All technicians know that an engine has to have good compression in order to run well. But an engine also has to "breathe" well, or be able to move the air in and out with minimal restriction. Restricted catalytic converters, improper cam timing, even clogged air filters, can impact the ability of the engine to do this with various drivability concerns as a result. In addition, many cars use Mass Airflow (MAF) sensors to tell the ECM how much air is being drawn in. If this information is incorrect, the ECM will not supply the correct fuel charge for optimum performance. There is an easy way to tell if the engine's ability to "breathe" or an inaccurate MAF sensor is the cause of the drivability complaint you are diagnosing.
It's called Volumetric Efficiency (VE) and is a measurement in percentage of the actual ability of the engine to move air versus what it could move in a perfect world. The scan tool PID "Calculated Load" is a rough representation of VE. To check VE for yourself, you’ll need a scan tool capable of recording data and a VE calculator. You can find one online – heck, there’s even a Google app!
Most calculators will require the engine size in cubic inch displacement, airflow in grams/second, engine rpm and intake air temperature when tested. Set your scan tool to record these PIDs (Parameter Identifications) and take the car out for a test drive. If you want to test the accuracy of the MAF sensor, record the fuel trim PIDs (STFT and LTFT) at the same time. Wait until you’re back in the shop before you review the data and be sure to perform the road test in a safe area and in accordance with all local laws and regulations.
Want to see how it’s done? In the March 2016 edition of the Trainer, I will show you just how easy it is to perform this test step-by-step and then share what to look for in the data you’ve captured. Be sure to “like” the video and subscribe to the YouTube channel when you’ve finished watching!
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About the Author
Pete Meier
Creative Director, Technical | Vehicle Repair Group
Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels.
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