.