My 90 Lumina 3.1 was having similar problems and it ended up being 2 bad fuel injectors. You can check this by taking off the plenum and removing the electrical connectors for the injectors. On the injector itself are two metal prongs. Using an Ohm Meter touch 1 prong with the positive line and the other with the negative line. Each injector should read between 11 and 13 ohms. If it reads 7.5 to 10.5 it will cause some issues but wont necessarily make your car not run but if it reads anything less your car will stall. I had also replaced the brain or ECM. While changing the injectors I replaced all the sensors in that area and the crankshaft position sensor which is located on the back side of the block to the side and down from the oxygen sensor under the spark plug on the passenger side. I used a mirror to locate it. It is easier to remove the alternator to get to it but not necessary. Be careful if you do change this sensor as it fits tightly and is known to break apart. The new ones have metal clips but the old original ones are plastic. I replaces this sensor, the oxygen sensor, the throttle position sensor, and coolant temperature sensor. I figured if I'm taking it off I'm putting on a new one! Good luck and I hope this helps!
Wednesday, October 26th, 2011 AT 2:10 PM