Ignition System:
* Worn spark plugs: This is a very common cause of misfires at higher RPMs. The increased speed demands more consistent and powerful sparks. Worn plugs can struggle to ignite the air/fuel mixture reliably.
* Worn or damaged spark plug wires: Cracks or corrosion in the wires can prevent proper spark delivery. High RPM increases the voltage demands, making worn wires more problematic.
* Ignition coil(s): The ignition coil(s) provide the high voltage for the spark plugs. A failing coil might not be able to generate enough power at higher RPMs, leading to misfires on one or more cylinders. These often fail at high temperatures.
* Ignition control module (ICM): This module controls the firing order of the spark plugs. A faulty ICM can cause misfires, especially at specific RPM ranges.
Fuel System:
* Fuel injectors: A clogged or failing fuel injector can't deliver enough fuel to a cylinder at higher RPMs, resulting in a lean condition and a misfire.
* Fuel pump: A weak fuel pump might not be able to supply sufficient fuel pressure at high demand.
* Fuel filter: A clogged fuel filter restricts fuel flow, similar to a failing fuel pump.
Engine Mechanical Issues:
* Valvetrain issues: Worn valve lifters, rocker arms, or valves themselves could cause a misfire at high RPM. The increased engine speed exaggerates any mechanical imperfections.
* Intake Air Restriction: A partially blocked air filter, MAF sensor issue, or other restriction in the intake system can cause a lean condition and misfires. This is more likely at higher RPMs due to increased air demand.
* Exhaust Restriction: A severely clogged catalytic converter can create back pressure that affects engine performance and might contribute to misfires.
Sensors:
* Mass Air Flow (MAF) sensor: This sensor measures the amount of air entering the engine. A faulty MAF sensor can lead to an incorrect air/fuel mixture, causing misfires.
* Crankshaft Position Sensor (CKP): This sensor tells the engine's computer where the crankshaft is. A faulty CKP can disrupt timing and cause misfires.
* Cam Position Sensor (CMP): Similar to the CKP, this sensor is crucial for proper timing.
Troubleshooting Steps:
1. Check for trouble codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs). This can provide valuable clues about the source of the problem.
2. Inspect spark plugs and wires: Check the condition of the spark plugs and wires visually for wear, damage, or cracks.
3. Check fuel pressure: A mechanic can measure fuel pressure to ensure it's within specifications.
4. Perform a compression test: This will check the health of your cylinders. Low compression could indicate a mechanical problem.
It's highly recommended to take your Expedition to a qualified mechanic for proper diagnosis and repair. Attempting complex repairs without the necessary knowledge and tools can lead to further damage. The systematic approach outlined above should help guide a mechanic to efficiently diagnose and fix the problem.
