My sister owns a 1990 Pontiac Firebird with the 3.1L v6 motor. It had been sitting for a few months because rather suddenly it couldn't hold an idle. It would crank and fire immediately but run really rough, and stall both if you let it run or if you gave it any throttle. I managed to pull two codes from the computer: 33 (MAP sensor high) and 35 (IAC valve failed).
The first code to show up was the MAP sensor code. I checked the sensor and the lines figuring that it could be those rather than the sensor itself. The vacuum line was in fact broken and so I replaced it. No dice. I then replaced the sensor. Still no dice. I replaced the MAP sensor. That caused code 35 to show up.
I pulled the IAC valve off and it looked bad. It was all caked up with carbon and I figured that maybe it just needed to be cleaned. Didn't help. I decided I wasn't going to dump any more cash into the project at the time so I let the car sit a few more months. After finally replacing the IAC valve there was still no progress. At this point I grew anxious. I was out of ideas. After scouring the 'net for a few hours I decided to pull the manifold off and check the injectors. Bingo. 4 of the 6 injectors had a resistance of 12 ohms, One had 6 ohms, and one had 8 ohms.
After a trip to two different Advance Auto Parts, I replaced the injectors and put the car back together. Initially the engine loped between racing and a rough idle -- but it ran. Eventually it stabilized and I was able to drive the car around the block. At which point it blew the O2 sensor out of the fitting on the exhaust pipe.