MAP sensor function and failure symptoms

August 26, 2017 | Electrical Repair Info

Here we will describe and discuss the manifold absolute pressure sensor of the engine. This part is commonly referred to as the MAP sensor. So the article below will explain about the MAP sensor function, working principle and the symptoms you can look out for if it were to go bad.

MAP sensor function

The MAP sensor can be found on most fuel injection engines made today. These are engines that use high pressure to pump gasoline which then vaporizes it. What the MAP sensor does is it assists the engine in figuring out what it needs to have the best combustion possible. This is done by calculating how much air is entering the engine, the number of rotations in the engine per minute, and the temperature of the air. Once this is all figured out, the electronic control unit of the engine calculates the air density within the fuel mixture. Based on this calculation, the electronic control unit will regulate the flow of fuel and the flow of air accordingly so that the best combustion possible can be performed.

Common symptoms of faulty MAP sensor

You will know when you have a bad MAP sensor because the signs will be easily noticeable. You may experience a weak engine performance while accelerating, decelerating or while leaving the car idle. Below is a bigger description of the main signs.

1) Reduced Power

If your MAP sensor is faulty then it may wrongfully determine that the intake manifold pressure is low. This will indicate to the powertrain control module that there is a much smaller engine load. As a result, the engine will have less fuel injected into it. This might sound good because it will help your fuel economy. However, your engine will not be as powerful because it doesn’t have enough fuel being injected into it. This also causes the temperatures in the combustion chamber to increase and the amount of nitrogen oxides being generated in the engine to increase. This creates smog too.

2) Low Fuel Economy

If your MAP sensor is faulty then it may wrongfully determine that the intake manifold pressure is high. This will indicate to the powertrain control module that there is a much bigger engine load. As a result, the engine will have more fuel injected into it. This means you’ll need to pump more fuel into the gas tank to keep up with this increased demand for fuel from your car. Not only will your fuel economy decrease, but the amount of carbon monoxide and hydrocarbon emissions will increase. These chemical components are what makes up smog.

3) Does Not Pass the Emissions Test

If you have a faulty MAP sensor in your vehicle, then it will not pass an emissions test. The emissions coming out of your tailpipe will have a lot of hydrocarbons, carbon monoxide and nitrogen oxides in them.

MAP sensor working principle

The outer barometric pressure is similar to the intake manifold pressure when the engine is off. But when you turn on the engine, the interior of the intake manifold has a vacuum created there because of the pistons’ pumping motion and the throttle plates creating more restriction. If the engine is running and there is a fully open throttle, then there is virtually no vacuum pressure in the interior of the intake manifold. This causes the pressure in there to be the same as the outer barometric pressure.

The amount of barometric pressure could be anywhere from 28 inches of Mercury all the way to 31 inches. The exact amount is dependent on where you are located and the current conditions of the climate. You will have lower air pressure in locations with higher elevations and higher air pressure in places with lower elevations. At sea level, the air pressure is about 14.7 pounds per square inch.

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That vacuum that gets created in the interior of the intake manifold can have 0 to 22 inches of Mercury. Depending on the conditions of operation, this number could be higher. An idle vacuum in the majority of cars will have between 16 and 20 inches of Mercury. You will have a vacuum at an even higher level if the throttle is closed while you’re decelerating. This happens because the pistons can’t bring in more air through the closed throttle. This results in a much larger vacuum forming in the interior of the intake manifold, probably about 5 inches of Mercury higher. When you accelerate and the throttle opens, large amounts of air are brought into the engine and the vacuum level falls all the way down to zero. As the throttle gradually closes, the vacuum gradually gets higher.

Read also: 5 Symptoms of a Bad Throttle Position Sensor

As you turn on the ignition key prior to the engine starting, the powertrain control module gets a reading of the MAP sensor. This tells the module what the barometric pressure is (also known as atmospheric pressure). The powertrain control module takes this information into consideration when regulating the mixture of fuel and air. This comes in handy when the air pressure outside changes due to weather or location elevation. You may have a vehicle that uses a BARO sensor to measure this while other vehicles use a BMAP sensor to measure both.

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Cost of Replacing the MAP Sensor

The cost of replacing the MAP sensor could be anywhere from $129 to $203. This includes the cost of labor which is from $17 to $23 and the cost of parts which is from $112 to $180. These features do not include the additional fees and taxes that you’ll have to pay too.

The replacement job itself is very fast. The MAP sensor is located on or near the intake manifold. It is only fastened down by a clip or screw so it will be easy to take this off and replace the sensor. The mechanic will check the sensor and make sure it is operating properly.

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