Under the black ECU cover there is a sensor with a pipe feeding to it from the plenum chamber, and two spade terminals on the other end.
Remove it, hook it up to a mityvac, connect a multimeter across the sensors set to continuity. It'll make a circuit as default, so pump up the mityvac until it cuts the circuit. note the pressure it cuts at, and adjust the screw between the 2 spade terminals to set the cut point to 18psi.
Do not just fuck with the screw without a calibration
Do not bridge the sensor out its there for a reason and you'll blow it up
If you can be arsed send it to me and ill calibrate it for you.