* IAC Valve: The IAC valve regulates the air entering the engine at idle. If it's dirty, sticking, or malfunctioning, it can't supply enough air to maintain a proper idle speed, leading to stalling. The rough idle and stalling are classic symptoms.
* Vacuum Leak: A vacuum leak after the throttle body will draw in unmetered air, upsetting the air/fuel mixture. This can cause a rough idle, stalling, and excessive fuel delivery (leading to the strong gas fumes). The AC compressor's vacuum assist might be temporarily plugging the leak, hence the smoother running with the AC on.
* Throttle Position Sensor (TPS): A faulty TPS can provide incorrect information to the engine's computer, leading to poor idle control. While less likely to cause the direct correlation with the AC, it can contribute to the overall rough idle and stalling issue.
* Fuel System Issues (Less Likely, but Possible): While the gas fumes point towards a rich mixture, a clogged fuel filter or other fuel delivery problems could also contribute. However, the strong correlation with the AC makes this less likely than the IAC or vacuum leak.
Here's how to troubleshoot:
1. Check for Vacuum Leaks: Visually inspect all vacuum hoses and connections for cracks, holes, or loose fittings. Pay close attention to the area around the throttle body and intake manifold. Use a propane torch (with the engine running) to carefully check for air leaks near suspected areas - the engine RPM will briefly increase if the propane is drawn into a leak. Safety first: Be extremely cautious with an open flame near fuel and moving engine parts. A better, safer option is a vacuum gauge connected to a known good vacuum port on the intake manifold; if it's fluctuating erratically, you have a vacuum leak.
2. Clean the IAC Valve: Locate the IAC valve (usually on the throttle body). Remove it and clean it thoroughly with a suitable cleaner (brake cleaner is often used, but follow the cleaner's instructions carefully). Reinstall it.
3. Check/Replace the IAC Valve: If cleaning doesn't solve the problem, the IAC valve might need replacement.
4. Check the TPS: Test the TPS voltage output with a multimeter. The output voltage should change smoothly as you move the throttle plate. If the voltage is erratic or out of spec, the TPS needs to be replaced.
5. Check the Fuel System (if other troubleshooting steps fail): Inspect the fuel filter and pressure regulator.
Important Note: Strong gas fumes are a serious safety concern. Don't continue driving the car until the problem is resolved. The unburnt fuel is a fire hazard. If you're not comfortable performing these checks yourself, take your Pontiac to a qualified mechanic for diagnosis and repair.
