Chapter 8 Ignition System Fundamentals 155 Copyright by Goodheart-Willcox Co., Inc. The ground electrode is attached to the steel outer shell, which is threaded into the engine. The top of the steel shell is crimped over to bear against a seal. The crimp tightly grips the insulator and also forms a pressure seal at both the top and bottom of the insulator to prevent combustion leaks and electrical arcing. Spark plug gaps can vary between 0.025″–0.100″ (0.635–2.54 mm), depending on the engine and ignition sys- tem. The spark gap is formed between the center and side, or ground, electrodes. Spark plug side electrodes come in a variety of designs and shapes. The center electrode is insulated from the rest of the plug by a ceramic insulator. The insulator is made of a ceramic material. A common material used for making spark plug insulators is aluminum oxide. The aluminum oxide is fired at high temperature to produce an insulator that is glassy, smooth, dense, and very hard. Spark plugs are manufactured in many heat ranges. Heat range is determined by the diameter and length of the insulator as measured from the sealing ring down to the plug tip and to the cooling system, Figure 8-24. Spark plug heat range is vital to proper engine operation. A plug that operates without becoming hot will be fouled by combus- tion byproducts. A plug that is too hot will begin to glow. This will ignite the air-fuel mixture too early, causing deto- nation and possibly engine damage. Spark plugs are manufactured in many thread types and lengths. The most common thread sizes are 14 mm and 10 mm. In a few older vehicles, 18 mm plugs are used. Directional Spark Plugs Some spark plugs used in distributorless ignition sys- tems are fired from the side (ground) electrode to the center electrode, which is opposite of conventional spark plugs. These plugs are known as directional spark plugs. They are equipped with special electrodes to prevent misfires. On many engines with waste spark or integrated direct ignition systems, half of the plugs are conventional and half the plugs are directional. When servicing the plugs on these types of engines, the technician must be sure to install all spark plugs in their original locations. Replacement spark plugs for these engines are usually the same for all cylinders. Misfire Monitor Engine misfiring due to ignition, fuel, or compression system problems can cause excessive emissions and per- formance problems. The OBD II protocol requires that the computer control system monitor the engine for misfiring. This system is called the misfire monitor and is an inter- nal function of the ECM. The monitor is sensitive enough to detect a misfire within 200 crankshaft revolutions. The OBD II protocol requires that misfires be recognized by the ECM under these conditions: • Misfire sufficient to result in damage to the catalytic converter. This is classified as a type A misfire. • Misfire sufficient to create exhaust emission levels exceeding 1.5 times Federal standards. This is classi- fied as a type B misfire. Each cylinder contributes to overall engine speed and power. When a misfire occurs, engine power drops, but, more importantly, the crankshaft rotational velocity momentarily decreases. To monitor the engine for misfir- ing, the ECM keeps a record of crankshaft revolutions. The ECM-monitored crankshaft readings are divided into separate, even samples, Figure 8-25. If a misfire occurs, one or more of the crankshaft speed samples will have an unex- pected drop or fluctuation. Some engines use an ion sensor, described later in this section and in Chapter 7, to check for misfires. Ion-sensing ignition systems eliminate most false misfire problems. Note: Some systems can set misfire trouble codes for individual cylinders. Misfire Monitor Operation When a misfire condition exists that will raise emis- sions levels, the ECM will set a trouble code and illuminate the MIL. If the problem is intermittent, the light will go out, but will illuminate again if the misfire reoccurs under the same conditions and within a certain number of engine warm-ups. If a misfire condition exists that can cause damage to the catalytic converter, a trouble code is set and, on most systems, the MIL flashes. When the condition no longer exists, the MIL stays on without flashing. All OBD II systems have trouble codes for random or multiple-cylinder misfires. Chapter 7 discusses OBD II sys- tem operation in more detail. Rough roads can sometimes cause a false misfire detection. Driving over a rough road can cause a varying load to be placed on the drive wheels, which can temporarily Figure 8-24. Spark plug heat ranges are determined by how far the heat must travel from the plug tip to the plug body. The far- ther the heat must travel, the higher the heat range of the plug.