Magazine for the auto service professional
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Powerplant 20 | April 2016 that's the data used for fuel control. It might also show the raw voltage or current signal, and you can always back-probe the sensor itself with an oscilloscope if you sus- pect there's a glitch in the signal. But the generic OBD II data stream will show you how the PCM is using the signal, and that's a clean and easy place to start your drive- ability diagnosis. What goes wrong By far the most common MAF failure is caused by accumulation of dirt on the hot wire. Even though the sensor is protected by the air flter (it's often mounted inside the air flter housing), oil mist or fne dust that contacts the hot wire tends to bake onto the wire. This insulates the wire from the air fowing past it so less heat is carried away. The MAF does not report all the air fowing into the engine and the PCM selects the wrong injector pulse width. On some sensors, the hot wire is momentarily heated to about 1,000 degrees when the ignition is turned off to burn it clean. Performance aftermarket air flters that use oil to trap fne dust particles are a prime source of MAF contamination because people tend to put too much oil on the flter. Usually with a dirty MAF, long-term fuel trim will be negative at idle and become more positive as rpm (airfow) increases. But we just noted that a dirty MAF under- reports airfow; why is long-term fuel trim negative at idle? Actually, given enough This shows both the "cold" and "hot" wires. Notice the dirt baked onto the hot wire: this one is dirty enough to affect engine perfor- mance, but it's easy to clean. Photo courtesy of Junius This sensor is in the intake duct just above the throttle plate. Cleaning it was easy, but the car came back in just a few months with similar MAF sensor issues. Photo courtesy of Junius