A 2000 Chevy 3500 Express 5.7 Vortek has always run fine but has a major problem with torque when towing uphill. By chance I stumbled across the MAP OBD II readings and they are way off. It shows 9" at idle when the actual vacuum is 19-20, and at WOT it shows 28". I found what voltages I should get off the sensor and it seems fine, 4.6vdc at WOT and 1.2vdc at idle. I obtained a wiring diagram and checked the continuity of the sense wire going to the VCM. Zero ohms resistance and no short to ground. Also checked the 0" vacuum with key on (but engine doesn't run with that plug off), but the 0" vacuum does give me 4.6vdc going to the VCM plug. Do I replace the VCM? Again, it seems to run fine but has always been low on torque. For instance, when towing a heavy load up my steep drive, there seems to be no difference when going from about 2/3 throttle to full throttle, except I can hear the sound in the TB change to a deeper tone. I also confirmed the odd readings with a different OBD II tool.
A partial answer. As for my actual vacuum readings vs. the VCM scanner readings. It was recently just drilled into my head that like it says in the MAP, "A" is for "Absolute", like the air pressure they talk about on the weather report! So the pressure reading is in reference to the ambient air pressure, not the same as a simple vacuum gauge. Take the idle vacuum of 19 and the air pressure of about 28", subtract 19 (from the vacuum gauge) from the ABSOLUTE air pressure of 28", and you get 9". Exactly what the VCM reads.
This doesn't fix the problem, but explains why the numbers on the vacuum gauge and the numbers from the scan tool did not match.