Most Common Causes:
* Dirty or Faulty Mass Airflow Sensor (MAF): The MAF sensor measures the amount of air entering the engine. A dirty sensor will give inaccurate readings, leading to a rough idle and poor performance. Cleaning it (with MAF sensor cleaner, *not* brake cleaner) is often the first step. If cleaning doesn't help, it might need replacement.
* Dirty Throttle Body: Similar to the MAF, a dirty throttle body can restrict airflow. Cleaning it with throttle body cleaner is a relatively easy DIY fix.
* Faulty PCV Valve: The Positive Crankcase Ventilation valve vents crankcase pressure. A faulty valve can cause excessive pressure buildup, leading to a rough idle and other problems. It's inexpensive and easy to replace.
* Spark Plugs and Wires: Worn-out spark plugs or damaged spark plug wires can cause misfires, resulting in a rough idle. Inspect them for wear, damage, or carbon buildup. Replacing them is relatively inexpensive.
* Ignition Coil(s): A failing ignition coil can prevent one or more cylinders from firing properly, causing a rough idle. This often requires more advanced diagnostics to pinpoint the faulty coil.
* Vacuum Leaks: Leaks in the vacuum lines can disrupt the engine's air-fuel mixture, leading to a rough idle. Inspect all vacuum lines for cracks or disconnections.
Less Common, But Possible Causes:
* Faulty Crankshaft Position Sensor (CKP): This sensor tells the engine's computer the position of the crankshaft. A faulty CKP will cause major problems, including a rough idle.
* Faulty Oxygen Sensor(s): While more likely to cause other issues like poor fuel economy, a faulty O2 sensor can contribute to a rough idle.
* Fuel Injectors: Clogged or malfunctioning fuel injectors can cause a rough idle. Diagnosing this usually requires specialized tools.
* Engine Mounts: Worn engine mounts can allow the engine to move excessively, leading to a rough idle, especially noticeable when the car is idling.
Troubleshooting Steps:
1. Check Engine Light (CEL): If the CEL is on, get the codes read using an OBD-II scanner. This will provide clues about the potential problem.
2. Visual Inspection: Look for any obvious problems like loose vacuum hoses, damaged wires, or anything out of the ordinary.
3. Clean MAF Sensor and Throttle Body: This is a low-cost, easy first step.
4. Check Spark Plugs and Wires: Inspect for wear and tear.
5. Check PCV Valve: Easy to replace and a common culprit.
6. Listen to the Engine: Pay close attention to the sound of the idle. A specific cylinder misfire might reveal itself through a distinct knocking or ticking sound.
When to See a Mechanic:
If you've tried the basic troubleshooting steps and the rough idle persists, it's best to take your Grand Am to a mechanic. They have the tools and expertise to diagnose more complex problems like faulty sensors, fuel injector issues, or vacuum leaks in hard-to-reach areas. Ignoring a rough idle can lead to more serious engine damage.
