Map Sensor
WARNING: This page is about a different car, the 1990 Chevrolet Corvette. However, it is still accessible from the selected car via links, so may be relevant.
An output that causes the control module to sense a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the control module to substitute a fixed value for the MAP sensor. If the condition disappears, substitute a different MAP sensor and continue testing.