A 3 bar sensor is supposed to give around 1.55 volt at key on, engine off.
That value doesn't change your problem. It's just good to know.

5 volt on the signal line with the map-sensor disconnected is false. It should be Zero.
So, your either measuring the feed, or there's a short circuit somewhere.
The readings are very contradictive. With the map-sensor connected the signal is way too low, with the sensor disconnected it's way too high.
Maybe it's time to get some professional help?