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What Is A MAP Sensor & Normal Manifold Absolute Pressure? (+Symptoms, Diagnosis, FAQs)

September 22, 2022

A manifold absolute pressure sensor (or MAP sensor) measures your car’s intake manifold pressure (sometimes called the intake manifold vacuum.)

So, what is normal manifold absolute pressure? 
What happens if the manifold absolute pressure sensor is faulty?

Let’s find out.

In this article, we’ll go through what the normal manifold absolute pressure and MAP sensor are, along with the symptoms of a bad MAP sensor. We’ll also show how to diagnose, fix a faulty MAP sensor, and answer some MAP sensor FAQs.  

This Article Contains:  

Let’s get started.

What Is A Normal Manifold Absolute Pressure?

The manifold absolute pressure (MAP) is the pressure inside the engine’s intake manifold (inlet manifold). 

When the engine is off, normal manifold absolute pressure is the same as the barometric pressure (atmospheric pressure) outside your car. Under normal conditions, actual atmospheric pressure or air pressure is usually around 14.7 PSI (29.9 inHg) at sea level.      

And when the engine is running, the plunging piston motion creates a vacuum. The intake manifold vacuum reduces the original barometric pressure by around 9.7 PSI (20 inHg), so the  MAP sensor reading drops to about 5 PSI (10.1 inHg). 

A running engine with a closed throttle creates negative pressure, while the closing and opening of the throttle body valves create positive pressure (though it’s still lower than atmospheric pressure).

Let’s find out more about the MAP sensor.

What Is A Manifold Absolute Pressure Sensor?

A manifold absolute pressure sensor measures the intake manifold pressure. Typically found on the inlet manifold (either next to or on the throttle body), the MAP sensor provides intake manifold pressure information to the Engine Control Unit (ECU). 

The readings allow the ECU to calculate air density and determine the airfuel mixture for the combustion process. 

The ECU uses this data to measure engine load, fuel injection pulse, and adjust ignition timing. In aircraft, the MAP sensor is known as the manifold pressure gauge.

Here are more uses of a manifold pressure sensor:

Note: Some vehicles use a barometric pressure sensor (baro sensor) or MAF sensor instead of a MAP sensor.

So how do you know if you have a bad MAP sensor?

What Are The Signs Of A Failing MAP Sensor?

A bad MAP sensor may send a MAP sensor output that makes no sense — for example, low engine vacuum when the engine is idle. 

Here are some more signs of a manifold absolute pressure sensor malfunction:

1. Rough Idle

If your MAP sensor malfunctions, the airfuel mixture may constantly alternate between lean (low fuel trim value) and rich (high fuel trim value) — causing a rough idle. 

2. Engine Stalling

Your car’s engine may receive insufficient fuel if a bad MAP sensor sends inaccurate manifold vacuum data to the ECM. 

This may cause your engine to stall when you step on the gas as the engine doesn’t have enough power for acceleration due to insufficient fuel.

3. Illuminated Check Engine Light

When your car’s MAP sensor malfunctions, the ECM may send a diagnostic signal that illuminates the check engine light.

Remember, an engine light can mean many things, like a leaky vacuum hose, not just a bad MAP sensor. 

4. Poor Fuel Economy

If the ECM reads low intake manifold pressure due to issues like a vacuum leak, it’ll assume that the engine load is high and send more fuel to compensate. This leads to excessive fuel consumption and poor fuel economy. 

However, if the ECM reads a high intake manifold vacuum, it’ll cut off fuel injection and sparks, leading to low fuel consumption and engine power.

But how do you find out what’s causing a MAP sensor malfunction? 
Let’s dive deeper.

How To Diagnose A MAP Sensor Failure?

A bad MAP sensor is a serious issue, affecting airfuel mixture and the ignition timing. So it’s advisable to get your car checked out by an expert once you spot an issue.

Here are two ways to diagnose a MAP sensor issue:

A. Physical Test

Let’s check out how to do a physical test to diagnose a bad MAP sensor: 

B. Multimeter Test

Here’s how a multimeter helps diagnose a MAP sensor issue:

1. Power Wire Test 

2. Ground Wire Test

3. Signal Wire Test

4. IAT Wire Test

Besides MAP sensor malfunction codes, engine codes like ‘calculate load value’ on an OBD II scan tool can indicate a MAP sensor issue as engine load is measured using inputs like the MAP sensor reading and engine speed. 

Now, let’s see how a manifold pressure sensor is replaced.

How To Replace A Bad MAP Sensor?

Replacing a manifold absolute pressure sensor requires specific technical knowledge. That’s why getting help from an expert mechanic is more convenient.

Here’s a general walkthrough of how to replace a malfunctioning MAP sensor:

Let’s look at some MAP sensor-related queries.

3 Manifold Absolute Pressure Sensor FAQs

Here are some common FAQs and their answers relating to MAP sensors.

1. How Much Does A MAP Sensor Replacement Cost?

You can expect to pay around $30-$70 for labor and $30-$100 for the sensor. Depending on your auto shop, location, and vehicle model, these repair costs may vary. 

2. What’s The Difference Between A Vacuum Gauge And MAP Sensor?

The vacuum gauge measures the engine vacuum pressure inside the intake manifold. Vacuum pressure refers to a pressure lower than surrounding air pressure (negative pressure).

Meanwhile, the MAP sensor measures absolute pressure inside the intake manifold. 

3. What Is A Boost Sensor?

A boost sensor measures the amount of pressure above set absolute pressure (boost pressure), which is usually 100 kPa in a turbocharged car. 

In vehicles without a boost sensor, the manifold pressure sensor acts as a boost sensor. You can calculate boost pressure by subtracting 100 kPa from the MAP sensor signal. That’s because most boost sensors read 1 atmosphere (around 100 kPa) less than MAP sensor. 

Closing Thoughts

Incorrect manifold absolute pressure readings could severely affect your car’s performance and fuel economy. So get experts to fix such suspected issues ASAP. 

To make things easier, contact RepairSmith as soon as you spot any symptoms.

We’re a mobile vehicle repair and maintenance solution offering upfront pricing, convenient online booking, and a 12-Month | 12,000-Mile warranty on all our auto repairs 一 24/7!