* Throttle Position Sensor (TPS): This sensor tells the computer the throttle position. A faulty TPS can cause erratic fuel delivery, particularly at idle. A bad TPS often leads to a car that only runs with the throttle partially open. This is a very strong suspect given your symptoms.
* Mass Air Flow Sensor (MAF) Sensor (if equipped): While less likely than the TPS in this scenario, a faulty MAF sensor can provide incorrect air/fuel mixture data leading to poor idle and stalling. The TBI system may not have a MAF, depending on exact specifications, but it's worth considering if your model does.
* Idle Air Control (IAC) Valve: This valve regulates airflow at idle. If it's stuck closed or malfunctioning, the engine won't get enough air to run smoothly at idle. This is another likely candidate for a car that won't idle properly.
* Fuel Pressure Regulator: Even though you replaced the pump and relay, the regulator could be faulty, causing inconsistent fuel pressure at idle. It might be delivering enough fuel under throttle but not enough at idle.
* Vacuum Leaks: A vacuum leak can disrupt the air/fuel mixture. Check all vacuum lines for cracks or disconnections. Pay close attention to lines connected to the intake manifold.
* Computer (ECM): Although less common, a failing ECM could be causing the problem. This is usually diagnosed last after checking all other components.
Troubleshooting Steps:
1. Check the TPS: This is the easiest and most likely starting point. You'll need a multimeter to test the voltage output of the TPS at various throttle positions. A Haynes or Chilton repair manual for your specific year and model will provide detailed instructions and voltage specifications for testing.
2. Check for Vacuum Leaks: Carefully inspect all vacuum lines for cracks, leaks, or loose connections. Use a spray bottle with carb cleaner (or brake cleaner, carefully!) and spray around the vacuum lines while the engine is running. A change in engine speed indicates a leak.
3. Check Fuel Pressure: You'll need a fuel pressure gauge to measure the fuel pressure at the fuel rail. This will determine if the fuel pressure is dropping or fluctuating erratically at idle.
4. Inspect the IAC Valve: Look for any signs of debris or sticking. Sometimes cleaning it with carb cleaner can resolve the issue.
5. Check the MAF (if equipped): If your car has a MAF sensor, testing it with a multimeter (or scanner) according to the manufacturer's specifications is crucial.
Important Note: If you are not comfortable working on your car's fuel system, it is best to take it to a qualified mechanic. Working with fuel systems can be dangerous if not done correctly. Incorrect fuel/air mixture can also damage the catalytic converter.
