1. Ignition System:
* Spark Plugs and Wires: Worn, fouled, or damaged spark plugs are the most likely suspects. Inspect them for wear, carbon buildup, cracks, or gaps that are out of specification. Similarly, cracked or loose spark plug wires can prevent a spark from reaching the cylinder. Check for proper connection and insulation integrity.
* Ignition Coil Packs: The 4.6L often uses coil-on-plug (COP) ignition, meaning each spark plug has its own coil. A faulty coil pack for cylinders 7 and 8 (typically the rearmost on this engine) will prevent firing. You might need a multimeter to check their resistance. A visual inspection for cracks or burns can also help.
* Ignition Control Module (ICM): While less common, a failing ICM can affect multiple cylinders, including the rear two. This is more likely if other cylinders also show intermittent misfires. Diagnosis requires specialized tools.
* Crankshaft Position Sensor (CKP): A faulty CKP won't allow the engine computer to time the ignition properly, potentially leading to misfires across multiple cylinders.
2. Fuel System:
* Fuel Injectors: Clogged or faulty fuel injectors in cylinders 7 and 8 will prevent fuel delivery. A fuel pressure test is needed to rule this out. While less likely to affect only those two cylinders simultaneously, it's still possible.
* Fuel Pump: A failing fuel pump could reduce fuel pressure, possibly affecting the cylinders furthest from the pump (the rear ones). However, this would likely cause issues across multiple cylinders, not just the back two.
3. Engine Management System (Computer):
* PCM (Powertrain Control Module): While uncommon, a faulty PCM can cause misfires in specific cylinders or cylinder banks. This would usually require a diagnostic scan tool to find a fault code.
4. Other Potential Issues:
* Vacuum Leaks: While less directly related, significant vacuum leaks could affect engine performance and cause misfires.
* Compression Issues: Low compression in cylinders 7 and 8 due to worn piston rings, valve problems, or head gasket failure could prevent proper combustion. A compression test is crucial to diagnose this.
Troubleshooting Steps:
1. Visual Inspection: Check spark plugs, wires, and coil packs for visible damage.
2. Diagnostic Scan: Use an OBD-II scanner to retrieve any diagnostic trouble codes (DTCs). This will give clues to the problem.
3. Spark Test: Check for spark at each of the rearmost plugs. A simple spark tester can determine if the ignition system is delivering spark.
4. Fuel Pressure Test: Check the fuel pressure to ensure adequate fuel is reaching the injectors.
5. Compression Test: Measure compression in cylinders 7 and 8 to check for mechanical issues.
It's highly recommended to start with the simplest and most likely causes (spark plugs and wires, then coil packs) and proceed systematically. If you're not comfortable performing these checks yourself, take the car to a qualified mechanic for diagnosis and repair. Trying to fix electrical problems without the right knowledge and safety precautions can be dangerous.
