* Idle Air Control (IAC) Valve: This is a very common cause of this symptom. The IAC valve regulates the air intake at idle. If it's dirty, sticking, or failing, the engine might not get enough air when you're off the throttle, leading to a stall. Cleaning it (carefully, with appropriate cleaner) is often the first step. Replacement is relatively inexpensive.
* Throttle Position Sensor (TPS): This sensor tells the computer the position of your throttle. A faulty TPS can provide incorrect information, leading to improper fuel delivery at idle. A bad TPS can cause erratic idle, stalling, and other driveability issues.
* Mass Airflow Sensor (MAF) Sensor: While less likely to cause a stall *only* when letting off the gas, a dirty or failing MAF sensor can provide inaccurate air flow readings. This affects the air/fuel mixture across the rev range, and could indirectly lead to stalling issues. Cleaning it (gently, with MAF sensor cleaner) is worth a try.
* Vacuum Leaks: A significant vacuum leak can disrupt the engine's ability to maintain idle speed. Check all vacuum lines and hoses for cracks, breaks, or loose connections. Pay close attention to lines near the intake manifold and throttle body.
* Fuel Pressure Regulator: Though less likely, a faulty fuel pressure regulator could be causing issues with fuel pressure dropping too low when the engine is idling, leading to a stall.
* Ignition System (despite changing plugs and wires): While you've changed plugs and wires, there could still be underlying ignition issues like a failing ignition coil, distributor cap (if equipped), rotor, or pickup coil. These components can be intermittent, and more likely to show problems at idle.
Troubleshooting Steps:
1. Check for Vacuum Leaks: Carefully inspect all vacuum lines and hoses. Use a vacuum gauge if you have one to check for proper vacuum at idle.
2. Clean the IAC Valve: This is the easiest and cheapest thing to try first. Look up a tutorial specific to your 1994 Pontiac Bonneville on YouTube.
3. Check the TPS: You might need a multimeter to test the voltage output of the TPS. Consult a repair manual for your vehicle to find out how to test it and the expected voltage readings.
4. Clean the MAF Sensor: Use a MAF sensor cleaner specifically designed for this purpose; don't use anything else.
5. Consider a fuel pressure test: This requires a fuel pressure gauge and some expertise, but it can rule out fuel delivery problems.
Important Note: Before attempting any repairs yourself, consult a repair manual specific to your 1994 Pontiac Bonneville. This will provide diagrams, specifications, and proper procedures. If you're not comfortable working on your car, take it to a qualified mechanic. They have the tools and experience to diagnose the problem accurately and efficiently.
