Q: What would cause the MPG gauge to fluctuate on a 1999 BMW 323i?

Hello, thank you for writing in. There are several different systems that interact with the fuel system which effect fuel economy. The first that you will want to check is the ignition system. Make sure your spark plugs are new and working correctly. From there make sure your air intake sensors are clean and working well. That would include your mass airflow sensor and throttle body. You may also want to check the EGR system and exhaust system. You may benefit from scanning the vehicle for diagnostic codes to make sure the computer has not already detected the problem. Beyond that, you will want to make sure your fuel pump and fuel filter are both working properly. Focus on the core aspects of the engine: fuel, air, and spark. For more help with diagnostics, contact our service department to schedule an appointment.