High-Probability Causes:
* Spark Plug: This is the most common cause. A fouled, damaged, or improperly gapped spark plug in cylinder 3 will prevent proper ignition. Inspect for wear, cracks, fouling (black or oily deposits), or incorrect gap.
* Ignition Coil: The ignition coil (or coil pack, depending on the engine) supplies the high voltage to the spark plug. A faulty coil for cylinder 3 will result in a misfire. These can crack internally, leading to inconsistent or no spark. A visual inspection might not reveal damage.
* Worn or Damaged Spark Plug Wire: If your Expedition uses individual spark plug wires, a cracked, worn, or loose wire to cylinder 3 will prevent proper spark delivery. Look for visible damage or poor connections.
* Injector Problem: A clogged or malfunctioning fuel injector in cylinder 3 prevents fuel from being delivered properly. This can lead to a lean condition and misfire. A fuel pressure test can help diagnose this.
* Compression Issues: Low compression in cylinder 3 indicates a problem with the piston rings, valves, or head gasket. This reduces the power of the combustion and can cause a misfire. A compression test is necessary to determine this.
Less-Likely but Still Possible Causes:
* Vacuum Leak: A significant vacuum leak can affect the air/fuel mixture across all cylinders, but it's more likely to cause multiple misfires. A vacuum leak could manifest as a misfire in one cylinder if that cylinder's intake system is particularly affected.
* Camshaft or Crankshaft Position Sensor: These sensors tell the engine's computer the position of the crankshaft and camshaft, crucial for timing the spark and fuel injection. A faulty sensor can disrupt the timing, leading to misfires.
* ECM (Engine Control Module): While less likely, a faulty ECM can cause misfires in specific cylinders. This is usually diagnosed after other possibilities have been ruled out.
Troubleshooting Steps:
1. Check the Spark Plug and Wire (if applicable): This is the easiest and cheapest place to start. Replace the spark plug if it shows any signs of wear or damage.
2. Inspect the Ignition Coil (or Coil Pack): Look for obvious cracks or damage. Testing with a multimeter might be necessary to confirm functionality.
3. Perform a Compression Test: This will tell you if there is a mechanical problem within the cylinder.
4. Fuel Injector Test: This often requires specialized tools or a professional diagnosis.
Important Note: Before working on your vehicle, disconnect the negative battery terminal to prevent accidental shorts or electrical shocks. If you're not comfortable performing these checks yourself, take your Expedition to a qualified mechanic for diagnosis and repair. They have the tools and expertise to accurately pinpoint the cause of the misfire.
