MAP Sensor
WARNING: This page is about a different car, the 1992 Pontiac Grand Prix, 1992 Oldsmobile Cutlass Supreme, and 1992 Chevrolet Lumina. 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.