How to Change Spark Plugs

Spark Plug Overview:

A spark plug is used to ignite vaporized fuel, in order to start an internal combustion engine. It does this by supplying a high voltage between two metal contacts.  The gap between the metal contacts is small which creates a high electric field (E = Volts/distance).  Note that this gap is different for different car makers, models, engines, etc.

Normally, gases are non-conductive.  When a sufficiently high electric field is applied (equal or greater than the ‘critical field’) the medium conducts. The spark plug creates an electric field greater than this critical field, causing current to flow (i.e. the “spark”) between the air gap.   

This current ignites the vaporized gasoline. Since the spark plug is placed next to the ignition chamber, the spark plug must be able to tolerate the extreme heat and high pressure environment. 

This harsh environment leads to a number of failure scenarios.  High absolute temperatures and thermal cycling can lead to melting of portions of the plug, mechanical failure, and the deposition of contaminants on the electrodes.  High temperatures and different ambient can lead to chemical reactions with the spark plug alloys and chemicals dissolved in the surrounding gases, ultimately leading to failure.  The chance of failure increases with the spark plugs number of miles driven. Typically, spark plugs are replaced after 30,000 miles. 

Changing the Spark Plugs:

To change your spark plugs, first obtain your car’s firing order.  Intuitively, the firing order describes the order in which cylinders (and therefore spark plugs) are fired during the operation of your engine.  The correct spark plug firing order must be used to ensure optimal operation.  Record the location of the spark plug wires and spark plugs prior to the changing process.  Taking a picture is another easy way to remember the correct location of everything. Alternatively, you can install one spark plug at a time.

After the wires are removed, use a torque wrench with a spark plug socket to remove the spark plugs.  Material deposited around the spark plug during engine operation may make removal difficult. If so, use penetrating oil such as those made by Kroil.

Next, check your factory service manual to find the optimal spark plug gaps for your engine.  You can use a wire or taper feeler gauge to measure the gap of your plugs. Measure the gap of your plug and ensure it is the correct gap for your engine.

It is advisable to apply some dielectric grease during the installation of your spark plug. The dielectric grease is applied to the ceramic casing of the spark plug. As its name implies, the dielectric grease is non-conductive, and improves lifetime of the device.  Apply some dielectric grease to the inside of the rubber spark plug wire, too.  Now, put the spark plug back in the socket.  You may also put a small dab of anti-seize on the spark plug threads and rail bolts.

Tighten the rail bolts until a torque of 20 to 30 foot-pounds.   Do so for all your spark plugs, and you’re done!