1. Verify your work:
* Double-check connections: Make absolutely sure all spark plug wires are securely connected to both the spark plugs and the coil pack. A loose connection is a very common cause of misfires. Look for any damage to the wire insulation as well.
* Check the coil pack's installation: While you've replaced it, ensure it is properly seated and all its connections are tight. Sometimes a seemingly minor misalignment can cause issues.
* Inspect the spark plugs closely: Even new plugs can be faulty. Make sure they're the correct type for your engine and are properly gapped.
2. Ignition System Diagnostics:
* Ignition coil (again): If you have individual coil-on-plug units (one per cylinder), and you only replaced the main coil pack, then you might need to replace the individual coils for cylinders 1 and 4. A faulty coil is a very common reason for misfires.
* Crankshaft Position Sensor (CKP): This sensor tells the computer where the engine is in its rotation cycle. A faulty CKP can lead to misfires, especially in a pattern like this.
* Camshaft Position Sensor (CMP): Similar to the CKP, this sensor monitors the camshaft's position. A faulty CMP can also result in misfires.
* PCM (Powertrain Control Module): The PCM controls the ignition system. Although rare, a faulty PCM could be the culprit. This is usually diagnosed last because it's more complex and costly to replace.
* Wiring harness: Inspect the wiring harness for any damage, breaks, or corrosion, particularly around cylinders 1 and 4.
3. Fuel System Diagnostics:
* Fuel injectors: While less likely if the other cylinders are firing, a clogged or faulty fuel injector for cylinders 1 and 4 could be the cause.
* Fuel pressure: Check the fuel pressure to ensure it's within the manufacturer's specifications. Low fuel pressure can cause misfires.
4. Diagnostic Tools:
* OBD-II Scanner: The most important step. Connect an OBD-II scanner to your car's diagnostic port. This will likely show diagnostic trouble codes (DTCs) indicating the specific problem. This is the best place to start. The codes will give you a much more precise diagnosis than guesswork.
* Multimeter: You can use a multimeter to test the voltage and resistance of the various components in the ignition system (coils, sensors, etc.) to confirm their functionality. However, you'll need a wiring diagram to know what to test.
5. Professional Help:
If you're not comfortable performing these tests or don't have the necessary tools, take your Focus to a qualified mechanic. They have diagnostic equipment that can pinpoint the problem much more efficiently.
Important Note: Cylinders 1 and 4 are often on opposite sides of the engine. While it's possible to have a problem in a shared component (like the PCM or fuel pump) that affects both, it's also possible that a single component failure on each bank of the engine is to blame. Focus on isolating the problem by testing components close to the cylinders.
