The spark plugs in your GS300 Lexus complete the compression process that makes your engine run. As fuel and oxygen are drawn into the cylinders, the piston comes up and at the top of the stroke, the spark plug ignites the mixture. The resulting explosion forces the piston back down. If the spark plug does not pass the electrical charge into the cylinder, the car will misfire and the engine will "sputter." Replacing spark plugs is not difficult. You can finish the project in about an hour.
Measure the gap for each of the new spark plugs with the feeler gauge. The "gap" is the space between the filament and the firing point at the top of the spark plug. You measure the space in between the firing point and the filament with the appropriate blade on feeler gauge. In this case, the gap for the Lexus spark plugs should be .044 thousandths. Spark plugs are factory set but you should still check each one.
Pull the spark plug wire off the plug by grasping the boot as close to the engine as possible and pulling it gently away from the spark plug. Remove the plug from the cylinder head using the spark plug socket and ratchet and discard it.
Place a new plug in the cylinder head of the GS300. Tighten it with the ratchet and spark plug socket. Be careful not to cross-thread the plug or you will damage the cylinder head. Plug the spark plug wire back on the plug. Repeat the process on the next plug.
Inspect the spark plug wires as you change each one of the plugs. If there is any sign of damage, you should replace the entire set of wires.
Do not over-tighten the spark plugs or you will damage the plug and possibly the cylinder head.