Can You Start Car With Bad Mass Air Flow Sensor Volumetric Efficiency – OR – How Well Does the Engine Breathe?

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Volumetric Efficiency – OR – How Well Does the Engine Breathe?

All technicians know that an engine must have good compression to run well. But the engine must also “breathe” well, that is, it must be able to move air in and out with minimal restrictions. Restricted catalytic converters, improper cam timing, even clogged air filters can affect the engine’s ability to do this, which in turn causes various drivability concerns.

Additionally, many cars today use mass air flow sensors to tell the ECM how much air is being sucked in. If this information is incorrect, the ECM will not ensure proper fueling for optimal performance and may also cause the Check Engine light to illuminate. There is an easy way to determine whether the engine’s ability to “breathe” or an inaccurate MAF sensor is the cause of the drivability complaint you are diagnosing.

This is called Volumetric efficiency and is a percentage measurement of the engine’s actual ability to move air compared to what it could move in a perfect world. To use this test, you need a review tool that can record data and a VE calculator. The calculator asks for engine size in cubic inches, airflow in grams/second, engine RPM, and intake air temperature when testing. Set the scan tool to record these PIDs (parameter identifications) and take the car for a test drive. Also note the fuel trim PIDs (STFT and LTFT) to test the accuracy of the MAF sensor.

From starting in first or second gear, perform wide open throttle during data recording. Do 3 or 4 passes to compare results. When you get back to the shop, find the point in your recording where the MAF-reported airflow and engine RPM are at their highest. Enter these values ​​into the calculator. The result is a percentage to “ideal”. Ideally, 100% of the engine’s displacement would be. However, “ideal” is not the real world, and normal efficiency is generally 80% or more. Turbocharged and supercharged engines will naturally run above 100%, so keep that in mind if you’re testing a so-equipped engine.

If the VE is low and the fuel corrections are positive (for a lean condition), suspect an inaccurate MAF sensor reading. If VE is low and fuel quantities are normal, look for airflow restrictions. A “flow” compression test can help determine whether the restriction is on the intake side of the combustion chamber or the exhaust side.

The VE calculator is another diagnostic tool that can provide quick direction in diagnosing various drivability issues. But as with other diagnostic tools, even more detailed tests should follow to verify the results… and errors.

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