Porsche The 1.8 air flow meter often fails when a cold engine lean backfires, warping the sensor flap. this warped flap sticks in the bore causing a rich mixture and an idle that goes away as the engine warms. replacement is the only solution, however it is important to cure the lean running engine that initially caused the problem, change the gas flooded oil and never again depress the accelerator pedal when starting a cold engine.