BMW MAF sensor, what does it do?
The purpose of the MAF (Mass Air Flow) sensor is to measure the volume of air entering the engine. Once this sensor determines the amount of air, it sends this information to the DME (Digital Motor Electronics, essentially the brain of the engine) which then determines the correct amount of fuel to be injected into the engine. Ensuring a proper air / fuel mixture ratio will ensure a smooth running engine optimizing performance. A faulty MAF sensor will hurt the performance of the vehicle.
Where is the MAF sensor located?
The mass air flow sensor is between the air box and the throttle body, typically with a wired connector attached to it. See image below for further clarification.
What are symptoms of a bad MAF sensor?
With a faulty MAF sensor, proper air fuel mixture will result in poor drivability. An air fuel ratio with too much fuel, referred to as “running rich” may result in a check engine light illuminated, a poor idle, poor fuel economy and or black smoke exiting the exhaust. An air fuel ratio with too little fuel, referred to as “running lean” may result in a check engine light, difficulty with engine starts and or acceleration hesitation or sudden jerking motions. Further symptoms may include bouncing off the engine RPM at idle.
Since the MAF sensor runs off of vacuum, it is critical all of the vacuum hoses in the engine are properly sealed with no vacuum leaks. If air is entering a cracked vacuum hose AFTER the MAF sensor, this air intake will not be taken into account in the proper fuel mixture. The DME only measures what passes through the MAF sensor, not air entering the engine through any other location.
How do I test for a faulty MAF sensor?
If you have any of the symptoms mentioned above, you can troubleshoot for a faulty MAF sensor as follows. Unplug the electrical connector located on the mass air flow sensor. At this point the DME will use default values to account for air flow. If the engine instantly runs smoother after unplugging the mass air flow sensor, you likely have a faulty unit. On the other hand, if the engine starts to run rough and or stall, the MAF is likely working as intended.
How do I repair a faulty MAF sensor?
The simplest way to repair a mass air flow sensor is to clean the delicate wire which measures the airflow. If this wire becomes covered with dirt or oil, an improper reading will occur. If you are running an air filter which requires oil (such as K&N) this oil may collect on the sensor. Ensure to use a cleaner which is specifically designed to clean the mass air flow sensor as an improper cleaning solution may damage the sensor. See the image below for an example cleaner.
If after cleaning the sensor a few times with the proper cleaning spray does not improve the symptoms you may have, the entire MAF sensor will likely have to be replaced if in fact you have found this sensor to be the root cause.