BMW X3 digital engine management (DME) systems use a hot-film mass air flow sensor, installed between the air filter housing and the intake manifold to monitor air flow into the engine intake. Inside the sensor, there is a thin metal film which is maintained at a constant temperature via electrical current from the DME. As ambient intake air flows over the film, it is cooled down and therefore additional current is needed to maintain its constant temperature. The DME monitors this additional current to determine the mass of the incoming air and to adjust fuel supply accordingly. Note that as the temperature of ambient air increases, its volume expands. Since the air flow sensor hot film monitors heat-loss to the passing air, it automatically adjusts for the increased volume (= lower density) of the incoming air.
Mass air flow sensor accuracy skews over time and can lead to fault codes or drivability problems. When replacing, inspect the plastic intake air ducts for cracks or dry rot. If the intake air ducts are faulty, replace them first and see if the problem you are having is remedied. An un-metered air leak downstream of the mass air flow sensor will set codes similar to a faulty mass air flow sensor. When you suspect a mass air flow sensor as the culprit for setting a fuel trim fault code, check when the fault code was set. You can do this using OBD II freeze frame data. Normally, a vacuum leak would set a fuel trim fault code at idle or very low engine RPMs. A mass air flow meter would usually set a fault code when you are driving, or at a higher RPM. You can check your fault code freeze frame data using an OBD II scan tool, similar to how you would check for fault codes.
In this tech article I will show you how to replace the mass air flow sensor on BMW X3 models with an M54 engines. Other engine equipped models are similar.
Keep in mind that when your car was serviced before, parts may have been replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have, so be prepared with different size sockets and wrenches.
Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Do not work on your vehicle if you feel the task is beyond your ability.
Vehicle models change and evolve as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Questions or want to add to the article? Leave a comment below. When leaving a comment, please include your vehicle information.
Turn the engine OFF and remove the key.
The mass air flow sensor (red arrow) is located at the air filter housing outlet on the left side of the engine. Fresh air is drawn in through the bottom of the intake air housing, through the air filter and measured by the mass air flow sensor.
If the O-ring (red arrow) gives you a hard time going in, apply a small amount of dish soap to the O-ring. It will slide right in and the soap will not damage the O-ring. Check the vehicle for fault codes. See our tech article on reading and clearing fault codes. Then test drive the vehicle and confirm that the engine runs well.