1. The Basics (Easiest & Cheapest First):
* Check Spark Plug and Wire: Start by inspecting the spark plug and wire for cylinder #3. Look for:
* Cracked or damaged insulator: On the spark plug itself.
* Corrosion or damage: On the spark plug or wire connector.
* Loose or disconnected wire: Ensure it's securely connected to both the spark plug and the distributor (if applicable, or coil pack). A loose connection is a common culprit.
* Replace Spark Plug and Wire (Recommended): Even if they look okay, a worn-out spark plug or damaged wire is a very likely cause. Replacing both is cheap and often solves the problem. Use the correct type of spark plug specified in your owner's manual.
* Check the Ignition Coil (Coil-on-Plug System): The 2003 Expedition likely has a coil-on-plug (COP) system where each cylinder has its own individual coil. Visually inspect the coil for cylinder #3 for any cracks or damage. While less common, a faulty coil can also cause a misfire. A bad coil is often more difficult to diagnose visually but will show up in testing.
2. More Advanced Diagnostics:
If the above doesn't solve the problem, you'll need more sophisticated tools and techniques:
* OBD-II Scanner: A code reader will provide diagnostic trouble codes (DTCs). These codes will often pinpoint the problem more accurately. A P0303 code specifically points to a misfire on cylinder 3. Other codes might indicate related issues like a faulty oxygen sensor or MAF sensor that could *indirectly* cause a misfire.
* Compression Test: This measures the pressure in each cylinder. Low compression in cylinder #3 indicates a problem with the valves, piston rings, or head gasket.
* Fuel Injector Test: A faulty fuel injector can prevent fuel from reaching the cylinder. This usually requires specialized tools or a professional mechanic. Injectors can be tested for proper resistance and spray pattern.
* Vacuum Leak: A significant vacuum leak in the intake manifold can affect the air/fuel mixture and cause a misfire.
3. Addressing Specific Potential Problems (Based on Test Results):
* Low Compression: This usually requires a more major repair involving head gasket replacement, valve work, or piston/ring replacement.
* Faulty Fuel Injector: Replacement of the fuel injector for cylinder #3 is necessary.
* Damaged Valve: Requires engine repair, possibly including valve replacement or head work.
* Other Sensor Issues (indicated by DTCs): Replacing the faulty sensor (MAF, oxygen sensor, etc.) may resolve the issue.
Important Considerations:
* Safety First: Disconnect the negative battery cable before working on any electrical components.
* Professional Help: If you're not comfortable working on your vehicle's engine, it's best to take it to a qualified mechanic. Incorrect repairs can cause significant damage.
* Owner's Manual: Consult your owner's manual for specific information about your vehicle's engine and recommended maintenance procedures.
By systematically checking these areas, you should be able to pinpoint the cause of the misfire on cylinder #3 in your 2003 Ford Expedition. Remember to always prioritize safety and seek professional assistance if needed.
