* Mass Airflow Sensor (MAF) Sensor: A dirty or failing MAF sensor provides inaccurate readings of the incoming air. This leads to a poor air/fuel mixture, especially at idle when the engine demands less air. The engine might run better under load because the engine's demands override the sensor's faulty signal to some extent.
* Throttle Position Sensor (TPS): Similar to the MAF sensor, a malfunctioning TPS provides inaccurate readings of the throttle plate position. This can cause a lean or rich condition at idle, leading to roughness. The issue might lessen under load as the throttle opens.
* Idle Air Control (IAC) Valve: This valve regulates the amount of air entering the engine at idle. A dirty or faulty IAC valve can cause erratic idle speeds and roughness.
* Vacuum Leaks: Small vacuum leaks can disrupt the air/fuel mixture, especially at idle when the vacuum is relatively high. These leaks can be hard to find.
* Fuel Injectors: Clogged or malfunctioning fuel injectors can cause a rough idle, particularly if only some injectors are affected. This is less likely to improve under load, but it's possible.
* Spark Plugs and Wires: Worn or fouled spark plugs, along with damaged or corroded spark plug wires, can create misfires that are more noticeable at idle. The load might mask this slightly.
* Computer (ECM) Issues: While less common, a problem with the engine control module (ECM) itself could also be the root cause. This is usually indicated by multiple other issues.
Troubleshooting Steps:
1. Read the Check Engine Codes: The most important first step is to obtain the diagnostic trouble codes (DTCs) stored in the ECM. You can do this with an OBD-I scanner (specific to your car's year) or have a mechanic read them. These codes will narrow down the possibilities significantly.
2. Inspect the MAF Sensor and Clean It (or Replace It): Gently clean the MAF sensor with MAF sensor cleaner (never use anything else). If cleaning doesn't solve the problem, it may need replacing.
3. Check for Vacuum Leaks: Carefully inspect all vacuum hoses and connections for cracks or loose fittings. Listen for hissing sounds near the intake manifold while the engine is idling.
4. Inspect Spark Plugs and Wires: Check the condition of your spark plugs and wires for signs of wear, damage, or fouling.
5. Check Fuel Pressure: If you suspect fuel injectors, you may want to test fuel pressure.
6. IAC Valve Cleaning/Replacement: The IAC valve can be cleaned with throttle body cleaner, but sometimes replacement is necessary.
7. TPS Test: You can test the TPS with a multimeter, following instructions specific to your vehicle.
If you are not comfortable working on your car yourself, a mechanic can diagnose and repair the issue more efficiently. Providing the diagnostic trouble codes to the mechanic will save them time and potentially money.
