MAP Sensor
WARNING: This page does not describe the selected car, but rather 24 other vehicles, including the 1992 GMC Yukon, 1992 GMC Vandura Special, 1992 GMC Vandura, 1992 GMC Value Van, and 1992 GMC Suburban. However, it is still accessible from the selected car via links, so may be relevant.
If ECM senses higher than normal manifold pressure (low vacuum) system can go rich. Disconnecting MAP sensor allows ECM to substitute a fixed value for the MAP sensor. If rich condition disappears, replace MAP sensor and continue testing.