WHAT IS A MASS AIR FLOW SENSOR? WHAT DOES A MAF DO?

WHAT IS A MASS AIR FLOW SENSOR?

The mass air flow sensor (MAF) is a key component of the electronic fuel injection system in a car. It is located between the air filter and the engine’s intake manifold. In some cars, the intake air temperature (IAT) sensor is built into the mass air flow sensor. However, there are various types of air flow sensors and all require different maintenance. Digital sensors send a frequency output, while analog mass air flow sensors produce a variable voltage output. Both outputs are sent to the engine control module to help regulate air-fuel mixture in the car’s automatic transmission system.

WHAT DOES IT DO?

Mass air flow sensors measure the air mass flowing into the engine intake. This is important for calculating the amount of fuel to add to achieve the proper air-fuel ration (AFR), Ideal AFR is 14.7:1 (14.7 lb of air to 1.0 lb. of gasoline), but AFR can vary. Acceleration requires AFR of up to 12:1, while cruising requires levels as low as 22:1. A faulty mass air flow sensor can make it difficult or impossible for the engine control module (ECM) to calculate fuel injection correctly. This can cause a myriad of other problems for your vehicle.

SYMPTOMS OF MASS AIR FLOW SENSOR FAILURE

Over time, your mass air flow sensor can degrade and become prone to failure. This is due to the fact that it is constantly exposed to moving air, which is full of exhaust fumes, debris, and dust. Even becoming too dirty can cause it to stop working, so it’s important to have a local mechanic take a look if you notice any of the following symptoms in your vehicle’s performance. In many vehicles, especially those made by Toyota, flow sensors built into the compressor have a high rate of failure. A bad air flow sensor may show as code B1479 when scanned by an automotive repair specialist.

Check Engine Light

The most common sign of mass air flow sensor failure is a lit-up check engine light. Regardless of whether there are any other signs of an issue, you should have your car checked out by your local mechanic if the light comes on. It could potentially signal a damaged or faulty mass air flow sensor.

Jerking and/or Hesitation

If you are noticing unusual hesitation and/or sudden abnormal jolts of power, this can be a sign of mass air flow sensor problems. This may happen while speeding up on the highway on-ramp or cruising down a street at regular speed. Operating a car this way is extremely dangerous, so it is important to have an auto repair expert take a look as soon as possible.

Misfiring/Rough Running

Without a proper signal from the mass air flow sensor, the powertrain control module (PCM), which manages the engine, transmission, and other systems in your car, won’t know how much fuel the engine needs. This can either cause the engine to run too lean (too little fuel) or too rich (too much fuel). The result is an incomplete combustion process, which can lead to rough engine running and misfiring.

Lean Idling

If your car seems to be struggling for power and stalling a lot, it may be getting too much air in the engine and too little fuel. This can cause the combustion process to fail, which is often affected by dirty buildup on the wires in the mass air flow sensor. A local automotive maintenance professional can get the wires back into working condition for you.

Black Exhaust Smoke

Lots of exhaust smoke coming from your car is probably a sign that something is wrong. While it can be the result of a few different issues, it could mean your mass air flow sensor is damaged, causing the engine control model to calculate improper fuel injection, resulting in black smoke.

Difficulty Starting/Turning the Engine Over

If you are having trouble starting your car or getting the engine to turn over, the air-fuel mixture might be off balance. All vehicles are very sensitive to proper air-fuel mixture and may have difficulty starting if it is thrown off by a bad mass air flow sensor. If you are experiencing this, you should take your car to an auto repair shop as soon as possible.

Poor Fuel Economy

If you’ve noticed your car suddenly needs more fuel than usual, this could be a sign of a serious issue. A fault mass air flow sensor might be sending inaccurate information to the powertrain control module (PCM), which can cause the vehicle to burn through fuel much more quickly than it should.

Full name Mass Air Flow Sensor, it’s more commonly known as a MAF sensor, air meter or sometimes simply MAF. While it might have many names, it’s responsible for just one, but still very important job: measuring the amount of air entering the engine. The ECU or PCM then uses this information to calculate the correct amount of fuel needed for an optimal air-fuel ratio. Of course, without this information the ECU will not be able to accurately control fuel injection, resulting in an engine that will either idle roughly or worse case, not at all. Since a number of other parts i.e. faulty spark plugs, wires, injectors, etc, may mirror these symptoms, MAF sensor failure can be difficult to diagnose. However, with some expert advice from OE manufacture Delphi, you’ll know what causes it, what to look out for, and crucially, how to replace it when it fails.

How does a MAF sensor work?

Installed in the intake pipe between the air filter housing and the intake manifold, most MAF sensors work on the hot wire principle. Put simply, a MAF has two sensing wires. One is heated by an electrical current, the other is not. As air flows across the heated wire, it cools down. When the temperature difference between the two sensing wires changes, the MAF sensor automatically increases or decreases the current to the heated wire to compensate. The current is then changed to a frequency or a voltage that is sent to the ECU and interpreted as air flow. The quantity of air entering the engine is adjusted accordingly.

Why do MAF sensors fail?

Since the MAF sensor is responsible for measuring air flow into the engine, they have a lot of air passing through them. In fact, more than 9000 liters of air can flow through the engine for every liter of fuel used.  That’s a lot of air! And with that comes the potential for a lot of contamination. Dust, dirt and other debris can all get into the sensor and are one of the primary causes of MAF failure.

Such contamination could occur as early as 18,000-25,000 miles, depending on the vehicle model. On small or compact cars, for example, the MAF sensor can clog quicker, as it is situated in a smaller engine bay subjected to more risk in critical areas (oil vapor flows and combustion debris). In this case, a replacement becomes the equivalent of a long drain oil service… it almost becomes a service-style repair.

Other common failure problems include:

• A contact fault at the electrical connections
• Damaged measuring elements
• Mechanical damage from vibrations or an accident
• Measuring element drift (exceeding the measuring framework)

What to look out for in a failing MAF sensor?

When a MAF sensor fails, the engine will not know the right amount of fuel to add, causing several common signs:

• Check engine light: as with most engine management components, a problem with the MAF sensor often causes the check engine light to come on.
• Engine runs rich at idle or lean under load: this would typically point towards a contaminated hot wire.
• Engine runs rich or lean:  caused by the MAF continuously misreporting airflow into the engine – a diagnostic procedure will be required to confirm this.
• Rough idle or stalls: a failed MAF sensor will not send any airflow information to the ECU preventing it from accurately controlling fuel.
• Excessive vibrations when stationary.
• RPMs changing noticeably without driver input.

Troubleshooting a MAF sensor

To identify the source of any MAF sensor faults, consider the following steps:

• Conduct an electronic test of the MAF sensor and read any fault codes using a diagnostic tool.
• Check the connector for a correct fit and good contact.
• Check the MAF sensor and measuring elements for damage.
• Check the voltage supply with the ignition switched on (circuit diagram for pin assignment is necessary). Ref. value: 7.5-14 V.
• Check the output voltage or frequency with the engine running (circuit diagram for pin assignment is necessary). Ref. value: 0.5V resp. 0 – 12.000 Hz.
• Check the connection cables between the removed control unit connector and sensor connector for transmission (circuit diagram for pain assignment necessary). Ref. value: approx. 0 ohm.

Common fault codes

Common fault codes and causes include:

• P0100: MAF circuit malfunction
• P0101: MAF circuit range/performance
• P0102: MAF circuit low input
• P0103: MAF circuit high input
• P0104: MAF circuit intermittent
• P0171 system too lean (bank 1) and P0174 system too lean (bank 2) are also often caused by a faulty or contaminated MAF sensor.

How to replace a faulty MAF sensor?

Once you’ve identified that the MAF sensor may be at fault, it’s best practice to follow these simple steps:

• To begin, connect a diagnostic tool to your vehicle.  Select the correct make, model, year, and engine code of the vehicle you’re working on.  Record the fault codes, and check the live data parameters for the MAF sensor.  Then exit the diagnostic software and switch off the ignition.
• You’ll also need to check the feeds, ground, and wiring. Connect the oscilloscope. Ideally, a break out lead should be used to prevent puncturing the wiring's insulation and causing a future wiring issue. To get a reading, snap the throttle open and observe the pattern.
• Once it’s determined that the MAF sensor is at fault, you’ll need to replace it. Remove the connector and then the fastening screws. Next, remove the sensor from its housing.
• Inspect the flow tube to ensure there are no cracks in the plastic housing. If there are, you will need to replace the entire unit – not just the probe. If the flow tube is crack-free, then you’re ok to replace the sensor probe only.
• Remember it’s important to handle the sensor connector only. Never touch the electronics as this may damage the sensor probe.
• Carefully slide the new sensor probe into the flow tube, then tighten the fasteners and replace the connector.
• Reconnect the diagnostic kit and delete any fault code(s). Run the engine, and recheck for any new fault codes.  Exit the diagnostic software and switch off the ignition. Finally, check that the check engine light has been extinguished, then carry out a road test.

3 SYMPTOMS OF A BAD MASS AIR FLOW SENSOR