* Fuel Injectors: One or more injectors could be stuck open, constantly spraying fuel into the cylinder even when the engine isn't demanding it. This is a common problem with age.
* Fuel Pressure Regulator: A faulty fuel pressure regulator could be maintaining excessively high fuel pressure in the fuel rail, leading to over-fueling.
* Mass Airflow Sensor (MAF) Sensor: A faulty MAF sensor provides incorrect readings to the computer about the amount of air entering the engine. If it's reading low, the computer will compensate by injecting more fuel than necessary.
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate information about the throttle position to the computer. This can also lead to incorrect fuel delivery.
* Computer (ECM): While less likely, a failing Engine Control Module (ECM) could be sending incorrect signals to the fuel injectors. This is generally diagnosed after checking the other components.
* Vacuum Leaks: A significant vacuum leak can cause the engine to run lean (lack of fuel), but if the computer is trying to compensate (which it does) it could initially make it run rich before completely shutting down.
Troubleshooting Steps:
1. Check for Vacuum Leaks: Carefully inspect all vacuum hoses and connections for cracks, holes, or loose fittings. Use a vacuum gauge to verify proper vacuum at idle.
2. Test Fuel Pressure: You'll need a fuel pressure gauge to check if the fuel pressure is within specifications (check your service manual for the correct pressure).
3. Inspect the MAF Sensor: Carefully examine the MAF sensor for dirt or debris. Cleaning it (with MAF sensor cleaner) might help if it's dirty. If it's visibly damaged or shows erratic readings, replacement is needed.
4. Check the TPS: Use a multimeter to test the TPS voltage output at various throttle positions. Compare your readings to the specifications in your repair manual.
5. Inspect Fuel Injectors: This is more involved. Ideally, a fuel injector flow test is required to assess their performance accurately. You can sometimes visually inspect them for leaks (with the engine off and the fuel rail disconnected), but this is less reliable.
6. Diagnostic Trouble Codes (DTCs): Use an OBD-I scanner (for a 1991 model) to retrieve any stored trouble codes. These codes can pinpoint the problem area.
Important Note: Before attempting any repairs yourself, consult a repair manual specific to your 1991 Firebird 3.1L V6. Working on fuel systems can be dangerous; fuel is flammable, and high pressure can cause injury. If you're not comfortable working with these systems, it's best to take your vehicle to a qualified mechanic.
