What did you replace it with? My truck did a similar thing. I hooked it up the way shown above, and it read 2.2, so far so good right. After I got the truck running, I noticed on the datalog that it was down to 1.3x, WTF. Now when I check it key on, engine off, I get around 1.5x.
Disconnect the vacuum line from the MAP sensor and then see what the voltage is with Key ON/Engine OFF.
What is your MAP sensor vacuum line feeding from? Are you sure it is from either your TB or Intake Manifold and NOT from your vacuum canister feed?
My understanding of MAP sensor voltage is like this:
All have a range of 0-5 volts although most if not all will EVER give you a reading of 5 volts so expect the max voltage to be closer to 4.8 or even a hair less than that. Key ON/Engine OFF reading is always that of 0 vacuum so depending on which MAP sensor you have, you simply divide 5 by the respective MAP sensor type to determine which one you actually have from your reading.
1BAR max voltage reading for 0 vacuum reading is that of 5 volts (5/1=5).
2BAR max voltage reading for 0 vacuum reading is that of 2.5 volts (5/2=2.5).
3BAR max voltage reading for 0 vacuum reading is that of 1.66 volts (5/3=1.66).
Again, the reading you get at 0 vacuum will be even less than these max values because of not being able to achieve a true 5 volt reading. Hence, on a 2BAR MAP reading at 0 vacuum it will most likely be between 2.1 - 2.4 instead of 2.5.
If your 0 vacuum reading isn't close to your MAP sensor type, then the sensor is either bad, wired incorrectly, or simply the wrong one.
With the engine running at idle, the voltage reading will be equivalent to your actual vacuum at idle. So someone with an aftermarket cam will most likely have less vacuum at idle than someone with a stock cam and will naturally have a higher MAP voltage reading at idle than someone with a stock cam given they both have the same type of MAP sensor.
