1. Preparation:
* Disconnect the negative battery terminal: This is crucial for safety.
* Gather your tools: You'll need a spark plug socket (usually 5/8" or 13/16", check your owner's manual), a ratchet, extension(s) (likely needed), and a spark plug gap tool (optional, but recommended for checking and setting the gap). Possibly a swivel socket extension to help maneuver in tight spaces. Gloves are also a good idea.
2. Accessing the Spark Plugs:
* Remove the air intake system: This is usually the biggest hurdle. It involves unclipping various hoses and possibly removing the air filter housing. Consult a repair manual or online resources (like YouTube videos specific to a 1999 Ford Explorer 4.0L) for the exact steps. Take photos as you go to help with reassembly.
* Remove the ignition coil pack(s): On the 4.0L, the ignition coils are usually grouped together. You'll likely need to unbolt or unplug them to get access to the spark plugs. Again, reference a repair manual or online guide. Be mindful of the delicate coil pack connectors.
* Access the spark plug tubes: Once the air intake and coils are out of the way, you should have better access to the spark plug tubes, which are the vertical cylinders containing the spark plugs.
3. Removing and Replacing the Spark Plugs:
* Remove the spark plugs: Carefully use your spark plug socket and ratchet to remove each plug. Go slow and steady to avoid stripping the threads. A swivel extension will help greatly here.
* Inspect the old spark plugs: Check their condition for wear, fouling, or damage.
* Install the new spark plugs: Gently hand-thread the new spark plugs into the holes before using the socket to tighten them. Don't overtighten! Use a torque wrench if possible (your repair manual will specify the correct torque setting). After installation, check the spark plug gap with a gap tool if needed.
* Reassemble everything: Carefully reinstall the ignition coils, air intake system, and any other components you removed.
Important Considerations:
* Repair Manual: A repair manual specific to your 1999 Ford Explorer is invaluable. It will provide detailed diagrams and instructions. A Haynes or Chilton manual is a good investment.
* Online Resources: YouTube is a great resource for visual guides. Search for "1999 Ford Explorer 4.0L spark plug replacement" to find videos that might show you the exact procedure.
* Safety First: Always disconnect the negative battery terminal before working on any electrical components.
* Patience: This is not necessarily a quick job. Be patient and methodical to avoid damaging anything.
If you are not comfortable working on your car yourself, take it to a qualified mechanic. Improperly installed spark plugs can cause serious engine damage.
