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I'll have to agree that it sounds like your MAP sensor output is biased for some reason.
FYI, inches of Hg isn't necessarily a vacuum measurement, it can go positive as well. It's just a different unit of pressure, commonly used for vacuum because it's a "finer" scale than psi.
"TEST RESULT: When I view datastream, at idle, MAP shows 21 " HG."
I suspect your scan tool is reading MAP (Manifold Absolute Pressure), but it's a bit low. At idle, MAP should be 14.7 psi +/- or about 29.9" Hg.
"Engine off MAP 0" would be correct if your scale is in "gauge" pressure. If in absolute pressure, it should read around 29" Hg or one atmosphere. The best way to check that is compare MGP (manifold gauge pressure) with EBP and BARO sensors. They should read within 1 psi (or 2" Hg) of each other with the engine off.
10 psi gauge should equal 20" Hg gauge or 50"Hg absolute pressure (adding 14.7 psi for atmospheric pressure). So if your 59"Hg is absolute, your MAP sensor is reporting about 15 psig versus your gauge reading of 10 psi. Maybe your gauge isn't properly calibrated, but 5 psi is a long ways off.
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