* Vacuum Leaks: A significant vacuum leak in the intake system will cause the engine to take in more air than expected, leading to a higher idle. Check all vacuum hoses for cracks, splits, or loose connections. Pay close attention to the PCV valve hose and the brake booster hose.
* Throttle Position Sensor (TPS): A faulty TPS can send incorrect signals to the engine control unit (ECU), causing it to increase the idle speed. The TPS needs to be properly calibrated; a simple cleaning might resolve the issue, but replacement may be necessary if the cleaning fails.
* Idle Air Control (IAC) Valve: This valve controls the amount of air entering the engine at idle. A dirty or malfunctioning IAC valve can cause erratic idle speeds, including high idle. Cleaning it is often a first step in troubleshooting; replacement might be needed if cleaning doesn't fix the problem.
* Mass Airflow Sensor (MAF) Sensor: A dirty or faulty MAF sensor can cause the ECU to add too much fuel, leading to high idle. Cleaning the sensor (carefully!) is a potential solution, but replacement is often necessary if cleaning is ineffective.
* Dirty Throttle Body: A buildup of carbon deposits on the throttle body can interfere with its smooth operation and cause irregular idle speeds. Cleaning the throttle body is relatively straightforward and should be considered.
* Faulty Engine Coolant Temperature Sensor (ECT): If the ECU thinks the engine is cold, it will enrich the fuel mixture and increase idle speed to help it warm up. A faulty ECT sensor can give a false reading, leading to a persistently high idle.
* Problems with the Accelerator Cable: Though less common with electronic throttle control systems (if your Grand Am has one), a sticking or improperly adjusted accelerator cable could keep the throttle slightly open, resulting in a high idle.
* Air Filter: While less likely to cause a drastically high idle, a severely clogged air filter can restrict airflow, potentially causing the engine to compensate by increasing the idle speed.
Troubleshooting Steps:
1. Check the obvious: Inspect vacuum lines, the air filter, and ensure nothing is mechanically interfering with the throttle plate.
2. Clean the throttle body and IAC valve: This is a relatively easy DIY task and often solves the problem. Use a throttle body cleaner specifically designed for this purpose.
3. Check and clean the MAF sensor: Use a MAF sensor cleaner; never touch the sensor element with your fingers.
4. Use a code reader: If you have access to an OBD-II code reader, scan for diagnostic trouble codes (DTCs). This can pinpoint specific issues.
If you're not comfortable performing these checks yourself, take your Grand Am to a qualified mechanic for diagnosis and repair. Trying to fix these issues without proper knowledge can potentially cause further damage. Remember to always disconnect the negative battery terminal before working on any electrical components in your vehicle.
