Replacing spark plugs may seem like a simple job, but proper procedures must be followed to ensure the best results. While you should always use the vehicle manufacturer’s service procedures, which may include removal of certain engine components to access the spark plugs, here is a basic a step-by-step guide.
It is always best to use a torque wrench to install spark plugs. If a torque wrench is not available, however, you may use this chart as a guide.
Note: These suggestions are for spark plugs with new gaskets.
Do not assume that the spark plug gap was preset at the factory when the spark plugs were made. While Autolite engineers pre-gap spark plugs for the most popular applications, the same spark plug may be designed for two different engines, and these may specify different gaps.
Spark plugs are wear items. Even though some of the most premium spark plugs can last over 100,000 miles (160,000 km), eventually all spark plugs wear out and must be replaced to ensure consistent engine performance.
The tip of the center electrode and the ground-strap electrode probably face the most severe conditions, so wear will be most apparent at these points. The extent of electrode wear will vary based on:
Owners should always follow the manufacturer’s suggested service interval unless there are driveability problems due to misfiring. Obviously the ideal service interval for a spark plug is to replace it before it starts to misfire.Some of the possible causes of misfiring are listed below.
On modern engines with computer-controlled ignition systems and onboard diagnostics, certain types of misfires will trip the Malfunction Indicator Light (MIL – also called a check engine light). The driver may experience a rough idle, sluggish performance and/or poor fuel economy as a result of a misfire.
If the MIL flashes, this generally indicates a catalyst-damaging misfire, which must be investigated and repaired immediately.Possible Causes:
To form a perfect seal in the combustion chamber, each spark plug must exactly match the dimensional characteristics of the cylinder head it is designed to fit in two important ways: (1) Thread Diameter, and (2) Reach.
Reach. Dimensional match is critical since it determines the position of the spark gap inside the combustion chamber.
Thread Diameter. Spark Plugs are made in 10, 12, 14, and 18 mm thread diameters. These thread dimensions have been standardized by the Society of Automotive Engineers (SAE) and are used by all spark plug manufacturers.
If the spark plug is too cold for its application, it could lead to misfires due to fouling
If the spark plug used in an engine is too hot for its application, it could result in: