Distributorless Ignition Systems

14-03 Explore the components of distributorless ignition systems.

The third type of ignition system is the DIS. DISs, also known as EI systems, eliminate the distributor replacing it multiple ignition coils. Distributorless systems use either one coil per cylinder or one coil for each pair of cylinders. In waste spark systems, there is one ignition coil for each pair of companion cylinders. For direct-ignition systems, there is one ignition coil for each cylinder. The spark plugs are directly fired from each coil. Spark timing is electronically controlled by a module or the PCM.

Multi-coil systems include waste spark (DIS), CNP, and COP systems (FIGURE 14-16). Waste spark systems share a single coil to fire a pair of companion spark plugs simultaneously, per each crankshaft rotation. The paired cylinder firing includes one that ignites the air-fuel mixture for the cylinder on the compression stroke while the other plug fires a cylinder nearing the end of the exhaust stroke. The cylinder that fires on the exhaust stroke is said to be “wasting” the spark, since it does not develop any power. A waste spark system can use either bypass control or up-integrated control.

FIGURE 14-16 Using multiple coils on an engine allows for a longer saturation point for every event, which increases the voltage that is being used by each individual spark plug.

The DISs that have a dedicated coil for each spark plug, direct-ignition systems, are referred to as CNP or COP (which can also mean coil-over-plug) systems. Direct-ignition systems are the current favorite by consensus among vehicle manufacturers. The main advantages of the DIS is increased dwell time (coil saturation) that produces a higher energy output and a longer cooldown time between firing events. Multi-coil ignition systems increase engine performance and reliability, allow flexibility in mounting due to their compact size, and reduce emissions and maintenance. COP systems may also be referred to as direct-ignition systems by some manufacturers. Coil-per-plug systems are typically up-integrated, meaning all ignition is controlled by the PCM.

Since there are no moving parts to contact each other in a DIS, mechanical wear is nonexistent, though electrical thermal cycling can cause electrical component failure. Removing mechanical components eliminates component wear, maintenance, and periodic adjustments. Eliminating the mechanical parts included in a distributor assembly, such as bushings, bearings, the breaker plate assembly, and advance mechanisms, improves ignition timing and performance while decreasing emissions. Also, the DIS reduces routine maintenance by phasing out wear items such as the distributor cap, rotor, contact points, and condenser.

Electronically Controlled Timing Systems

Ignition systems are also classified by how the system processes the trigger signal. The triggering device sends its signal either to the PCM or to an ICM. A vehicle that uses the PCM for complete control of the ignition systems has an up-integrated system. The trigger output on an up-integrated system directly connects to the PCM. Conversely, bypass systems route the trigger signal to the ICM. A bypass ignition system can operate independently of or in combination with the PCM. Splitting system control allows either the module or the PCM to take command of the ignition system based on operating conditions.

The PCM uses signals from a CKP sensor and, if equipped, a CMP sensor as a trigger input to control the on/off signal to the primary windings of the coils. Both sensors are either Hall-effect sensors or induction-type sensors. They let the PCM know the positions of the crankshaft and the camshaft. Not all manufacturers use a CMP sensor. On waste spark systems, the PCM does not need to know when Cylinder 1 is the compression stroke; it needs to know only when Cylinder 1 is at top dead center (TDC), to give the proper spark timing on waste spark systems. During the operation of any coil-per-plug system, the PCM must know the CKP and which stroke each cylinder is on, for ignition control (FIGURE 14-17). The PCM uses the CKP and CMP sensors to identify which cylinder is on the compression stroke. For many manufacturers, the waste spark system was the first ignition system controlled entirely by the PCM.

FIGURE 14-17 Knowing the precise location of the camshaft and crankshaft allows the PCM to control ignition events so that they can happen at the proper time.

Bypass ignition systems use an ICM to process timing information from inputs and to process the trigger signal. The module sends the calculated timing signal to the PCM, as a reference. Depending on the triggering device, the module may have to convert the signal from an AC voltage to a digital voltage before routing it to the PCM. PM sensors produce a square wave that requires processing and amplification by an analog-to-digital (AD) converter before sending the signal to the PCM. Digital triggers, including Hall-effect, MR, and optical, do not require signal conditioning.

One or several circuits in the ICM control primary current flow in a bypass system. Most bypass systems operate off module timing during start-up and low-rpm operation. When the rpm reaches a predetermined level, the PCM takes over and sends ignition timing control information to the module based on direct inputs or conditioned signals from the ICM.

Ignition System Inputs

The PCM uses numerous sensors to control spark timing. Using the triggering circuit to control the primary section directly effects and controls the secondary side of the system. The inputs are as follows:

Electronic Distributorless Ignition Systems

Electronic DISs are the next advancement in the ongoing development of ignition system design. DISs provide significant improvements/upgrades over distributor-based systems:

Waste Spark Ignition System

Waste spark systems, otherwise known as DISs, have been found on in-line and V-type engines since being introduced by Pontiac in 1985. Waste spark systems use one coil for every two cylinders. Cylinders are grouped in pairs, with each pair having its own ignition coil (FIGURE 14-24). Individual coils have their own primary and secondary windings. They can be combined to form one coil pack, or they can have a separate coil for each pair of cylinders. Each coil fires once for every revolution of the crankshaft, creating a high-voltage spark in two opposite cylinders at the same time (FIGURE 14-25). Each ignition coil serves two cylinders, with each end of the secondary winding attached by a high-tension lead to a spark plug. These two plugs are on companion cylinders—that is, cylinders where the pistons reach TDC at the same time. The cylinder on the compression stroke is said to be the event cylinder, because it is the one getting ready to produce power, and the cylinder on the exhaust stroke is the waste cylinder, since the spark has no effect on engine operation (hence the term “waste spark”).

FIGURE 14-24 Paired cylinders in a waste spark system will always have one spark plug that fires from the center terminal to the side terminal, while the other cylinder will fire in reverse, from the side to the center electrode. The spark plug’s polarity does not change.

FIGURE 14-25 Four-cylinder, and some six-cylinder, engines have one coil pack. Others have one two-terminal coil for each pair of cylinders. Eight-, ten- and other six-cylinder engines use multiple coil packs featuring two or four coil towers per pack. On waste spark systems, all ignition timing is module controlled.

TECHNICIAN TIP

In a waste spark system, remember that when one spark plug fires on the compression stroke, the other fires on the exhaust. The cylinders reverse strokes during the next crankshaft revolution. However, the spark always flows in the same direction, either center to the side electrode or the side to the center electrode. The voltage always wants to return to its source, the secondary coil.

Waste Spark Components

The components of a typical waste spark/coil pack ignition system include the CKP sensor, CMP sensor, KS, coil pack, capacitor, spark plug wires, and spark plugs (FIGURE 14-26). Coil packs are generally attached to the engine, combining several coils into a single component. Each coil in a pack fires two spark plugs in different cylinders simultaneously; therefore, there are half as many coils as cylinders. Four- and six-cylinder engines may have one coil pack consisting of two or three coils or multiple two-tower coils to match the number of cylinders. Eight-cylinder engines may use four two-tower coils or two four-tower coils.

FIGURE 14-26 The components in a typical waste spark system ignition system. Not all systems will use a CMP sensor. Another name for a waste spark system is a DIS.

A waste spark coil pack operates differently than a conventional coil. The primary and secondary windings of these coils are not electrically connected, making them a true transformer design. The secondary winding is wired in series and connected to two spark plugs. Secondary current that leaves the coil must overcome both spark plug gaps when firing to return to the negative side of the coil.

Waste Spark Operation

In a waste spark system, spark plugs are paired with the cylinder opposite it in the firing order. Each coil in the multi-coil pack assembly fires both spark plugs for each set of paired cylinders. Both companion cylinders are at TDC at the same time. The cylinders are paired opposites since they are always at opposite ends of the four-stroke engine cycle. One cylinder is on the compression stroke and the other the exhaust stroke (FIGURE 14-27). During the next crankshaft revolution, the roles are reversed, and thus the waste cylinder becomes the event cylinder. The cylinder on compression is now on the exhaust stroke, and the cylinder that was on the exhaust stroke is now on compression. This same process repeats for each pair of companion cylinders as they approach the TDC position. The primary circuit in each coil must therefore trigger at the correct time during each crankshaft revolution. The PCM controls ignition based on sensors inputs.

FIGURE 14-27 A single coil fires a pair of spark plugs that are opposite one another in the firing order. DISs have two spark plugs wired in series, with one coil that may be separate or contained in a coil pack. When the coil fires, one plug fires during the compression stroke, the event cylinder, while the companion cylinder fires on the exhaust stroke, the waste spark cylinder. The next time the coil fires, the cylinder that was on compression will now be on the exhaust stroke and the one on the exhaust stroke will now be on the compression stroke.

Each set of paired cylinders, the spark plugs, engine block, and coil form a series circuit. With every ignition coil discharge, both spark plugs fire at the same time, completing the series circuit. Switching off the current flow through the primary circuit induces a high voltage in the coil’s secondary windings. High-tension wires carry the high voltage generated in the secondary coil windings to the spark plug, supplying the energy needed to jump the air gap between the electrodes. Current travels in a forward direction in one cylinder from the center electrode to the side or ground electrode to the metal plug casing then the cylinder head. Next, the current transfers through the cylinder block to the other paired cylinder. In the companion (paired) cylinder, current flows in the opposite direction, bridging the gap from the side (ground) electrode to the center electrode through the spark plug wire and back to the coil, completing the series circuit. The cylinder on the compression stroke with its charge of fuel and air is “fired” by its spark, driving the piston down on the power stroke, while the spark at the plug of the cylinder on exhaust simply serves to complete the circuit and is thus “wasted.”

Since both plugs in companion cylinders fire at the same time, the module does not need to identify which cylinder is on which stroke (FIGURE 14-28). Because of the lower pressure in the cylinder on the exhaust stroke, its plug requires less voltage to produce an arc. Therefore, most of the available coil’s energy is used to fire the plug in the cylinder that is on the compression stroke. The cylinder on the compression stroke uses most of the available voltage to ignite the air-fuel mixture due to the high cylinder pressures. These pressures create an environment that is more conductive than the inert gases in the exhaust stroke on the opposing cylinder.

FIGURE 14-28 Waste spark ignition systems typically use a coil with separate primary and secondary windings or a common primary circuit for all the coils with an isolated secondary circuit. A single coil fires a pair of spark plugs that are opposite one another in the firing order.

When the coil discharges, the secondary current creates a high-energy, high-voltage spark across the gaps of both spark plugs at the same time, completing the series circuit. The polarity of the coils is fixed and thus remains constant. One plug always fires in a forward direction, while the other fires in reverse (FIGURE 14-29). This means that one plug always fires in a traditional manner from the center electrode to the side electrode, whereas the spark in the other cylinder travels in reversed polarity, always firing from the side electrode to the center electrode. This is different from conventional ignition systems, where the spark plugs fire in the same direction each time. Firing the spark plug in reverse polarity requires more voltage than firing the plug in a conventional manner. Coil design, primary current flow, and coil saturation time are all intended to provide the voltage capacity needed under all operating conditions to fire the coils properly. Most waste spark coils can deliver 40,000 volts or more. In addition to requiring more energy to fire the coil, firing in reverse polarity wears out the spark plug faster.

FIGURE 14-29 DISs have two spark plugs wired in series, with one coil that may be separate or contained in a coil pack. The coil, paired cylinders, high-tension leads, and engine block form a series circuit. The current flows from the coil tower and high-tension lead to the event cylinder, jumping the plug gap in a conventional method from the center electrode to the side electrode, through the block to the waste spark cylinder, across the plug gap in the reverse direction from the side electrode to the center electrode, through its high-tension lead, and back to the coil to complete the series circuit. The coil polarity does not change. One spark plug will always have the spark going from the center to the side electrode and the other spark plug from the side to the center electrode. The plug that fires using the opposite polarity requires more energy to create the spark across the gap.

Output voltage from a DIS coil can reach about 40,000 volts (40 kV). However, this voltage is hardly ever needed. During normal operation, the voltage output only rises to what is needed to jump the spark plug gap. The amount of voltage required depends on several factors. As compression pressure builds, the firing kV climbs on average to between 8 and 22 kV (FIGURE 14-30). On the waste spark, noncompression side, the voltage needed is approximately 3 to 4 kV. Other items that affect firing voltage include the air-fuel mixture, the engine load, and the shape of the spark plug electrode. Lean air-fuel mixtures require more firing voltage to ignite. As load increases, the demands on the ignition system increase, requiring additional voltage to fire the plug. The sharper and finer the tip of the center and side electrodes, the lower the voltage demand. Any combination of these conditions can cause the firing voltage to rise to the point of approaching the coil’s maximum output.

FIGURE 14-30 Scope image showing the difference in firing voltage between the cylinder on compression (power) and the cylinder on exhaust. As compression rises, the amount of voltage needed to jump the spark plug gap increases.

Hybrid Waste Spark Systems

Toyota and Mercedes-Benz are two manufacturers, among others, that use a hybrid-style waste spark system. While coil and ignition system operation are the same, each coil firing a paired group of cylinders, the coils are physically different from most waste spark coils. These systems use a pencil-style coil similar to a COP coil, but they add a high-voltage tower on top of the coil. Ordinarily found on V-type and some in-line engines, the hybrid arrangement directly mounts ignition coils onto the spark plugs of on one bank (like a COP system) (FIGURE 14-31). The second high-voltage tower uses a spark plug wire to connect the coil to the spark plugs on the opposite bank. This arrangement reduces the number of spark plug wires and their potential faults.

FIGURE 14-31 Hybrid-style waste spark system and ignition coils. The coils on the left are from a V6 Toyota engine. The picture on the right is a Mercedes-Benz in-line six-cylinder setup. These coils have an additional high-tension tower connecting each coil to its paired cylinder.

Dual Plug Systems

Given all the advancements in engine design, electronics, and fuel formulations, the thermal efficiency of an internal combustion engine (ICE) is still very low. ICEs are astonishingly inefficient at burning the air-fuel mixture thoroughly and turning the burned fuel into usable energy. The design of ICEs is inefficient since perfect combustion is not possible. Losses from the design of the combustion chamber to thermal and pumping losses all account for efficiency ratings of 25% for most gasoline-fueled engines. Furthermore, the fuel is not burned instantaneous, meaning that there is always fuel left unburned after the ignition and power cycles. This unused fuel is wasted energy and contributes to increased emissions. Using dual plugs, however, can help to overcome some of these inefficiencies.

Various manufacturers currently employ, or have in the past employed, an ignition system with two plugs per cylinder. The theory behind the dual plug design is that firing two spark plugs increases the combustion speed of the fuel, resulting in rapid, intense combustion. Firing the plugs can occur simultaneously or sequentially, the interval between the firings depending on engine load and rpm. Firing two spark plugs increases the diameter of the flame front, burning the fuel more efficiently and almost instantaneously. Dual plug systems are typically found in engines with large combustion chambers, in those that are somewhat inefficient, or in high-performance applications.

Ordinarily, the plugs are in two different areas in the combustion chamber, usually on opposite ends of the cylinder. Generally, sequentially fired systems ignite the intake plug first. As the flame front propagates across the cylinder but before the piston reaches TDC, the exhaust side plug is fired. When the second plug fires, the flame expands rapidly across the combustion chamber. This results in a more complete, more efficient burn and higher cylinder pressures, increasing engine output.

The timing of the interval between firings is a balance between economy and power. For instance, at high rpm and engine load, the ignition events may be almost simultaneous. During acceleration with a large throttle opening and engine rpm below a preset value, the intake spark may be advanced and the exhaust side slightly retarded. If the engine is accelerating during a midrange cruise, the exhaust spark can be retarded further. At part throttle, depending on engine speed, the ignition may be simultaneous. Some dual plug systems may use an inhibit mode during cranking. On these systems, only one spark plug fires until engine rpm reaches a preset value. Once the engine starts, normal dual plug operation starts.

Using two spark plugs per cylinder burns the air-fuel mixture more efficiently in the combustion chamber, reducing HC emissions and improving engine torque and fuel economy. Using two plugs also helps maintain a smooth stable idle and helps reduce detonation. In addition to being used in the automotive world, dual plug designs are found in motorcycles and air planes.

Combination Coil-on-Plug/Distributorless Ignition Systems

Dual spark plug configuration varies by manufacturer. Some may use a waste spark system with a coil that resembles a COP coil, only with an additional high-voltage output terminal, such as early 5.7 L Chrysler Hemi engines. This configuration uses one dual output coil for each pair of cylinders (FIGURE 14-32). Each cylinder shares a coil pack with another cylinder on the opposing bank. The coil sits over the spark plug and uses a high-tension lead that connects the additional high-voltage output terminal to the coil’s paired cylinder on the opposite bank. In other words, an eight-cylinder engine, for example, has eight coils but 16 spark plugs. Both plugs in a cylinder are fired at the same time but by separate coils: one by the coil sitting on top of a spark plug and the other by way of a spark plug wire connected to a coil on the opposite cylinder bank. The additional plug fires during the power stroke to promote more complete combustion, fully burning the HCs.

FIGURE 14-32 Dual spark coil on a Dodge engine. A boot and a spring, similar to a COP design, sit on top of a spark plug. The additional high-voltage output terminal uses a spark plug wire to carry the high-energy spark to a paired cylinder on the opposite bank. Each coil will fire two different cylinders: one on the compression stroke and the other on the exhaust stroke. For this reason, this system takes two coils to fire both spark plugs for the event cylinder.

This early Hemi system requires firing two coils to ignite two spark plugs. Since this is a waste spark system, the cylinders are paired. The firing order of Hemi is 1-8-4-3-6-5-7-2. The paired cylinders, then, are 1 and 6, 5 and 8, 4 and 7, and 3 and 2. To fire Cylinder 1, then, Coils 1 and 6 need to be triggered. Triggering two coils fires four spark plugs: two on Cylinder 1, which is on the compression stroke, and two on Cylinder 6, which is on the exhaust stroke. Operating like a waste spark system, the spark plugs firing on the exhaust stroke for Cylinder 6 do not perform any work and therefore waste spark.

The complexities of this system compared to a standard waste spark system are obvious. Chrysler chose this arrangement to reduce the stress and added current consumption of using a single coil to ignite both spark plugs. Firing a coil in a cylinder under compression requires a higher output voltage. Using a single coil per cylinder to fire both plugs on the compression stroke would lead to each coil consuming much more current to fire the plugs. Using a separate coil to fire each of the two spark plugs results in flowing less current through each coil, reducing stress on the coils.

Chrysler adopted this technology due to trade-offs associated with the hemispherical head (FIGURE 14-33). The opposed valve arrangement of a Hemi delivers good power and breathes well but emits high emissions. A true Hemi, or flattened hemispherical combustion chamber, has quench areas that are problematic for collecting unburned fuel, resulting in high HC content from the exhaust. The additional plug helps promote combustion by shortening how far the flame front must travel. Adding the additional spark plug prevented the need to use a more restrictive, power-robbing catalytic converter to reduce emissions. Dual plugs also work well with lean air-fuel mixtures. The flame front from a single spark plug does not propagate well with lean mixtures, leaving fuel unburned in the exhaust stream, increasing emissions. Dual flame fronts ignite the air-fuel mixture more easily, reducing flame travel and thus improving combustion.

FIGURE 14-33 Because of the design of the combustion chamber on a hemispherical engine, dual spark plugs must be used to guarantee complete combustion.

Because this system uses spark plug wires, it is vulnerable to secondary ignition failures. Common faults on these systems include misfires due to open or high-resistance plug wires. Crossfire issues from high-voltage bleed-off of the plug wires are even worse because they cause more damage. The high-tension leads being improperly routed is a common cause of crossfire issues and misfires. Consistent crossfire can cause severe engine damage. Whenever a misfire issue arises, before delving too deep into looking for an issue, inspect the plug wire condition and routing. Ensure that the wires have been routed correctly and that the retaining clips have been installed in the routing tray that runs over the intake manifold to prevent crossfire.

To determine which cylinder is missing, check the PCM for diagnostic trouble codes (DTCs). If any codes are present, the simplest check is a coil swap, to determine whether the miss moves with the coil. After blowing off the area around the coil with compressed air, remove the coil and spark plugs. Inspect for signs of arcing, carbon tracking, and other physical damage to the boot(s) and spark plugs. Swap the coil with a known-good cylinder. If the problem follows the coil, then it is likely damaged. If moving the coil does not help, swap the spark plug. Another option is to watch the misfire data parameter identification (PID) counters on the scan tool. On many of Chrysler’s vehicles, the manufacturer provides PIDs for misfire data and coil burn times. Select all the coils, then using graph mode, watch for glitches in a cylinder(s) that are different from the others. Check for any one cylinder missing more than the others. This can be done at idle while power braking or during a test drive. If any codes are present, the simplest check is to swap coils with a known-good cylinder and then determine whether the miss moves with the coil. If the problem follows the coil, then it is likely damaged. If moving the coil does not help, move the injector to a known-good cylinder and then determine whether the problem still follows. Note: Don’t forget the basics—a clogged/failed injector, injector driver, or mechanical engine fault can also cause a misfire. Not all misfires are ignition related.

If there are no codes present, perform an rpm drop test. Disconnect the electrical connector of companion coils to determine which cylinder and coils are responsible for the misfire. A misfiring cylinder will have less of an rpm drop than the good cylinders. If the plug wires are not the source of the misfire, check the coil driver and circuit. A common fault on these systems is a PCM output driver that fails to cycle the coil’s negative terminal to ground. To test this fault, verify the integrity of the battery supply circuit first. Perform a voltage drop test or a load test on the B+ supply circuit for the coil. If it’s faulty, repair as necessary. If no problems are found, check for cycling of the control circuit to ground at the coil connector. With the engine running or cranking, check the coil primary to determine whether it cycles on and off. Use a light bulb across the coil connector and watch for it to flash, or monitor the control circuit (negative side of the coil) with a scope. If the signal does not cycle on and off, verify the continuity of the control circuit from the coil to the PCM. If the wire is OK, replace the PCM. If the wire is open, repair the circuit and retest.

If a lab scope is available, perform a current ramp test on the coil. The current ramp test will show coil supply voltage, switching of the coil on and off, and the condition of the coil driver all in one quick and easy test (FIGURE 14-34). Expect to see around 8 amps or slightly over that, with a steady slope upward and a rapid completely vertical drop when the coil is turned off. An open coil, supply, or ground circuit or a failed driver will not show the correct current draw. A failing driver will show as a slow, hash, or jumpy vertical drop when the coil turns off.

FIGURE 14-34 Partial wiring diagram for a 2010 5.7 L dual plug Chrysler. The image shows a current probe and primary circuit connection points. Since current is the same everywhere in the circuit, the current probe can be placed anywhere in the fused supply or control circuits. To use a scope to check the primary waveform, back probe the control circuit (negative side) at the coil connector by using a lubricated pin as shown by the yellow arrow. Compare the suspect cylinder against a known-good cylinder to find any faults in the pattern. Technicians should store a known-good waveform on their scopes for future use.

Other options include a dual plug engine using separate coil packs for each pair of cylinders. Each coil terminal on each coil pack feeds a single cylinder. For example, a four-cylinder engine would use two coil packs with four high-tension terminals each and eight spark plugs (FIGURE 14-35). The coils are controlled by the PCM or ICM. Another alternative is a COP design that uses two terminals and boots per cylinder (FIGURE 14-36). This arrangement is much less complicated and removes potential problems from high-tension wires, including RFI. When the coil in this system fires, both plugs fire simultaneously. In this configuration, the coils are fired half as often as the previously discussed system, which used high-tension wires and two coils to fire the spark plugs. Reducing the number of times the coil fires more than makes up for the additional current flow required.

FIGURE 14-35 Line drawing of a dual plug ignition system that uses two separate coil packs and spark plug wires for each cylinder.

FIGURE 14-36 COP design dual plug ignition coils. This style of coil fires both plugs in a cylinder simultaneously, which improves engine efficiency and fuel economy while reducing HC emissions.

SAFETY TIP

Due to the high-voltage output of waste spark and COP ignition coils, exercise caution when working around either. Stray voltage from a leaking coil or spark plug wire can be harmful to both people and vehicles. Do not let a spark plug wire or coil simply arc to the atmosphere. The stress placed on the coil to find ground can damage the coil and—depending on the system’s setup—the ICM, PCM, or other vehicle electronics.

Diagnosing this system is the same for the coil primary and current testing as with all ignition coils. Each coil has two boots, and a fault in either one will cause a misfire. The misfire can be seen in a primary waveform pattern on the scope. The absence of easily accessible secondary wires removes issues from crossfires, routing, and cable replacements. Not having spark plug wires also makes secondary waveform analysis using a scope more difficult.

TECHNICIAN TIP

Due to the design of a waste spark coil, a faulty coil can result in a misfire on more than one cylinder. Misfire codes and scan data may show the engine has misfires on two paired cylinders. In other words, using the four-cylinder engine example above, Cylinders 1 and 4 or 3 and 2 may show misfires at the same time. An open spark plug wire or faulty spark plug will still typically affect only one cylinder.