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 driveability concerns as a result.
In addition, many of today's cars use Mass Airflow 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 and may also result in the Check Engine light illuminating. There is an easy way to tell if the engine's ability to "breathe" or an inaccurate MAF sensor are the cause of the driveability complaint you are diagnosing.
It's called Volumetric Efficiency 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. To use this test, you need a scan tool capable of recording data and a VE Calculator like this one. The calculator requires the engine size in cubic inch displacement, airflow in grams/second, engine rpm and intake air temperaure 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) as well.
From a rolling start in first or second gear, perform a wide open throttle run while recording the data. Make 3 or 4 passes in order to compare results. Back at the shop, find the point in your recording where airflow reported by the MAF and engine rpm are at their highest. Enter these values into the calculator. The result is a percentage to "ideal". Ideal would be 100% of the engine's displacement. However, "ideal" is not real world and normal efficiency is generally 80% or more. Turbocharged and supercharged engines will run higher than 100%, naturally, so bear that in mind if testing an engine so equipped.
If VE is low, and fuel trims are correcting positive (for a lean condition), then suspect an inaccurate reading from the MAF sensor. If VE is low, and fuel trims are normal, then look for restrictions to air flow. A "running" compression test can help isolate 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 driveability complaints. But just is as the case with other diagnositic tools, it should be followed by more pinpoint tests to verify the results...and the failure.
