Simple Checks (Do these first!):
* Correct Gap: Did you gap the new spark plugs correctly? An incorrect gap can lead to misfires. Check your owner's manual or a reliable online source for the correct gap specification for your engine. Use a feeler gauge to measure.
* Proper Plug Installation: Ensure the plugs are seated firmly and correctly. Cross-threading can cause misfires. If you suspect cross-threading, you may need to carefully remove the plug and inspect the threads.
* Secure Wire Connections: Double-check that all spark plug wires are securely connected to both the plugs and the distributor (or coil packs, depending on your engine). A loose connection is a common cause of misfires.
* Worn Distributor Cap/Rotor (if applicable): If your Expedition uses a distributor, the cap and rotor can wear down, leading to poor spark delivery even with new wires. Inspect them for cracks, carbon buildup, or wear.
* Ignition Coil(s): A faulty ignition coil (or individual coil packs) can cause misfires in one or more cylinders. These can be tested with a multimeter or by swapping them with known good coils (if you have spares or can borrow some).
* Bad Spark Plug Wires (despite being new): While unlikely, it's possible you received a faulty set of wires. Visually inspect them for any damage (cuts, cracks).
More Complex Issues (If simple checks don't solve the problem):
* Ignition Control Module (ICM): The ICM controls the ignition system. A faulty ICM can cause intermittent or consistent misfires.
* Crankshaft Position Sensor (CKP): This sensor tells the computer the engine's rotational speed and position. A faulty CKP can disrupt ignition timing and cause misfires.
* Camshaft Position Sensor (CMP): Similar to the CKP, a faulty CMP can cause misfiring issues by disrupting the timing of the valves and ignition.
* Mass Airflow Sensor (MAF): A dirty or faulty MAF sensor can provide inaccurate readings to the engine computer, leading to improper fuel/air mixture and misfires.
* Fuel System Problems: While less likely after only replacing plugs and wires, issues like a clogged fuel filter, low fuel pressure, or faulty fuel injectors can contribute to misfires.
* Vacuum Leaks: Leaks in the vacuum system can affect the engine's performance and contribute to misfires.
* Computer Issues: In rare cases, a problem with the engine control module (ECM) could be causing the misfire.
Troubleshooting Steps:
1. Start with the simple checks. These are the easiest and cheapest to address.
2. Use a code reader: A code reader (OBD-II scanner) will retrieve diagnostic trouble codes (DTCs) from your Expedition's computer. These codes can pinpoint the likely source of the misfire.
3. Visual inspection: Carefully inspect all components related to the ignition system for any signs of damage or wear.
4. Systematic testing: If you have some basic automotive knowledge, you can start testing components individually (e.g., checking for spark at each plug, testing ignition coils).
5. Professional help: If you're not comfortable performing these tests yourself, take your Expedition to a qualified mechanic for diagnosis and repair.
Remember safety first! Disconnect the negative battery terminal before working on any electrical components in your vehicle.
