* Idle Air Control (IAC) Valve: This valve regulates the amount of air entering the engine at idle. A dirty, sticking, or faulty IAC valve is a very common cause of rough idling and stalling. When the AC compressor kicks on, it increases the load on the engine, requiring more airflow. The increased demand might be enough to overcome the restriction caused by the malfunctioning IAC, allowing the engine to run smoothly.
* Vacuum Leak: A leak in the vacuum system can drastically affect idle. With the AC on, the compressor's vacuum assist might partially seal the leak or change air pressure enough to temporarily stabilize the idle. Look for cracked vacuum hoses, loose connections, or leaks around the intake manifold.
* Throttle Position Sensor (TPS): A faulty TPS can provide incorrect information to the engine computer, leading to poor idle control. While less likely to be specifically tied to the AC, a bad TPS can cause similar symptoms.
* Mass Airflow Sensor (MAF) Sensor (If equipped): A dirty or failing MAF sensor can cause inconsistent air/fuel mixtures, leading to rough idling and stalling. Again, the increased load from the AC might compensate for a minor MAF issue.
* Low Fuel Pressure: While less probable given the AC correlation, a drop in fuel pressure could manifest as rough idle and stalling. However, this usually doesn't improve with increased engine load.
* Ignition System Issues: Worn spark plugs, bad plug wires, or a failing ignition coil can also cause rough idling. However, these issues usually persist regardless of the AC being on or off.
Troubleshooting Steps:
1. Check for Vacuum Leaks: Visually inspect all vacuum hoses and connections for cracks, leaks, or loose fittings. Use a vacuum gauge to test the system's pressure if you have one.
2. Clean the IAC Valve: This is the easiest and often most effective first step. Remove the IAC valve, clean it with carburetor cleaner (following the cleaner's instructions), and reinstall it.
3. Inspect and Clean the Throttle Body: While you have the IAC valve out, clean the throttle body thoroughly. Dirt and debris can interfere with its operation.
4. Check the TPS: Use a multimeter to check the voltage output of the TPS at various throttle positions. Compare your readings to the specifications in your vehicle's repair manual.
5. Check the MAF Sensor (if equipped): Clean the MAF sensor carefully with a MAF sensor cleaner. Do *not* use anything abrasive.
6. Check Spark Plugs and Wires: Inspect spark plugs for wear and replace them if necessary. Check spark plug wires for cracks or damage.
If you're not comfortable performing these checks yourself, take your Blazer to a qualified mechanic for diagnosis and repair. They have the tools and expertise to properly diagnose the issue. Remember to always disconnect the battery's negative terminal before working on any electrical components.
