* Faulty Injector: A clogged or malfunctioning injector on cylinder 3 prevents proper fuel delivery. This is a common cause of misfires. You'll need a fuel pressure test and potentially an injector flow test to diagnose this.
* Compression Issues: Low compression in cylinder 3 means the engine isn't effectively igniting the air/fuel mixture. A compression test is crucial here. Possible causes of low compression include worn piston rings, a blown head gasket, or a valve problem.
* Wiring Harness Problem: While you replaced the coil pack wires, there might be a break or short in the main wiring harness leading to the injector or coil pack on cylinder 3. A thorough visual inspection of the harness is necessary.
* PCM (Powertrain Control Module) Issue: The PCM controls fuel injection and ignition timing. A faulty PCM can cause misfires in specific cylinders. This is less likely but possible. Diagnostics would be needed to check for PCM fault codes.
* Vacuum Leak: A significant vacuum leak can affect the engine's performance and cause misfires, especially if the leak is near the intake manifold for cylinder 3.
* Camshaft Position Sensor (CMP) or Crankshaft Position Sensor (CKP): These sensors tell the PCM the engine's position. A faulty sensor can lead to incorrect timing and misfires.
* Exhaust Restriction: A severely clogged catalytic converter or exhaust system could restrict the exhaust flow enough to cause a misfire.
Troubleshooting Steps:
1. Retrieve Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to read any stored trouble codes. This is the first and most important step. The codes might point directly to the problem.
2. Visual Inspection: Carefully examine the wiring harness leading to cylinder 3, looking for any damage, breaks, or corrosion.
3. Compression Test: Perform a compression test to check the compression in all cylinders. A significantly lower reading on cylinder 3 confirms a problem within the cylinder itself.
4. Fuel Injector Test: Test the injector on cylinder 3 to ensure it's delivering the correct amount of fuel. A fuel pressure test is often done first to rule out overall fuel supply issues.
5. Vacuum Leak Test: Use a vacuum gauge or smoke test to check for vacuum leaks in the intake system.
If you're not comfortable performing these tests yourself, take your Impala to a qualified mechanic for proper diagnosis and repair. Replacing parts haphazardly without proper diagnosis will only waste time and money. The DTCs will be the most helpful piece of information to get you started.
