How the MAP sensor works
External air pressure is similar to the pressure in the intake manifold when the engine is off. However, when you start the engine, the pumping action of the piston and throttle valve creates more restriction, creating a vacuum in the intake manifold.
When the engine is running and the throttle is wide open, there is virtually no vacuum pressure in the intake manifold, which means the pressure there is equivalent to the air pressure outside.
Air pressure ranges from 28 inches of mercury to 31 inches of mercury. The exact amount depends on your location and current weather conditions.
Higher altitudes have less air pressure and lower altitudes have more air pressure. At sea level, the air pressure is about 14.7 pounds per square inch.
The vacuum created in the intake manifold can be anywhere between 0 and 22 inches of mercury. Depending on your operating conditions, this value can be higher. Most cars have an idle vacuum of 16 to 20 inches of mercury. When you close the throttle while braking, the vacuum is even higher.
This is because the piston can’t draw air when the throttle valve is closed. This creates a much larger vacuum in the intake manifold, perhaps about 5 inches higher.
When you accelerate and open the throttle, a lot of air enters the engine, causing the vacuum to go to zero.
When you turn the ignition key on before starting the engine, the Engine Control Unit receives a reading from the MAP sensor. This lets the module know the air pressure (also called atmospheric pressure).
The Engine Control Unit takes this information into account when adjusting the fuel-air mixture. This is useful when outside air pressure changes due to weather or altitude. Some vehicles measure using the BARO sensor, while others measure both sensors using the BMAP sensor.
MAP Sensor Replacement Cost
The cost to replace a MAP sensor ranges from $200 to $300. This includes $100-200 in labor and $100-200 in parts.
The replacement itself is often very quick. The MAP sensor is located on or near the intake manifold. The sensor is easy to remove and replace since it is only attached with a clip or screw. A mechanic will check the sensor to ensure it is working properly. It is such a simple job that many amateur mechanics can replace the MAP sensor themselves.
