1. Recheck all Vacuum Lines:
* Thorough Inspection: This is the most likely culprit. Carefully examine *every* vacuum line connected to the intake manifold and throttle body. Look for:
* Disconnected lines: Obvious visual check.
* Cracked or damaged lines: Look for small cracks, especially near clamps. Replace any suspect lines.
* Incorrect routing: Ensure each line is connected to the correct port. Consult a repair manual (Chilton's or Haynes) or a reliable online resource (like a Ford forum specific to your year and engine) for a vacuum line diagram.
* Loose clamps: Tighten all clamps securely.
2. Check for Intake Manifold Leaks:
* Visual Inspection: Look for any gaps or cracks around the intake manifold gasket. Use a flashlight to get a good look into all crevices.
* Listen for Leaks: With the engine running, carefully listen around the intake manifold for any hissing sounds indicating a leak.
* Spray Test (Optional): With the engine running, carefully spray a mixture of soapy water around the intake manifold gasket. Bubbles will indicate a leak. Caution: Don't spray near any electrical connections or hot surfaces.
3. Check Ignition System:
While less likely, if vacuum lines are all good:
* Spark Plugs and Wires: Inspect the spark plugs for fouling, damage, or incorrect gap. Check the spark plug wires for cracks, damage, or loose connections. Consider replacing them if they are old or damaged.
* Distributor Cap and Rotor (if applicable): If your engine uses a distributor, check the cap and rotor for wear, cracks, or carbon buildup.
4. Mass Airflow Sensor (MAF) and Throttle Position Sensor (TPS):
* These sensors are crucial for proper engine operation. Disconnecting and reconnecting the intake manifold could have jarred them loose or affected them. Disconnect the MAF sensor, carefully clean it with MAF sensor cleaner (following the instructions), and reconnect. Inspect the TPS for damage or loose connections.
* Consider a sensor cleaning or replacement if the problem persists.
5. Check for Codes:
Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the vehicle's computer. This will provide valuable clues about the cause of the misfire.
6. Fuel System (Less Likely):
If all else fails, a fuel delivery problem is a possibility, though less likely given the recent intake work. You could check fuel pressure if you suspect a problem in this area, but this should be left to someone with experience unless you are comfortable working with fuel systems.
Important Note: A repair manual for your specific year and model Ford Expedition will be invaluable for this troubleshooting process. It will have detailed diagrams and procedures. If you're uncomfortable working on the engine yourself, it's best to take it to a qualified mechanic. Misfires can lead to further engine damage if not addressed correctly.
