Auto Service Professional

APR 2016

Magazine for the auto service professional

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Powerplant 22 | April 2016 time it might start to increase, but on most models it won't change at idle as long as the short-term trim is able to keep total fuel trim below the code-set cri- teria (25%). So think about what happens when the MAF is under-reporting: As the car cruises down the road, the PCM "sees" less air and calculates a shorter injector pulse width, but the oxygen sensor "sees" a lean air/fuel ratio so the PCM trims the pulse width longer. The engine will cruise smoothly, but when the throttle opens sud- denly for acceleration, the sudden inrush of air isn't detected. The result: Fuel delivery doesn't increase as much as it should for acceleration and the engine hesitates or stumbles. At wide open throttle, the fuel system goes into open-loop to provide maximum power. With a dirty MAF, the engine will run lean because of the under-reported airfow. Fuel trim will become more extremely positive but no code will set because of the (programmed) open- loop condition. Still, the engine will run so lean that it might not accelerate beyond a certain rpm, and this often leads to random misfre. Dirt accumulates on the MAF gradually, and the PCM is able to compensate well enough to avoid setting codes for thousands of miles. Some drivers may never notice the loss of engine performance, but those who do notice might come to your shop asking for a "tune-up." That's another clue that the mass air fow sensor has failed in-spec, meaning it's not functioning correctly but the malfunction is not bad enough to set a fault code. Testing A faulty MAF will have the same effect on both banks of a two-bank engine. If a fault code or some other symptom shows up only on one bank, the MAF is probably OK. A dirty MAF can be diagnosed quickly and accurately with a scan tool running in generic OBD II mode. Set the tool to graph Many older-style MAF sensors have a screen in front, not so much for protection but to straighten the fow of air through the hous- ing. If this screen is damaged, the MAF sensor will tend to over- report airfow. Engine performance will actually improve but the driver will probably notice higher fuel consumption. Photo courtesy of Junius

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