Download the MAF calculator spreadsheet for help in diagnosing faulty MAF sensors
and other volumetric efficiency problems such as plugged catalytic converters. Plug
in a few variables and the spreadsheet will tell you how many gm/sec the MAF sensor
SHOULD be reporting. If the readings are too low, suspect a faulty MAF sensor or
plugged exhaust. This is a great tool when diagnosing P0171, P0174 or other drivability
conditions such as low power or spark knock.
Below is an example of what the spreadsheet looks like with Excel 2007 from Microsoft.
If you do not have the Excel program, then you can download openoffice from openoffice.org
and use the “calc” spreadsheet program to run the MAF calculator.
The spreadsheet is very simple to use and most of the instructions are written directly on the page. Below are a few things to keep in mind for improved accuracy when using the calculator.
- The calculator is only accurate at wide open throttle (ZERO intake manifold vacuum).
- Only input your altitude OR Barometric pressure, NOT BOTH. Also, don’t mistake true barometric pressure with the calculated barometric pressure reported by most weather and news stations. In almost all cases, altitude should be sufficient, just leave baro at 0 unless you are certain of your true baro.
- For best test results, use a graphing scan tool. Use manual PID selection to deselect all parameters except RPM and MAF gm/sec. By deselecting all other parameters, the scan tool will update faster and give more accurate results.
- With the engine fully warmed, drive with wide open throttle through first gear until the transmission shifts (or near redline for manual transmissions). The spreadsheet is more accurate at higher RPMs(over 4000).
- Most vehicles will have about 75% to 85% volumetric efficiency. The calculator assumes that 80% is correct. Typically, when a low power problem exists that is caused by a MAF, the readings will be very low as they are in the image above. Notice that the 2.0 engine shown above is reading about 30% lower than it should. The actual gm/sec input by the user was 64.32, and the calculator shows that at 5856 RPM that engine should be breathing about 92 gm/sec of air.
- A MAF sensor that is causing lean codes, but is not yet bad enough to cause a low power issue may read about 15% lower than normal. Remember, not all engines have the same “normal” volumetric efficiency. The calculator is a “close enough” tool, it is not perfect.