GM 3-bar MAP sensor reading

[FarNorthDSMer]

Hey guys, I just decided to begin the switch over to Speed Density, got everything wired in and what not, but I'm running into a problem. The sensor came with the car when I bought it from the previous owner, so I have no idea how many sets of stairs it fell down, bottom of boxes it saw, etc. At idle, it doesn't read anything below -0.8 inHg, which is basically logging my idle half way down my SD table, while my gauge reads 20-21 -inHg. All the logs I've pulled up have been people running the Omni 4bar, and showing a normal amount of negative pressure. Faulty sensor? Common problem with the GM's? As boost picks up, the MAP sensor follows right along once I'm out of the vacuum range. Any ideas would be well appreciated!