Auto Service Professional

APR 2016

Magazine for the auto service professional

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Powerplant 24 | April 2016 rpm, both long- and short-term fuel trims and the oxygen sensor or air/fuel ratio sensor. The fuel trims will show you if the PCM is compensating for inaccurate air- fow measurement, and the oxygen sensor readings show you if that compensation is successful. Watch how the short- and long-term fuel trims change during a test drive. If the PCM is subtracting fuel at idle but adding fuel as rpm climbs, that's a typical sign of a dirty MAF. During a steady cruise, expect the O2 sensor to cycle normally if there are no lean codes. During a full-throttle acceleration, the O2 sensor would normally indicate rich A/F ratio, but it won't if the MAF is dirty. Of course, the most accurate diagnostic technique is to read the actual airfow on the scan tool and compare it to a known- good value. The problem is, few auto manufacturers provide airfow specifcations in their service/diagnostic information. A volumetric effciency (VE) calculator will tell you what the airfow should be for any engine at any rpm. There are a several easy- to-use VE calculators on the Web and even a few smartphone apps, but be careful with the results, since a low VE could be caused by a dirty MAF or by mechanical problems in the engine (low compression, etc). There is also a rule-of-thumb VE that says airfow at idle should be roughly equal to the engine's displacement. That means a four-liter engine should inhale roughly 3.7 to 4.3 grams of air per second at idle. A reading that's signifcantly different is not enough to condemn a MAF, but at least it will tell you if you're on the right diagnos- tic path. Probably the easiest diagnostic technique is to unplug the MAF and then start the engine. It doesn't work on all engines, but on many models the PCM will substitute airfow data if the MAF fails completely, and the scan tool might actually display those numbers so you can compare them to live data. This is a good way to test the MAF on older models if the malfunction indicator light (MIL) is illuminated and lean codes are set (P0171 and P0174). Of course, this test will also set some codes, but if the engine starts easier and/or runs Thanks to proper engine maintenance, the original mass airfow sensor on this Chevy van is still working perfectly after running 20 years and 140,000 miles.

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