1. Gather the necessary tools and parts. You will need:
* A new set of spark plugs (8 for the LS400)
* A spark plug wrench
* A ratchet and socket set
* A gap gauge
* A Phillips head screwdriver
* A wire brush
2. Prepare the engine.
* Turn off the engine and allow it to cool down completely.
* Disconnect the negative terminal of the battery.
3. Locate the spark plugs.
* The spark plugs are located on top of the engine, near the front of the vehicle.
*There is a large plastic cover over the spark plugs. Use the screwdriver to unscrew the four screws holding this cover down, and then remove the cover.
* On each of the spark plug wire looms there is a black tab sticking up. Press down on that tab and pull to detach the spark plug wires.
4. Remove the old spark plugs.
* Use the spark plug wrench to loosen and remove the old spark plugs.
* Be careful not to drop the old spark plugs into the engine.
5. Gap the new spark plugs.
* Use a gap gauge to set the gap between the center electrode and the side electrode of each new spark plug.
* The gap should be .044 inches (1.1 mm).
6. Install the new spark plugs.
* Screw the new spark plugs into the cylinder heads by hand until they are snug.
* Use the spark plug wrench to tighten the spark plugs until they are snug.
* Do not overtighten the spark plugs.
7. Reattach the spark plug wires.
* Push the spark plug wires back onto the spark plugs.
* Ensure that the wires are fully seated.
8. Replace the spark plug cover.
* Replace the plastic cover that you removed in step 3.
* Secure the cover using the four screws that you removed earlier.
9. Reconnect the battery.
* Reconnect the negative terminal of the battery.
10. Start the engine.
* Start the engine and let it run for a few minutes.
* Listen for any unusual noises or vibrations.
* If you hear anything unusual, turn off the engine and investigate the problem.
