* Fuel System: A clogged fuel filter, a faulty fuel pump, or issues with fuel pressure regulation can cause similar symptoms. The engine might not be getting enough fuel, or it might be getting inconsistent fuel delivery. Have fuel pressure tested.
* Mass Airflow Sensor (MAF) Sensor: This sensor measures the amount of air entering the engine. A faulty MAF sensor provides inaccurate data to the computer, leading to incorrect fuel delivery and potentially causing surging and hard starting.
* Crankshaft Position Sensor (CKP) or Camshaft Position Sensor (CMP): These sensors tell the computer where the engine is in its cycle. Faulty sensors can cause misfires, poor starting, and rough running.
* Vacuum Leaks: A leak in the intake manifold or vacuum lines can disrupt the air/fuel mixture and cause erratic engine behavior. Inspect all vacuum lines and the intake manifold for cracks or leaks.
* Throttle Body: Even with a new IAC valve, the throttle body itself might be dirty or malfunctioning. Clean the throttle body thoroughly. A sticky throttle plate can cause surging.
* PCV Valve: A faulty Positive Crankcase Ventilation valve can cause excessive pressure in the crankcase, leading to vacuum leaks and other issues.
* Engine Wiring Harness: While less likely, a short circuit or damaged wire in the engine's wiring harness could be sending incorrect signals to the engine control unit (ECU). A professional inspection might be needed to check this.
* ECU (Engine Control Unit): In rare cases, the ECU itself might be faulty, although this is less likely given the other potential issues.
Troubleshooting Steps:
1. Fuel Pressure Test: This is crucial. A mechanic can easily perform this test.
2. Visual Inspection: Thoroughly inspect all vacuum lines, the intake manifold, and the throttle body for leaks, cracks, or debris.
3. MAF Sensor Check: Have the MAF sensor tested or replace it. It's relatively inexpensive.
4. CKP/CMP Sensor Check: These sensors can be tested with a multimeter or by a mechanic.
5. PCV Valve Inspection/Replacement: This is a relatively cheap and easy part to replace.
Because you've already replaced several parts, it's strongly recommended to take your vehicle to a qualified mechanic. They have the tools and expertise to diagnose the problem more accurately and efficiently than trying to troubleshoot it yourself further. They can perform diagnostic tests to pinpoint the exact cause of the surging and hard starting.
