1. Fuel System:
* Low Fuel: Sounds obvious, but check your fuel level.
* Clogged Fuel Filter: A restricted filter prevents sufficient fuel flow. Replace it.
* Fuel Pump Issues: A weak or failing fuel pump can't deliver enough pressure. You might hear a weak humming or no humming from the pump (listen near the fuel tank). A fuel pressure gauge is needed for accurate diagnosis.
* Fuel Injectors: Clogged or faulty injectors can cause poor idle. Cleaning or replacing them may be necessary.
* Fuel Pressure Regulator: A malfunctioning regulator can cause incorrect fuel pressure.
2. Ignition System:
* Spark Plugs and Wires: Worn, fouled, or damaged spark plugs and wires can disrupt the spark. Inspect and replace as needed. Look for cracks in the insulator of the spark plugs.
* Ignition Coil: A failing coil won't deliver sufficient voltage to the plugs.
* Distributor (if applicable): Check the distributor cap and rotor for cracks, wear, or carbon buildup. The ignition module inside the distributor can also fail.
* Ignition Control Module (ICM): This module controls the ignition timing. A failing ICM can cause erratic idling.
3. Air Intake System:
* Vacuum Leaks: Cracks or holes in vacuum hoses can significantly affect idle. Inspect all hoses carefully. Pay close attention to the brake booster hose, PCV valve hose and the manifold vacuum lines.
* Mass Airflow Sensor (MAF) Sensor: A dirty or faulty MAF sensor provides incorrect air flow readings to the computer. Clean it with MAF sensor cleaner (follow instructions carefully) or replace it.
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate throttle position information to the computer. Test it with a multimeter or replace it.
* Air Filter: A clogged air filter restricts airflow. Replace it with a new one.
* IAC (Idle Air Control) Valve: This valve controls idle speed. A dirty or faulty IAC valve is a common cause of idle problems. Try cleaning it with carb cleaner first.
4. Engine Components:
* EGR Valve: Exhaust Gas Recirculation valve problems can cause rough idling. Inspect and clean or replace if necessary.
* PCV Valve (Positive Crankcase Ventilation): A clogged PCV valve can cause excessive crankcase pressure and affect idling.
* Throttle Body: A dirty throttle body can restrict airflow. Clean it thoroughly with throttle body cleaner.
5. Computer System:
* ECM (Engine Control Module): While less common, a faulty ECM can cause various problems, including idling issues. This requires professional diagnostics.
Troubleshooting Steps:
1. Start with the easy checks: Fuel level, air filter, visual inspection of vacuum hoses and spark plug wires.
2. Listen to the engine: Are there any unusual sounds like ticking, knocking, or hissing?
3. Check engine codes: Use an OBD-I scanner (specific to your vehicle year) to retrieve any diagnostic trouble codes (DTCs). This will point you to the most likely culprit.
4. Systematic approach: Address potential problems one by one, starting with the most likely causes. After each repair or replacement, check if the idling issue is resolved.
If you're not comfortable working on your car yourself, take it to a qualified mechanic. They have the tools and expertise to diagnose the problem accurately and efficiently. Providing them with any codes you've found will help.
