Remove and replace all eight spark plugs on the 390ci V8 engine to restore proper ignition and engine performance.

Warnings

Tools required

Parts

• Spark plug × 8 — Champion N12Y or equivalent (0.035" gap)

Preparation

1. Ensure engine is completely cool to the touch
2. Park vehicle on level surface and set parking brake
3. Open hood and secure with prop rod
4. Clean any debris from around valve covers and spark plug area to prevent contamination

Procedure

1. 1

   Access driver side spark plugs
   Identify the four spark plugs on the driver side (odd cylinders 1, 3, 5, 7 from front to rear). The plugs are accessible from above without removing any major components on this engine.
2. 2

   Remove first spark plug wire
   Starting with cylinder #1 (front driver side), grasp the spark plug wire boot firmly and twist slightly while pulling straight up to disconnect from the spark plug. Do not pull on the wire itself. If boots are stuck, use boot pliers to twist and remove.
3. 3

   Clean spark plug area
   Use compressed air or a vacuum to clean around the exposed spark plug to prevent dirt and debris from falling into the cylinder when the plug is removed. This is critical to prevent scoring the cylinder walls.
4. 4

   Remove spark plug
   Install the 5/8" spark plug socket with appropriate extension onto the spark plug. Turn counterclockwise to loosen. Once broken free, unscrew by hand using the ratchet to prevent cross-threading or dropping the plug. Remove the plug and inspect for fouling, wear, or unusual deposits.
5. 5

   Prepare new spark plug
   Verify the gap on the new spark plug is set to 0.035" using a gap tool. Adjust if necessary by gently bending the ground electrode. Apply a small amount of anti-seize compound to the threads (avoid getting any on the electrode or insulator). Do not over-apply as excess can foul the plug.
6. 6

   Install new spark plug
   Thread the new spark plug by hand into the cylinder head using the socket and extension, turning clockwise. This ensures proper thread engagement and prevents cross-threading. Once hand-tight, use the ratchet to tighten. Tighten to approximately 25-30 ft-lbs or 1/16 turn past hand-tight with a tapered seat plug. Do not overtighten as this can damage the aluminum cylinder head threads.
7. 7

   Reconnect spark plug wire
   Apply a small amount of dielectric grease inside the spark plug wire boot. Press the boot firmly onto the spark plug until it clicks or seats completely. Ensure the wire is properly routed away from exhaust manifolds and moving components.
8. 8

   Complete driver side plugs
   Repeat steps 2 through 7 for the remaining three driver side spark plugs (cylinders 3, 5, and 7, moving from front to rear). Work on one plug at a time to maintain organization and prevent mixing up wires.
9. 9

   Access passenger side spark plugs
   Move to the passenger side of the engine. The four spark plugs (even cylinders 2, 4, 6, 8 from front to rear) are accessible from above. Cylinder #2 may require reaching past or moving the air cleaner assembly slightly for better access.
10. 10

    Replace passenger side spark plugs
    Repeat the plug replacement procedure (steps 2 through 7) for all four passenger side spark plugs. Start with cylinder #2 at the front and work toward cylinder #8 at the rear. Use extensions as needed to reach plugs comfortably. The rear plugs may require a longer extension for proper leverage.
11. 11

    Verify all connections
    Double-check that all eight spark plug wires are firmly seated on their respective plugs with a distinct click or firm connection. Verify wires are properly routed in their factory looms and separated from heat sources. Ensure no tools or parts were left in the engine bay.

Reassembly

1. No disassembly of major components required for this procedure
2. Ensure all spark plug wire boots are fully seated and wires are properly routed
3. Replace air cleaner assembly if it was moved during access to passenger side plugs

Verification

• Start the engine and listen for smooth, even idle with no misfiring or hesitation
• Verify no vacuum leaks or unusual sounds from spark plug areas
• Allow engine to reach operating temperature and confirm smooth acceleration with no stumbling
• Visually inspect all plug wire connections one final time with engine running to ensure no arcing or sparking visible in low light conditions

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