Modern-day cars are an amalgamation of thousands of smaller, individual components working together in unison. There are several systems in place, mechanical and electronic, that are interdependent and essential to the normal functioning of a vehicle. One of these components is called the Mass Air Flow (MAF) Sensor, and it is perhaps the most crucial element in a car’s electrical fuel injection system. If something were to go wrong, it could result in serious problems.
Bad MAF Sensor symptoms include rough idling, loss of power while accelerating, starting trouble, and more. This guide will tell you everything there is to know about a faulty MAF sensor, from why it’s an integral part of your car to what you should look for while replacing one.
What is a MAF Sensor?
A mass airflow sensor determines the airflow rate entering an engine. It then relays this data to the ECU, which delivers the right amount of fuel to the engine.
As we all know, an engine requires two things to operate — fuel and oxygen (air). When an engine is running, the density of the air (and thus oxygen concentration) is constantly changing thanks to parameters like ambient temperature, altitude, and the induction method on the engine.
The MAF sensor, placed between the engine’s intake manifold and the air filter, determines the oxygen entering the combustion chamber. As you can imagine, this is an indispensable piece of engineering as it’s responsible for maintaining the proper air-fuel mixture in the combustion chamber.
How Does a MAF Sensor Work?
Today’s BMWs use a hot-wire type of MAF sensor. Essentially, it measures the temperature drop on a consistently heated wire to assess the density of air passing over it.
A hot-wire mass airflow sensor has a small wire suspended in the engine’s air stream. This wire is electrically heated using a constant voltage and has another sensor to measure its temperature. When the car is turned on, and the engine is idling, a small amount of air flows over the wire. This cools the wire and reduces its electrical resistance, allowing more current to flow through the circuit (since supply voltage is constant.)
Similarly, the engine draws more air that passes over the wire, cooling it even more when you accelerate. A higher current passes through the wire to keep it hot when this happens. This increase and decrease in current is directly proportional to the mass of air flowing past the wire and is monitored by an integrated electronic chip in the sensor.
The chop converts the electrical signal into a digital one. It relays it to the ECU, which uses this data to determine how much fuel it should inject into the cylinder to keep the air-fuel ratio optimal.
Bad MAF Sensor Symptoms
If the MAF sensor fails, the ECU or PCM (Powertrain Control Module) will not be able to send the right amount of fuel into the engine. As a result, it can end up in a lean or rich mixture inside the cylinders, affecting the normal functioning of the engine. Other bad MAF sensor symptoms include difficulty starting the engine, rough idling, and a drop in performance.
Check Engine Light Turns On
If there’s something wrong with the MAF sensor, it will likely illuminate the check engine light (CEL) on your dash. An illuminated CEL is not something you should ignore in any case and may be triggered by several issues, apart from a bad MAF sensor.
You can use an OBD-II tool to figure out the exact source of the problem. The most common MAF sensor-related diagnostic trouble code (DTC) is P0101, which means “Mass Air Flow (MAF) Circuit Operating Range or Performance Problem.”
Misfiring and Rough Running
As we mentioned above, a faulty MAF sensor can result in the air-fuel mixture being too rich or too lean. In such a case, the engine will continue to run, but since the mix isn’t optimal, it will cause inefficient combustion and misfiring or rough running.
You may also notice this when the engine is idling. If the air-flow mixture isn’t steady, it will cause improper idling and engine ‘knocking.’ In some cases, if the mixture is too lean, it may result in the engine dying.
Black Exhaust Smoke
When the air-fuel mixture is excessively rich, it can result in black smoke from the tailpipe. This is because the engine won’t be able to appropriately combust all the fuel, resulting in excessive emission. Thick, black exhaust smoke can also cause your cat-con to wear out faster.
Difficulty Starting/Turning the Engine Over
When you turn the key or push a button to start your engine, extra fuel is sprayed into the cylinder to initiate combustion. If the MAF sensor is bad, it may not realize that a momentary spurt of a rich mixture is required, resulting in starting trouble.
Bad Fuel Economy
This one is relatively self-explanatory. A bad MAF sensor signal to the ECU can inject extra fuel into the cylinder, reducing fuel efficiency. Alternatively, a lean mixture may even result in increased fuel efficiency. However, this is likely damaging your engine, so it’s not necessarily a reason to celebrate.
Loss of Power
While accelerating, the engine demands more air and fuel. If the MAF sensor can’t communicate this requirement to the ECU, the latter will not be able to send the right amount of fuel. This can cause a drop in power while accelerating.
Can You Clean a MAF Sensor
Sometimes, bad MAF sensor symptoms are brought about by an accumulation of dirt and dust that gets past the air filter. Cleaning the sensor may resolve some of these issues, but in many cases, it may just end up delaying the inevitable replacement of the system. Nevertheless, it is worth considering if you don’t want to spend the money on a new sensor. To clean the MAF sensor:
Remove the Sensor
Cleaning the MAF sensor requires you to remove it. Getting access to it on your BMW is pretty straightforward; simply open the airbox and use a screwdriver to get it out. Be careful about all the wires and components surrounding the mass airflow sensor, as they’re pretty delicate and can be easily damaged.
Clean the Sensor
We recommend using a dedicated mass airflow sensor cleaner. They are more expensive than just wiping or blowing your sensor clean, but they will do a much better job at getting rid of the grime that’s affecting the normal functioning of the sensor.
Reinstall the Sensor
Once you’ve cleaned the sensor, you should let it dry before installing it back into your car. Installing the sensor when it still has the cleaning solution on it can damage the engine when you start it up.
A mass flow sensor is designed to last as long as the car. However, its lifecycle is dependent on the environmental factors, driving style, overall condition of the vehicle, your maintenance routine, and more.
Is a MAF Sensor Expensive?
A MAF sensor can cost anywhere between $50 to $400 depending on the quality of the part you buy and if there are labor costs involved. There are cheap, aftermarket parts out there that you can get for as low as $30, but we strongly recommend that you steer clear of buying one of these options.
The MAF sensor is one of the most critical components in the car’s fuel injection system. It’s constantly measuring air flow rate and needs to relay accurate data to the car’s computer. A cheaper system may not be as efficient in doing so. It’s also worth noting that every MAF is manufactured and tested with the car it will be a part of. It’s unlikely that an aftermarket one has the same level of calibration.
Quality Matters
While buying a cheap MAF sensor may seem like the smarter financial decision, it will likely end up costing you more in the long run. Having your MAF sensor replaced by a BMW service center can set you back around $400 to $500, and if you’re willing to spend that much money, you should.
You can also get your hands on a more affordable part from a trusted OEM brand like Bosch. The Bosch 0280218266, for instance, is our top recommendation for a MAF sensor replacement; if you replace it yourself, you’ll end up spending a fraction of what you would have if you got it replaced by a technician.
Buy a Genuine MAF Sensor
One last thing you should know is that a MAF sensor can be duplicated effortlessly. Several rip-offs are cast plastic bodies with fake labels and packaging. You should always buy your parts from a trusted source. Our catalog at Bimmers.com is filled with genuine, quality spares that are reliable and will prevent any of the bad MAF sensor symptoms we’ve discussed above. All you need to do is input your car’s data and search for the part you need!
