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!
SPECIAL MESSAGE FROM PETE
March 7, 2016 - It was my mistake in using this particular vehicle for this video. The 2013 Dodge Ram 1500 equipped with the 5.7 liter Hemi is a "speed density" system that does not use a MAF sensor (that's why it wasn't on the OBDII PID list! - duh) and rather than verify it or use another vehicle I succumbed to deadline pressure and pressed forward. I am shooting an official "apology" video later today to explain the difference between an inferred PID (the airflow PID discussed in the video) and a true PID to all of our subscribers and readers. However, the process demonstrated is correct, and to clarify for everyone the short version is this: 1. Select Global OBD II on your scan tool. 2. Select the MAF PID (airflow in grams/second), the IAT PID (Intake Air Temperature) and the Engine RPM PID. For more diagnostic information, also select Long Term and Short Term Fuel Trim for both banks (LTFT and STFT). 3. Select graphing mode for all PIDS and record during the test drive. 4. In a safe location, perform 2-3 WOT accelerations from a rolling start in first gear. 5. On the recording, look for the data PID values where they intersect with maximum RPM and enter those values into a VE calculator.
Hope that clarifies it for everyone! And thanks to all those who commented or emailed me for keeping me honest and supporting my efforts on YouTube and in the pages of Motor Age!
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