Here is my understanding (which may be wrong of course): This engine does not have a MAP sensor or a separate baro sensor built into the MAF sensor like some Fords have. It calculates air pressure based on MAF output at WOT and assumes air pressure equals barometric pressure at WOT. One indication of a properly working MAF is the indicated barometric pressure on an OBDII scanner equals actual barometric pressure.

This sounds good but I can't figure out how the ECM knows how to do this. The electrical output of the MAF is proportional to the square root of air density x air velocity (basic hot wire anemometer). Air density can be calculated easily from the ideal gas law (PV = nRT). At WOT, the ECM knows IAT so it can calculate air pressure if it knows the air density. BUT, the MAF reading does not separate air density and air velocity since the output is based on the product of both. If the ECM assumes an air velocity across the MAF at WOT, then the air pressure can be calculated. However, how can the ECM make this assumption when it doesn't know if the air filter is clean? Maybe it assumes the air filter offers insignificant resistance meaning the MAF barometric pressure test is only good if you have a clean air filter?

Bottom line: Do you agree that a scanner barometric pressure compared to actual barometric pressure tells us anything about MAF performance? If yes, how does it do that? Thanks.