Fuel System:
* Low Fuel Pressure: The fuel pump may be weak, the fuel filter clogged, or there's a leak in the fuel lines. This restricts fuel delivery, especially when the engine's demand drops (like when you let off the gas).
* Dirty or failing fuel injectors: Clogged injectors can't deliver fuel efficiently, leading to rough idling and stalling.
* Vacuum Leak in the Fuel System: A leak in the vacuum lines related to the fuel pressure regulator can disrupt fuel delivery.
Air Intake System:
* Vacuum Leaks: Leaks in the intake manifold gaskets, vacuum hoses, or PCV system can cause erratic idling and stalling. The engine is getting too much or too little air, disrupting the air/fuel mixture.
* Mass Airflow Sensor (MAF) Issues: A dirty or failing MAF sensor provides inaccurate air flow readings to the computer, resulting in a poor air/fuel mixture.
* Throttle Body: A dirty throttle body can restrict airflow. Cleaning it is often a simple fix.
Idle Air Control System (IAC):
* Faulty IAC Valve: This valve controls the amount of air entering the engine at idle. A malfunctioning IAC valve can't maintain the correct idle speed, leading to stalling.
* IAC Valve Passage Clogged: The passages within the throttle body where the IAC valve operates can become clogged with carbon buildup.
Other Possibilities:
* Crankshaft Position Sensor (CKP): A failing CKP sensor prevents the engine from receiving the correct signals about crankshaft position, leading to misfires and stalling.
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate readings to the engine control module (ECM), leading to poor idle and stalling.
* Ignition System Problems: Weak spark plugs, failing ignition coil, or worn distributor cap and rotor (if applicable) can cause misfires, rough idle, and stalling.
* Engine Coolant Temperature (ECT) Sensor: An inaccurate ECT sensor can cause the ECM to adjust the air/fuel mixture incorrectly.
Troubleshooting Steps:
1. Check for obvious vacuum leaks: Visually inspect all vacuum hoses and connections for cracks, splits, or loose connections.
2. Check the fuel pressure: This requires a fuel pressure gauge.
3. Clean the throttle body: This is a relatively easy task and often resolves idling issues.
4. Inspect and clean the MAF sensor: Be careful not to touch the sensor element.
5. Check the spark plugs and wires: Replace if worn or damaged.
6. Have the codes read: Use an OBD-II scanner to check for diagnostic trouble codes (DTCs). These codes can pinpoint the problem area.
This is not an exhaustive list, but it covers the most common causes. It's recommended to start with the simpler checks (vacuum leaks, throttle body cleaning) and then proceed to more involved diagnostics if necessary. If you're not comfortable performing these checks yourself, take it to a qualified mechanic for diagnosis and repair.
