What is a coil-on-plug ignition coil?
Most late model engines use coil-on-plug (COP) ignition coils. COP ignition coil provide higher voltage than coil packs because they operate with longer dwell time. In other words, they have more time to build a magnetic field, so it produces a higher voltage spark when it collapses. Car makers need that hotter/longer spark to ignite leaner fuel mixtures in modern engines.
The COP units are most commonly installed directly over each spark plug, sometimes sitting deep in the cylinder head. That placement eliminates the need for spark plug wires and that reduces the potential for spark plug wire voltage drop. In theory, COP units should operate far more efficiently and much longer than traditional coil pack designs. Unfortunately, we’re seeing high failure rates in COP units and replacing multiple COP units can be expensive.
What causes COP ignition coil failures?
The most common cause of COP ignition coil failure is worn spark plugs! Yup, car owners are ignoring normal maintenance and driving too long on factory spark plugs. As the spark plug gap erodes, it take more voltage to jump the larger gap. In fact, a worn spark plug can require almost 80,000 volts to fire. A COP ignition coil can deliver that kind of voltage, but not for extended periods. Repeated 80,000 volt firings overheats the secondary windings and degrades the insulating materials inside the ignition coil. Then, the COP ignition coil begins to misfire.
Extra lean air/fuel mixtures can also cause ignition coil failures. A vacuum or air duct leak can cause a lean mixture that can’t be corrected by the PCM. Continued operation causes the ignition coil to overheat and fail.
Water or oil in the spark plug tubes can also cause ignition coil failures. A failed valve cover gasket, for example, can allow engine oil to seep into the spark plug tube and cause a short to ground. Or, faulty sealing gaskets at the top of a COP coil can allow water to enter the spark plug tube, especially during a pressure wash engine cleaning operation.
What happens when ignition coils misfire?
Once you create 80,000 volts, it has to seek ground somewhere. If it can’t jump the spark plug gap, it will find ground through degraded coil insulation. The high voltage can shoot through the plastic case or through the spark plug boot. In the most severe instances, the high voltage can seek ground through the ignition controller—either a free-standing ignition module, or the ignition “driver” inside the PCM.
Once the ignition module or the driver is destroyed, replacing the module or PCM without fixing the underlying cause of the failure will just damage the replacement components.
When does an ignition coil misfire?
Ignition coil misfire occurs when the engine is under heavy load, if the air/fuel mixture is consistently too lean, when spark plugs are worn, or when engine temperatures are too high.
Diagnose the root cause of the ignition coil failure before replacing
Always check the condition of the spark plugs before replacing an ignition coil. If the plug gap is too wide, the new coil will prematurely fail. Also, check fuel trim readings to see if the PCM is attempting to compensate for a vacuum or air duct leak.
Read this post to learn how to diagnose a misfire
©, 2017 Rick Muscopalt
Posted on by Rick Muscoplat