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Sometimes simple = complex. For example, the modern coil-on-plug (COP) ignition system is perhaps among the simplest to diagnose, but to illustrate the complexities, let’s look at how a 2008 Toyota Tundra 4.0L V6 might record various misfire codes.
Understanding Coil-on-Plug Issues for the 2008 Tundra
Sometimes simple = complex. For example, the modern coil-on-plug (COP) ignition system is perhaps among the simplest to diagnose, but to illustrate the complexities, let’s look at how a 2008 Toyota Tundra 4.0L V6 might record various misfire codes. According to Toyota, four conditions are required to efficiently burn an air/fuel mixture contained in a closed cylinder: 1) The cylinder must have within 80% of specified cranking compression; 2) the cylinder must contain the correct air/fuel ratio for the operating conditions; 3) the intake air and exhaust flow must not be restricted; and 4) the ignition system must produce an electrical spark capable of arcing across a specified spark plug gap at high cylinder pressures. (See Photo 1.)
When one or more of these operating conditions isn’t present, one or more P0301-P0306-series misfire codes can be stored in the Engine Control Unit’s (ECU’s) diagnostic memory. A P0300 multiple-cylinder misfire code can also be stored due to a condition that affects multiple cylinders, such as an over-rich or over-lean air/fuel ratio or a restricted intake or exhaust system.
In contrast, a P0300-series code for a specific cylinder, such as a P0302, will be stored because a single-cylinder component, such as a fuel injector, cam follower, valve spring, intake or exhaust valve, cylinder head gasket, spark plug or a coil-on-plug unit has failed. As always, component accessibility can create diagnostic issues in any application and, of the above possibilities, the coil-on-plug unit is most often the easiest to diagnose.
Keep in mind that specific thresholds for a misfire code will vary according to application. Misfire detection systems are based upon the crankshaft position sensor (the NE signal) measuring the relative speed of each piston as it decelerates on compression stroke and accelerates on power stroke.
To illustrate, we might have a rough-idling engine that doesn’t store a misfire code. In this case, the acceleration and deceleration profiles of the rough idle don’t meet the enabling criteria programmed into the engine’s misfire monitor. A leaking EGR valve might, for example, create such a condition. On the other hand, when the acceleration and deceleration profile meets the misfire monitor criteria, a P0300-series misfire trouble code will be stored in the engine’s diagnostic memory.
Basic Ignition Concepts
In brief, spark ignition systems operate on a common principle of a magnetic field being created by wrapping many coils of copper wire around a soft-iron core. In response to a timed signal from the crankshaft position sensor (NE signal) and, in most cases, the camshaft position (G signal) sensor, the ECU switches the primary circuit “on” through a power transistor or coil driver to saturate the primary windings with 5 to 10 amperes of electrical current. When the resulting magnetic field builds to maximum potential, the ECU abruptly switches off the primary circuit, which causes the magnetic field to collapse into the secondary ignition windings.
Since the secondary circuit contains many more windings than the primary, the primary voltage is multiplied many times (from about 12.5 primary volts to well over 25,000 secondary volts) when the magnetic field collapses into the secondary circuit. The crankshaft position (NE) signal identifies a piston’s position in relation to TDC on both waste-spark and sequential ignitions. With sequential ignition systems, the camshaft position (G) signal additionally identifies the piston’s compression stroke.
When diagnosing coil-on-plug ignitions, it’s important to differentiate between three basic coil-on-plug designs. For instance, the 3.4L V6 truck engines use a waste-spark ignition in which the coils are installed onto the right-bank spark plugs and are connected to the left-bank spark plugs by three conventional spark plug wires.
Again, while this configuration might appear to be a coil-on-plug design, it is a waste-spark, “simultaneous ignition” that fires companion cylinders — one on compression stroke, and the other on exhaust stroke. (See Photos 2 and 3.)
In contrast, many early coil-on-plug ignition coils are two-wire units, with one primary wire connected to a B+ source, and the other wire grounded through an ignition driver located in the ECU. The two-wire designs are more easily diagnosed with a volt-ohm meter or a labscope connected to an inductive secondary pick-up. But, more about that in my sidebar story, “Some Final Thoughts on Coil-On-Plug Diagnostics,” found at the end of this article.
Later coil-on-plug or “Direct Ignition” systems incorporate the igniter into the coil-on-plug assembly. A four-wire connector is used consisting of a B+ voltage source, a B- ground, an IGT (ignition timing) signal from the ECU, and an IGF (ignition confirmation) return signal to the ECU.
In this configuration, the coil-on-plug’s igniter switches the coil primary circuit on/off in response to the IGT signal from the ECU. As the igniter shuts off the coil primary current, an IGF signal from the coil igniter to the ECU confirms that an ignition event has taken place.
To illustrate with a specific example, if the IGT circuit fails to report on a 2005 Tacoma 2.7L, four-cylinder 2TKFE engine, a P0350-series circuit malfunction code will be stored in the ECU’s diagnostic memory. A P0351 code would indicate a problem within the IGT/IGF circuit in the #1 igniter. The P0351 is a one-trip continuous code that occurs under 1,500 rpm. (See Photo 4).
Scan Tool Diagnostics
A good first step for diagnosing any coil-on-plug failure is to examine the current misfire and history misfire menus on your professional scan tool. Current misfires taken with the engine running can provide some valuable clues. For example, a high-misfire count on cylinder #2 of a V6 engine, and fewer misfire counts on the remaining cylinders, might indicate what we call “sympathetic” misfires, which are the reactions of an adjacent cylinder to the misfire of another cylinder. If, for example, the firing order on a 2005 V6 engine is 123456, the cylinders adjacent to #2 (#1 and #3) might show higher misfire counts than the remaining cylinders.
Many coil-on-plug failures are caused by worn or incorrect spark plugs. A spark plug gapped at 0.040” usually requires about 12,000 volts (12kV) at idle speeds and low cylinder pressures. The kV demand will increase with engine speed and cylinder pressure to about 25kV, which is enough to ignite a 14.7:1 air/fuel charge under normal cylinder pressures. As the spark plug gap widens due to normal electrode erosion, the kV demand rises higher and begins to overheat the coil, break down the coil winding insulation, and possibly perforate the coil boot or spark plug wire connecting the coil to the spark plug.
When removing any coil-on-plug assembly, carefully examine the boot or wire for small white spots that indicate voltage perforation. If the coil has failed due to badly worn spark plugs, the remaining coils are difficult to access due to interference from the intake manifold or accessories, or the coils demonstrate a higher-than-normal failure rate, replacing all coils is generally the preferred recommendation. In addition, it’s always a wise practice to replace all coil boots, especially if one or more spark plugs show carbon tracking down the side of the insulator, or if one or more boots are perforated.
Next, check the spark plug for correct application. In some applications, substituting a dual-electrode spark plug with a single-electrode plug can cause a random cylinder misfire on Toyota engines.
Since Toyota’s coil-on-plug ignition systems might contain the igniter, I highly recommend following Toyota service information (SI) for diagnosing modern coil-on-plug systems. We know, for example, that a coil is defective when the misfire follows the coil as it is swapped from cylinder to cylinder. But, if the misfire doesn’t follow the coil, and the coil has a two-wire connector, then it’s time to test the B+ wire for battery voltage. A quick test on the B- wire to the ECU with a voltmeter should indicate less than battery voltage with the engine running. If B+ voltage is observed, the B- connection might be defective or the ECU’s coil driver might be ruined due to excessive current draw caused by shorted coil windings.
Four-wire connectors require a different diagnostic strategy. While B+ must be available in any coil design, remember that B- will ground either on the engine or inside the engine compartment. In any case, a diagnostic code will be set if the ECU doesn’t receive an ignition confirmation signal (IGF). In this case, examine the coil connector for damaged contacts and examine the ECU connections for corrosion or a loose pin fit. If the process of elimination reveals no coil or circuit failures, the ECU might be at fault.
Complex = Simple
Since 2008, all passenger cars and light trucks utilize more sophisticated on-board engine diagnostic capability. The four-wire coil-on-plug unit is capable of sensing an ignition circuit failure and setting an appropriate trouble code. This self-diagnostic capability makes the scan tool a vital part of coil-on-plug diagnosis, which makes the accurate diagnosis of any P0300-series misfire code faster and easier.
Some Final Thoughts on Coil-on-Plug Diagnostics
The underlying problem with coil-on-plug ignition diagnostics is that we’re now confronted with issues such as variable dwell, double-strike ignitions, variable valve timing and direct fuel injection — all of which can affect the secondary ignition waveform. Unless you are a nameplate specialist, I generally don’t recommend using secondary ignition as a diagnostic tool because the results are often open to interpretation. And, because ignition coils are notoriously heat-sensitive, I don’t recommend measuring primary and secondary winding resistance in any ignition coil as the final word in any coil diagnosis.
In contrast, a low-amperage, inductive current probe connected to a labscope can produce a reliable test by measuring the rate of current flow into the coil primary circuit. In some applications, the primary circuit can be accessed through an “ignition” fuse located in the underhood fuse box. If the ignition and fuel injection share a common fuse, the fuel injector waveforms will be intermixed with the ignition coil waveforms, which can be confusing. On some engine designs, attaching a jumper wire between the coil and connector will allow access for current-ramp testing. In any case, you will see a non-current limiting or a current-limiting waveform. If the ignition is a “double-strike” system, you will see two similar and consecutive waveforms.
A normal non-current-limiting signal is displayed in Photo 5. The current “ramp” is straight and peaks at about 4.4 amps. If the coil winding was shorted, the scope would display a curved rather than straight-line current ramp.
Photo 6 displays a current-limiting waveform peaking “clipped” at about 6.1 amps that prevents coil overheating. While there is much more to current ramp testing than can be discussed in this space, it is most useful for locating bad coil drivers and shorted or open coil primary windings.
Courtesy Import Car.