1.1b) Symptoms of faulty MAF:
Associated OBD2 error codes DTCs: P0100 - P0104 (Often P0171-P0174)
How to check? Disconnecting the faulty MAF will halt the symptoms of 'surge and slow' and the engine will resume a steady idle which maybe higher or lower than usual. The ECU will take its signals from the throttle position sensor (TPS) to maintain the engine idle in the absence of MAF input. Visually inspect the MAF when it is out, as dirt residue will be easy to spot. The MAF has 3 or 4 pins in its socket connector:- 3 pins: one is 'ground', one is 12 volt (or 5 volt) 'reference' and the third is 'signal' 4 pins: as above but additional 'hot wire ground' The 'signal' pin should output about 1 volt at idle and nearer 5 volts at open throttle. If the output looks good but engine still reports bad MAF then check wiring continuity between plug connector and ECU.
- Idle 'Surge and slow': The idling engine often exhibits a rhythmic pattern of 'surge and slow' every two or three seconds when the MAF is at fault. In older cars the carbon track in the air vane MAF can wear through leading to errors in voltage signal to the ECU. In modern MAF sensors the delicate heated coil or film can become contaminated by dirt or simply break or burn out leading to an error or an absent signal.
- Erratic idle/near stall: If the MAF gets dirty the signal indicates low air mass and the ECU responds by reducing the fuel mix to the engine to an extent that it struggles to maintain a steady idle or even to keep running.
How to fix? With the older variety of MAF, a possible fix consists of first running a knife around the silicone seal and opening the box unit, then easing the contact-arm body along the air vane shaft by a few millimeters and then bending the contact arm downwards to again make contact with a fresh concentric carbon track to that of the worn region.For the modern hot film or hot wire MAF there are no moving parts. Dirt and oil carried in the air stream are the only hazards and this can be cleaned off by spraying with a residue-free electrical cleaner and allowing it to air dry; ideally direct contact with the MAF ought to be avoided as it is easily damaged, although a very gently wipe with a soaked cotton bud should be OK. If the MAF is beyond reviving through cleaning then it must be exchanged for new one.
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