Ignition System:
* Spark Plug: This is the most frequent cause. A fouled, damaged, or improperly gapped spark plug in cylinder 4 will prevent or weaken combustion. Check for fouling (black, wet, or oily deposits), cracks, or electrode wear.
* Ignition Coil: The coil pack supplies the high voltage to the spark plug. A faulty coil pack for cylinder 4 will lead to a misfire. Testing the coil's resistance is necessary to confirm its health.
* Ignition Wire (if applicable): While less common in modern vehicles with coil-on-plug systems, a damaged or corroded ignition wire can impede spark delivery. Inspect for cracks, fraying, or corrosion.
* Crankshaft Position Sensor (CKP): This sensor tells the engine's computer the crankshaft's position, crucial for precise ignition timing. A faulty CKP can cause misfires in various cylinders, including #4.
* Cam Position Sensor (CMP): Similar to the CKP, this sensor monitors the camshaft's position for valve timing. Issues can disrupt the precise ignition sequence.
Fuel System:
* Fuel Injector: A clogged or faulty fuel injector in cylinder 4 will prevent sufficient fuel delivery, resulting in a misfire. A fuel injector cleaning or replacement might be needed.
* Fuel Pressure: Low fuel pressure across the entire system can affect all cylinders, but a partially clogged fuel filter or a weak fuel pump can manifest as a misfire in one cylinder initially.
Engine Mechanical Issues:
* Valve Problems: A burnt, bent, or sticking valve in cylinder 4 will prevent proper combustion. This requires a more involved diagnosis and often engine inspection.
* Compression: Low compression in cylinder 4 indicates a leak in the cylinder – possibly caused by worn piston rings, a blown head gasket, or a damaged valve. A compression test is essential here.
* Piston/Connecting Rod Issues: While less likely, damage to the piston or connecting rod can lead to a misfire. This usually involves more serious engine damage.
Other Potential Factors:
* Vacuum Leaks: Leaks in the intake manifold or vacuum lines can affect engine performance and cause misfires.
* ECM/PCM Issues: Problems with the Engine Control Module (ECM) or Powertrain Control Module (PCM) (the computer) can lead to misfires. This usually requires professional diagnosis using scan tools.
Troubleshooting Steps:
1. Check for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to retrieve any stored trouble codes. These codes will often point directly to the problem area.
2. Visual Inspection: Inspect spark plugs, wires (if applicable), and the coil pack for cylinder 4 for visible damage.
3. Perform a Compression Test: This will identify if there's a mechanical problem within the cylinder.
4. Test the Fuel Injectors (if possible): This may require specialized equipment or professional help.
5. Check Fuel Pressure: Ensure fuel pressure is within specifications.
Important Note: Without proper diagnostic tools and knowledge, attempting repairs can be risky. If you are not comfortable performing these checks yourself, it's strongly recommended to take your vehicle to a qualified mechanic for diagnosis and repair. A misfire, if left unchecked, can cause further engine damage.
