Low fuel pressure would make you go lean rather than rich in most cases (a lot of experience with THAT problem personally).
The emission/drivability documents are essential reading since they are what GM published specifically for troubleshooting these systems. The only drawback is, in a lot of cases they assume access to tools and equipment that has not been seen since the late 1980's, which made (at least for me) following the procedures difficult.
A big key is to know exactly what the sensors are telling the computer, which is hard to do without a real time OBD1/ALDL data logger. You can measure voltages at the sensors, but if there are bad connections between the sensor and computer that doesn't always reflect an accurate picture. Being able to directly read the sensors may not give you the complete answer as to what is wrong (these systems can tell you there is an error but can't tell you why there is an error), but if you can get your hands affordably on an old Win7 or Win10 laptop with an ALDL interface cable, that will make following the procedures in the drivability/emissions manual much easier (it did for me anyway). The sensors aren't a complete picture though since they don't measure everything...if GM had provided a fuel pressure sensor I'd have solved my problems in days instead of months.
The key elements that are likely to impact running rich are the TPS, MAP and CTS readings, with the O2 sensor playing a role as well. The computer takes the values of those 1st 3, looks up what the fuel flow should be based on those values and pulses the injectors accordingly. Adjustments are made to the lookup table over time using the O2 sensor, changing the fuel flow values up or down based on the O2 measurements. If the TPS or MAP are incorrectly reading high, or the CTS is reading incorrectly low, the computer is going to push too much fuel thru the injectors. If the O2 sensor is bad, it could also be incorrectly telling the computer to add more fuel that necessary.
Something that cannot be controlled by the computer or sensors would be faulty injectors dumping more fuel than commanded, or (unlikely but possible) fuel pressure is too high.
