All Spark Plugs Are Not Created Equal
What are Your Spark Plugs Telling You?
Spark plugs provide the spark to ignite the air-fuel mixture in the combustion chamber of a gasoline engine. They consist of a steel shell, an insulator core, and a minimum of two electrodes (some plugs have more than one ground electrode). Even though the spark plug’s basic purpose and component definitions have not changed for more than a hundred years, all spark plugs are not created equal. Let’s look at a few ways they can vary.
HEAT RANGE – Spark plugs are very heat sensitive. As a result they are designed with various heat ranges to handle a wide range of engine cylinder temperatures. For example, a spark plug for a daily-driver engine that idles a lot requires a hot plug to keep the center electrode at optimal temperature to burn off deposits that would otherwise cause poor performance. Whereas a high-performance engine, that turns at high rpm, requires a colder plug to transfer heat away from the electrode.
ELECTRODE METAL – There are at least two electrodes in a spark plug. They are the center and ground electrodes. The electrodes may be made of various metals and metal alloys, such as nickel and chromium, copper or platinum tipped. Platinum-tipped spark plugs having a higher melting point and longer service life than the aforementioned electrode metals.
GAP – The spark plug’s gap refers to the space between the ground electrode and the center electrode (see Figure 1). It’s important to set the gap to the manufacture’s specification for optimal engine operation. A gap set too wide will require a higher voltage to jump the gap. If there is not enough available voltage to perform this task, a misfire will result. Furthermore, a gap that is too narrow increases current flow which may burn or pit the electrodes.
REACH – Engines require spark plugs with a specific “reach.” The reach is the distance from the seat of the spark plug to the bottom of the metal shell. The correct reach allows the electrodes to be in the optimal firing location in the combustion chamber.
THREAD DIAMETER & SEAT TYPE – Most automotive spark plugs are either 14 or 18 millimeters. Some spark plugs have a tapered seat while others are a flat type. A tapered seat does not require a gasket, whereas spark plugs with a flat seat require a gasket.
When Should They Be Replaced?
All spark plugs eventually wear out. While certain materials, like platinum, have extended the service life of spark plugs, they still need to be replaced. Refer to the vehicle’s Owner’s Manual for the replacement interval for your vehicle. For the proper spark plug recommendation, refer to the Vehicle Emission Control Information label located in the engine compartment or the vehicle’s owner’s manual.
Removal and Installation
After the engine has cooled, remove the spark plugs while keeping them arranged in the corresponding order to the cylinders of the engine. Examine them for burned electrodes, fouling, or cracked or broken porcelain insulators. An isolated plug displaying an abnormal condition indicates a problem exists in the corresponding cylinder.
Spark plugs that have low mileage may be cleaned and reinstalled if they are not damaged. After cleaning, file the center electrodes flat with a small file or jeweler’s file. Adjust the gap between the electrodes (Figure 1) to the dimension specified on the Vehicle Emission Control Information label located in the engine compartment of the vehicle. Some labels may not show spark plug information. If this is the case, then refer to the vehicle’s owner’s manual.
Always tighten spark plugs to the specified torque. Over tightening can cause distortion resulting in a change to the spark plug gap.
What Are Your Spark Plugs Telling You?
NORMAL – A spark plug with a few deposits present will probably be light tan or slightly gray in color. The spark plug will not show evidence of electrode burning. Spark plugs with normal wear can usually be cleaned. File the electrodes, re-gap the plug, and then reinstall.
COLD (CARBON) FOULING – A dry, black deposit on one or two plugs in a set may be caused by sticking valves or defective spark plug cables (if equipped). Cold fouling of the entire set may be caused by a clogged air cleaner. Note that cold (carbon) fouling is normal after short operating periods as the spark plugs do not reach a high enough operating temperature.
WET FOULING (Not shown) – A spark plug coated with excessive wet fuel or oil is wet fouled. In older engines, worn pistons rings or excessive cylinder wear can cause wet fouling. Break-in fouling of new engines may occur before normal oil control is achieved. In new or recently overhauled engines, wet fouled spark plugs can be cleaned and reinstalled.
OIL OR ASH ENCRUSTED – If one or more spark plug(s) are oil or ash encrusted the engine should be evaluated for the cause of oil entry into the combustion chamber.
ELECTRODE GAP BRIDGING – Loose deposits in the combustion camber can cause electrode gap bridging. The deposits accumulate on the spark plugs during continuous stop-and-go driving. When the engine is suddenly subjected to a high torque load, the deposits partially liquefy and bridge the gap between the electrodes.
SCAVENGER DEPOSITS – Fuel scavenger deposits may appear either white or yellow. They are a normal condition caused by chemical additives in certain gasoline. Spark plugs with this condition can be cleaned and reinstalled.
CHIPPED ELECTRODE INSULATOR – A chipped electrode insulator usually results from bending the center electrode while adjusting the spark plug gap. However, under certain conditions, severe detonation can separate the insulator from the center electrode. Replace spark plugs when this condition arises.
PREIGNITION DAMAGE – Excessive combustion chamber temperature can cause preignition damage. Determine if the spark plug has the correct heat range rating for the engine. This condition can also be caused by excessive combustion chamber deposits.
SPARK PLUG OVERHEATING – Overheating is indicated by a white or gray center electrode insulator that also appears blistered. Verify that the correct heat range spark plug is used. Over time, advanced ignition timing, detonation and cooling system malfunctions can also cause spark plug overheating.
