I m betting either a bad map sensor or the wrong part altogether.
Gm 3 bar map sensor voltage chart.
Linear output based in this info chart shown is for a 2 bar scaling to 29 4 psi.
886 012 539 609 701 3 bar.
3 digit numbers for identification.
The map sensor manifold absolute pressure has a 5 volt input and then reduces this voltage according to throttle opening altitude etc and sends the signal to the ecu.
The op stated the map voltage was high at idle and went down as vacuum decreased.
It is possible that detroit is slightly above sea level and you can use 12 11 in the formula for 1 mile above sea level.
But all of these sensors must still operate within the same voltage output specs as a 1 bar.
So by supplying a higher or lower voltage the return signal is varied by the same amount.
749 861 the logic module uses the manifold absolute pressure map sensor to determine the absolute pressure not the relative to atmospheric pressure of the air inside the intake manifold and the atmosphere barometric pressure.
039 460 883 876 2 bar.
Formula for the gm 3 bar map sensor is v 8 94 14 53 i have no idea why it s 14 53 and not 14 696 rounds up to 14 7 but it works out on gm s chart every time.
If i choose the holley it goes to 31 2 psi.
Formula for the gm 3 bar map sensor is v 8 94 14 53 i have no idea why it s 14 53 and not 14 696 rounds up to 14 7 but it works out on gm s chart every time.
This means the output voltage of a 2 bar map sensor with the key.
I have looked around to try and find the calibration data for the gm so that i can choose custom 3 bar and get it to work up to 31 2.
A 2 bar map sensor will read up to 2 bars of atmospheric pressure difference and a 3 bar will read up to 3.
On my fuel table it only goes up to 26 8 psi.
It is possible that detroit is.
Now at a slightly higher elevation reading flickers between 1 78 and 1 79v giving a reading of 0 9 psig engine load with the car off.
Still not 3 bar but a little better.
I have the gm 12592525 map sensor and have it selected as my map sensor.
So in order to accomplish this the output voltage must be scaled accordingly.
It shows right on the chart top part idle to be to the right side high vacuum low map voltage and left to be wot low vacuum high map voltage.
This is the opposite of what should really happen.