How Can You Tell If A MAP Sensor Is Bad?

A failing MAP sensor will leave your engine running badly. Early symptoms of a failing MAP sensor include engine misfires, decreased fuel efficiency, rough engine idle and decreased power especially when accelerating. 

The MAP sensor is one of a group of realtime sensors that the ECU relies on for up-to-date information to keep everything running as it should. When it comes to troubleshooting an engine problem, I always start by checking for error codes in the ECU that may point to a faulty sensor. 

When it comes to the smooth running of your car’s engine, there are a few important sensors that must work properly. Many engine problems are caused by dirty, failing and worn out sensors so checking for an error code is a great place to start.

9 Symptoms Of A Bad MAP Sensor

You should always start troubleshooting by hooking the engine up to a diagnostic reader. This can save you a lot of time by giving you an error code that will point you in the right direction. In my experience, this is the quickest way to fix an engine problem and in many cases will lead you directly to a faulty sensor.

Evaporative Emission Control System Incorrect Purge Flow

Checking the ECU for error codes can save you lots of time when troubleshooting a problem with engine performance

If you are going to rely on engine performance related symptoms just remember that these can be caused by faults with other parts of the engine. Problems such as faulty fuel injectors, dirty fuel filters, a faulty MAF sensor, vacuum leaks and problems with the fuel pump can manifest themselves as hard to start engines or engines that are low on power, especially when accelerating.

A faulty MAP sensor may show up as an intermittent problem, and sometimes simply cleaning the sensor will resolve the issue. It can be difficult to pinpoint exactly where the problem lies, and it may be necessary to get an expert opinion and to monitor the engine performance using real time diagnostics. 

Here are some general performance related symptoms that may point to a faulty MAP sensor:

1. Check Engine Light 

Should not be a surprise that many trouble codes are related to the MAP sensor. Over the years, ECU’s self-diagnostics routines have been improved to isolate almost any imaginable MAP condition like ground line loss, intermittent signal, reference voltage, bad sensor, and so on. However, keep in mind that its particular characteristics make this sensor sensible to false positives, especially when a vacuum leak is present.

2. Decreased Engine Performance 

Depending on  ECU’s programming and emergency mode strategies you may experience variable levels of lack of power during acceleration. In general terms, the ECU cannot determine the engine’s load anymore and thus it needs to extrapolate the information from the rest of its sensors to calculate an approximate value. That value is then used and depending on the results compensated using the oxygen sensor closed-loop feedback.

3. Hard Starting 

Once again, this is highly conditioned to ECU’s software. It’s not uncommon finding cars with a no-start condition when the MAP is not reporting to the ECU, however, the contrary is also true. Many vehicles start without any problem even if the MAP is disconnected or faulty. There are infinite variations to the aforementioned extreme conditions, as well as a myriad of other reason that could affect the engine’s starting. Keep all this in mind when facing a possible MAP problem.

4. Poor Fuel Economy 

Without MAP’s crucial information the ECU will have a hard time adjusting the air-fuel ratio. Expect a noticeable drop in fuel economy as a direct consequence. 

5. Rough Idle 

As part of the compensation process employed by the ECU, the fuel mixture will be constantly changing between lean and rich. This variation may cause a rough idle in some vehicles.

It is difficult to predict how rough the idle could be due to other engine conditions that could be accompanying the MAP failure. A clogged air filter, a restricted exhaust system or a vacuum leak could also impact idle performance. Unfortunately, without the MAP signal, the ECU won’t be able to compensate for such combination of issues.

6. Engine Misfires 

As a collateral problem derived from MAP failure and constant air-fuel ratio changes are engine misfires.

Engine misfires should be taken seriously as they can potentially damage your engine. If you are experiencing noticeable misfires and/or exhaust backfiring then it’s highly recommended to stop using the vehicle until it can be checked out.

7. Gas Smell Coming From The Exhaust Pipe 

Another direct outcome of a bad MAP is the gas smell coming from the exhaust. A typical problem when combustion is not completed in the chamber is the fuel particles flowing through the exhaust system.

Don’t overlook this symptom because it’s more delicate than you may think. Besides the smell and possible smoke coming from the exhaust, the catalytic converters might be suffering irreversible damage as liquid particles such as unburnt fuel are very bad for catalytic converters.

8. Barometric Issues 

This symptom may be difficult to experience unless you are making a long trip with noticeable barometric changes.

For example, if you turn on the engine at the base of a mountain and start driving uphill you will experience a progressive lack of power due to the inability of the ECU to compensate for the variation in barometric pressure.

9. Failed Emission Tests 

Last but not less your car won’t be able to pass any emission test under these conditions. If you live in an area with strict smog regulations a faulty MAP could cause you legal problems. Keep this in mind if you are plan using your car with the MAP problem present. 

What is a MAP sensor?

The manifold absolute pressure sensor (MAP sensor) is a small sensor and is located on the manifold (often at the top or back of the engine under the windshield)

MAP Sensor
A typical MAP sensor

The MAP sensor is responsible for calculating the internal air pressure of the engine manifold and relaying this information to the ECU in realtime. 

Air always comes in to the engine via the air filter and mass air flow sensor. The information captured by the MAP sensor is then used to calculate the air density of the air coming into the engine. 

When the ECU knows the amount of air that is coming in it can then calculate the correct fuel to air ratio, helping to keep the engine running smoothly under all current conditions.

How Does the ECU use MAP Sensor Information?

The MAP sensor information is crucial during the air-fuel ratio calculation. This has to do with how internal combustion engines work.

The internal pressure of the intake manifold  is a direct indicator of the engine’s load (amount of power used by the engine).

The ECU uses the data provided by the MAP sensor in two ways:

1. Barometric Pressure readings

Many vehicles use the MAP sensor to get barometric pressure values. This is possible because there are at least two times when the intake manifold pressure equals barometric pressure (atmospheric pressure).

The first one is just before starting the engine. When you turn the ignition switch on the ECU takes a “snapshot” of all sensor readings. The MAP pressure at this point is exactly the atmospheric pressure.

The second occasion when the MAP equals barometric pressure is during Wide Open Throttle (WOT) acceleration. For a fraction of second, the intake manifold loses its vacuum and equalizes with the atmospheric pressure.

Since the oxygen level in the air varies with altitude, the barometric pressure provided by the MAP/BARO sensor is used for corrections in the air-fuel delivery calculations.

2. Engine Load Calculations

As mentioned earlier, intake manifold pressure is directly proportional to the engine’s load. As you increase speed, the engine will burn fuel at a higher rate and will need more air to do so.

Thanks to the MAP sensor the ECU can compensate the amount of fuel needed as the engine demands it.

Each time you press the accelerator pedal you are asking for more power and thus increasing the engine’s load.  The opposite is also true. As you release the accelerator pedal the throttle closes and thus intake manifold pressure decreases.

How To Fix a Bad MAP Sensor

If your code reader detects P0106 or P0105, then the next step is to remove and inspect the MAP sensor. You will need to check your vehicle’s repair manual for it’s location. Usually, the MAP sensor is directly placed on the intake manifold or near it (when it uses a vacuum hose).

Once you have located the sensor, you will need to remove it and inspect it for dirt or oil, and also make sure the wiring is not frayed or broken.

It’s very common over time for MAP sensors to get dirty. The inside of the intake manifold can become clogged up with dirt and carbonated deposits, especially in diesel engines. This dirt then transfers onto the tip of the MAP sensor and can mess up the readings.

Sometimes, you can clean a MAP sensor and this will improve performance and because it’s an easy fix it’s always worth a shot.

How To Clean A MAP Sensor:

  1. Locate and remove the MAP sensor from the intake manifold (refer to a repair manual if you can’t locate it). Be careful when removing it that you don’t drop it down the back of the engine or break the wiring or connector. If it’s an older car, wiring can become brittle and lose its flexibility.
  2. Inspect the sensor for cranks in its housing, scorch marks or problems with the connector. If it all looks good, then it may be worth cleaning.
  3. To clean the sensor, you’ll need a good quality MAP sensor spray cleaner. If you can’t find one then a mass air flow cleaner will do the job too.
  4. Spray the cleaner into the nozzle part of the sensor ( the part that pushes into the manifold). Don’t be tempted to push a screwdriver or anything pointy in there, let the cleaner do the work.
  5. After a few minutes, spray some more cleaner in there and gently shake the sensor, nozzle down to dislodge any dirt.
  6. You may need to repeat this a few times until it’s clean.
  7. It’s also a good idea to clean the inlet manifold too. If you are capable, then the best way to do this is to remove it from the car and give it a thorough cleaning.
  8. If you can’t remove it from the car, then spray a clean rag with some MAP cleaner and clean around the MAP sensor mounting hole. You may not have access to it if it’s at the back of the engine, in this case you’ll have to remove the manifold to clean it.
  9. Once everything is clean and dry, refit the MAP sensor, clear any engine codes and bring the car for a drive to see if the error codes reappear.

If  cleaning it doesn’t work, then it’s not possible to actually fix a MAP sensor. Over time, engine sensors tend to lose their sensitivity. Usually the best option is to just replace the sensor that is throwing up error codes.

In some cases, replacing the MAP sensor will not fix the problem. There may be a leak somewhere in the air intake system that is causing the error.

You may need to inspect the vacuum hoses, air filter or EGR valve to find the cause of the problem. Check out my article on error code P0106 and how to diagnose a faulty EGR valve for more detailed information.

Here’s a brief synopsis of what’s involved:

 Vacuum hoses: perform a meticulous visual inspection of all vacuum lines, including MAP sensor, PCV, brakes vacuum booster, fuel pressure regulator, etc. Also, check the air intake hose looking for any sign of deterioration or leak. When the MAP/BARO sensor is installed directly on the intake manifold it uses a rubber o-ring type seal. Check its condition and replace as necessary.

Exhaust Gas Recirculation valve inspection: perform a thorough inspection of the EGR valve exterior, tubing, and connections. Look for possible exhaust leaks in the EGR gasket and connections. Replace the gasket and/or EGR valve if necessary.