Most Likely Culprits:
* Ignition System: Worn spark plugs, failing ignition coils (especially common on Expeditions of that era), or damaged ignition wires are prime suspects. Under load, a weak component might fail to ignite the fuel-air mixture consistently.
* Fuel System: A clogged fuel filter restricting fuel flow under pressure (especially uphill) can lead to misfires. A failing fuel pump might not supply enough fuel at higher demand. Low fuel pressure is a major suspect.
* Mass Airflow Sensor (MAF) or Throttle Position Sensor (TPS): These sensors provide critical information to the engine's computer. A faulty sensor can cause the computer to deliver the wrong fuel/air mixture, resulting in misfires under load.
* Engine Vacuum Leaks: A leak in the intake manifold or vacuum lines can disrupt the proper air-fuel mixture, leading to misfires, especially under increased engine load.
* Catalytic Converter: A clogged catalytic converter can create back pressure, leading to misfires. This is less likely to be the *primary* cause but could exacerbate an existing issue.
Troubleshooting Steps:
1. Check Engine Light (CEL): Is the CEL on? If so, have the codes read using an OBD-II scanner. The codes will pinpoint potential problem areas.
2. Inspect Spark Plugs and Wires: Visually inspect the spark plugs for wear, fouling, or damage. Check the ignition wires for cracks, fraying, or loose connections. Replace as needed. This is a relatively easy and inexpensive first step.
3. Test Ignition Coils: This often requires a multimeter or a professional test. Faulty coils are a common cause of misfires, particularly on older vehicles.
4. Check Fuel Pressure: A fuel pressure gauge is needed to test fuel pressure. Low pressure indicates a potential problem with the fuel pump or filter.
5. Inspect the MAF and TPS Sensors: These can be cleaned (with MAF cleaner specifically for the MAF sensor) or tested with a multimeter or scanner. Replacement might be necessary if they're faulty.
6. Check for Vacuum Leaks: Carefully inspect all vacuum hoses and the intake manifold for cracks or leaks. Use a propane torch (carefully!) to check for changes in engine idle speed near potential leak points. A rising RPM indicates a leak.
7. Inspect the Catalytic Converter: This requires a visual inspection and potentially a pressure test. A severely clogged converter will restrict exhaust flow, leading to problems.
Important Considerations:
* Professional Diagnosis: If you're not comfortable performing these checks yourself, take your Expedition to a qualified mechanic. A proper diagnosis is crucial to avoid unnecessary repairs.
* Specific to Uphill: The fact that the misfire only occurs uphill at a higher RPM suggests a problem with the engine's ability to handle increased load and fuel demands. This narrows down the possibilities.
By systematically checking these components, you should be able to pinpoint the cause of the misfire. Remember safety first – always disconnect the negative battery terminal before working on any electrical components.
