1. Ignition System:
* Spark Plug: This is the most common culprit. A fouled, damaged, or worn-out spark plug in cylinder #4 will prevent proper combustion. Check for cracks, excessive wear, or fouling (oil, carbon buildup). Gap should be correct.
* Spark Plug Wire: A cracked, frayed, or loose spark plug wire can prevent the spark from reaching the plug effectively. Inspect for damage and ensure a secure connection.
* Ignition Coil: The ignition coil (or coil pack, depending on the engine) supplies the high voltage to the spark plugs. A failing coil can weaken or eliminate the spark to one or more cylinders. A coil test is necessary to diagnose this. If it's a coil-on-plug system, only cylinder 4's coil needs to be tested.
* Ignition Control Module (ICM): This module controls the firing sequence of the ignition coils. A faulty ICM can cause misfires in one or more cylinders.
2. Fuel System:
* Fuel Injector: A clogged or malfunctioning fuel injector in cylinder #4 won't deliver the necessary fuel for combustion. A fuel injector test or cleaning is needed.
* Fuel Pressure: Low fuel pressure can affect all cylinders, but it's possible low pressure could be particularly noticeable in one cylinder. A fuel pressure test is necessary.
* Fuel Filter: A clogged fuel filter can restrict fuel flow, leading to insufficient fuel delivery.
3. Engine Mechanical Issues:
* Valve Problem: A burnt, bent, or sticking valve in cylinder #4 can prevent proper air/fuel mixture from entering the combustion chamber. A compression test is crucial to diagnose this.
* Low Compression: Low compression in cylinder #4 indicates a problem with the valves, piston rings, or head gasket. A compression test is essential.
* Vacuum Leaks: A vacuum leak can disrupt the air/fuel mixture, leading to a misfire.
4. Other Potential Causes:
* Mass Airflow Sensor (MAF): A faulty MAF sensor can provide inaccurate air flow readings to the computer, resulting in improper fuel mixture.
* PCV Valve: A clogged Positive Crankcase Ventilation valve can cause excessive crankcase pressure, affecting combustion.
* ECM/PCM (Engine Control Module/Powertrain Control Module): While less likely, a faulty ECM can cause misfires, though this is usually accompanied by other symptoms.
Troubleshooting Steps:
1. Check for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to read any stored codes. This will often point to the most likely culprit.
2. Visual Inspection: Inspect the spark plug, wire, and coil pack for cylinder #4. Look for any obvious damage.
3. Compression Test: This is crucial for determining if there are mechanical issues within the cylinder.
4. Spark Test: Test the spark plug wire for adequate spark.
5. Fuel Injector Test: Test the fuel injector for proper operation and spray pattern.
It's important to diagnose the problem systematically. Starting with the simplest and most common causes (spark plug, wire, coil) and working your way through the more complex issues is recommended. If you're not comfortable performing these tests yourself, it's best to take your Expedition to a qualified mechanic for diagnosis and repair.
