High Probability Issues:
* Idle Air Control (IAC) Valve: This is a very common cause of this symptom. The IAC valve controls the air intake at idle. If it's dirty, malfunctioning, or stuck, the engine won't maintain a consistent idle. Cleaning it (with carb cleaner) is often the first step, but replacement might be necessary.
* Throttle Position Sensor (TPS): This sensor tells the engine's computer the position of the throttle. A faulty TPS can lead to erratic idle and stalling. Testing is usually required to confirm this.
* Mass Airflow Sensor (MAF) Sensor: This measures the amount of air entering the engine. A dirty or faulty MAF sensor can give the computer incorrect information, leading to poor idle and stalling. Cleaning it (carefully, with MAF sensor cleaner) is worth trying.
* Vacuum Leaks: A vacuum leak anywhere in the intake system can cause similar problems. Inspect all vacuum hoses and connections for cracks, breaks, or loose fittings. Pay particular attention to the hoses connected to the PCV valve.
Less Likely (but possible) Issues given what you've already done:
* Fuel Pump (despite changing the filter): While you changed the *filter*, the pump itself could be weak or failing, especially if it struggles to provide enough fuel under load (when idling). Fuel pressure testing is needed to diagnose this.
* Fuel Injectors: A clogged or intermittently failing injector could cause inconsistent running, but it's less likely to be the *sole* cause of the problem. Again, testing is needed.
* Coolant Temperature Sensor (CTS): While a faulty CTS can affect engine performance, it's less likely to cause the specific symptom of only running with the throttle held down. A faulty CTS would likely cause other drivability issues as well.
Troubleshooting Steps:
1. Check for Vacuum Leaks: This is the easiest and cheapest first step. Visually inspect all vacuum lines.
2. Clean the IAC Valve: This is a relatively easy and inexpensive fix. If possible, replace it.
3. Clean the MAF Sensor: Use a specialized MAF sensor cleaner. Do not use anything else.
4. Test the TPS: This requires a multimeter and a bit more technical knowledge. You can often find videos online showing how to test this sensor.
5. Test Fuel Pressure: This requires a fuel pressure gauge and will require more in-depth knowledge.
Recommendation:
Start with the easiest and cheapest options first: checking for vacuum leaks and cleaning the IAC and MAF sensors. If these don't resolve the issue, consider having a mechanic perform a diagnostic scan and fuel pressure test to pinpoint the problem more accurately. A scan tool can often reveal codes that directly point to a faulty sensor or component.
