* Idle Air Control (IAC) Valve: This is the most common cause. The IAC valve regulates the amount of air entering the engine at idle. If it's dirty, sticking, or malfunctioning, it can't provide the correct amount of air for a stable idle. Cleaning it (with appropriate cleaner) is often a first step, but replacement might be necessary.
* Throttle Position Sensor (TPS): The TPS tells the computer the position of the throttle plate. A faulty TPS can send incorrect signals, leading to poor idle and starting problems. It needs to be tested or replaced.
* Mass Airflow Sensor (MAF) Sensor: The MAF sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can cause a lean or rich mixture, preventing a stable idle. Cleaning (carefully, with the correct MAF cleaner) or replacement may be needed.
* Vacuum Leaks: A vacuum leak in the intake system can disrupt the air/fuel mixture, making it difficult to idle. Inspect all vacuum lines and hoses for cracks, holes, or loose connections.
* Faulty Crankshaft Position Sensor (CKP): While less likely to cause this specific symptom directly, a failing CKP sensor can disrupt the engine's timing and lead to erratic operation, including idling issues.
* Fuel Delivery Issues (less likely): Though less probable given the specific symptom, problems with fuel pressure, a clogged fuel filter, or a failing fuel pump *could* contribute. This is more likely if the engine struggles even when the gas pedal is held down.
Troubleshooting Steps:
1. Check for obvious vacuum leaks: Visually inspect all vacuum lines and hoses. Listen for hissing sounds near the intake manifold when the engine is running (or attempted to be running).
2. Clean the IAC valve: This is a relatively easy and inexpensive first step. Consult a repair manual or online resources for your specific engine to locate the IAC valve and the proper cleaning procedure.
3. Check the TPS and MAF sensor: These usually require a multimeter to test their voltage output. Again, consult a repair manual or online guides for your specific Neon model. These can be relatively expensive to replace.
4. Check fuel pressure (if other steps fail): This will require a fuel pressure gauge and may need a mechanic to perform accurately.
Important Note: If you're not comfortable working on your car, take it to a qualified mechanic for diagnosis and repair. Improperly diagnosing and fixing these problems could cause further damage to your engine.
