* Idle Air Control (IAC) Valve: This valve regulates the amount of air entering the engine at idle. A faulty IAC valve, either stuck closed or malfunctioning, prevents the engine from idling properly. Without the gas pedal providing additional fuel, the engine starves for air.
* Throttle Position Sensor (TPS): This sensor tells the computer the position of the throttle plate. A faulty TPS can send incorrect signals, leading the computer to believe the throttle is always closed, even when it's not. This can result in the engine not getting the right fuel/air mixture at idle.
* Mass Airflow Sensor (MAF) Sensor: This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can provide inaccurate readings, leading to a lean fuel mixture and preventing idling.
* Vacuum Leaks: A significant vacuum leak in the intake system can disrupt the fuel/air mixture, preventing a stable idle. The engine might only run when the throttle is open because the extra airflow partially compensates for the leak.
* Computer Problems (PCM): While less likely, a problem with the Powertrain Control Module (PCM) itself could cause misinterpretations of sensor data, resulting in an inability to idle.
* Fuel Delivery Issues (Less Likely, but possible): While less probable given the symptom, a weak fuel pump or clogged fuel filter could restrict fuel delivery at low engine speeds, making it only run when the gas pedal is depressed, increasing fuel pressure.
Troubleshooting Steps (Do NOT attempt if you lack mechanical experience):
1. Check for obvious vacuum leaks: Visually inspect all vacuum hoses for cracks, breaks, or disconnections. Listen for hissing sounds near the intake manifold.
2. Clean the MAF sensor: Use MAF sensor cleaner (NOT regular cleaner) to gently clean the sensor element. Be careful not to touch the sensor's delicate wires.
3. Check the IAC valve: Sometimes a gentle cleaning can help, but often replacement is necessary.
4. Have the TPS sensor checked: A diagnostic scan tool can check if the TPS is providing the correct voltage readings.
5. Have the vehicle scanned for diagnostic trouble codes (DTCs): A scan tool can pinpoint specific problems within the engine's electronic control system. This is the most important first step.
Important Note: Attempting to diagnose and repair this issue yourself without experience could lead to further damage. It's best to have a qualified mechanic diagnose and repair the problem. Driving a car that only runs with the gas pedal pressed is dangerous and should be avoided.
