MAP Sensor
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 1992 Pontiac Firebird, 1992 Oldsmobile Custom Cruiser, 1992 Chevrolet Caprice, 1992 Chevrolet Camaro, and 1992 Cadillac Brougham. However, it is still accessible from the selected car via links, so may be relevant.
An output causing control module to sense a higher than normal manifold pressure (low vacuum) can cause system to go rich. Disconnecting MAP sensor will allow control module to substitute a fixed value for MAP sensor. If condition disappears, substitute a different MAP sensor, and continue testing.