With Bosch Aftermarket reporting that ignition failure is more prevalent in cold weather conditions, you might wish to prepare now for an increase in demand. Rob Marshall gives an overview of spark plugs and coils, investigates how they fail, describes routine checks and examines low-cost tools to help you with dismantling and diagnosis.
Despite the huge advances that have been made in engine electronics, the spark ignition internal combustion engine remains reliant on a high voltage circuit to generate the several thousands of volts necessary to jump-across the spark plug electrodes’ gap. The vast majority of modern ignition systems employ a coil to generate this high voltage (or High Tension – HT ) output from a low voltage (or Low Tension – LT ) input, employing Faraday’s Law for readers that have either good memories, or an O-level/GCSE Physics textbook to hand. Essentially, once power is cut to a primary coil of wire within the coil assembly, the resultant magnetic field collapses and high voltage is subsequently generated within another internal coil winding. The spark plug is connected either directly to the coil unit(s), or via a series of high voltage flexible HT leads, or both. Naturally, the type of coil fitted depends on the car model being worked upon, the attributes of each type will be examined in a future issue. Even so, a mechanical distributor might be employed on some older cars, made until the early 2000s, despite lacking ‘Old-Skool’ vacuum-advance timing mechanisms and contact breaker points that are present on most historic vehicles.
Rendering contact breaker points obsolete, a transistor within the engine ECU supplies the LT to the coil(s). Naturally, this is controlled by a microprocessor that considers the many other signals received by other sensors fitted to the running gear. Among many benefits, modern transistorised ignition systems can ensure an optimum dwell angle, an advantage of which is that the coil is supplied with LT voltage for long enough to ensure optimum spark strength. The HT voltage tended to reduce gradually on older cars with contact breaker points and is one reason why the driver would notice a significant improvement in engine performance after a service had been completed, which is not always that obvious today.
AVOIDING CORNER CUTTING
Even so, a modern ignition system is not immune to neglect and premature ignition coil failure tends to be the result of neither making the appropriate checks at service time, nor the owner heeding maintenance schedules. Mobiletron advises that poor fuel economy, stalling, back-firing and difficult starting are typical symptoms of coil issues. SMPE reports that the main checks tend to be restricted to visual inspections that might not be stated specifically in maintenance literature, which comprise checking the insulating gaiters and the LT electrical connections. More detailed advice is included within our earlier feature, www.autotechnician.co.uk/servicing-be-a- bright-spark.
“High resistance in the ignition system will prematurely kill any coil,” reports Morten Hansgaard Jensen, Product Specialist of Ignition at the Bosch Aftermarket Division. He adds: “While a resultant misfire might cause unburnt fuel to overheat the catalytic converter, the coil has not had a chance to release the energy that it has ready for the spark plug. Instead, it is transferred as heat within the coil that will shorten its life.”
Apart from internal short and high resistance, Denso adds that defective cables, low battery power, vibration and mechanical damage also reduce coil service life.
Walker Products says that other factors within the cylinder, such as a faulty fuel injector producing an incorrect spray pattern, or even low compression, can vary the voltage demand on the coil. This means that a spark plug can be used as a combustion chamber sensor. Coil-on plug systems that employ Delphi’s Ion Sense Technology, for example, can monitor conductivity at the spark plug electrodes to detect misfires, knock and even fuel mixture/quality. The resultant signals are then sent to the engine ECU. While skilled technicians have used oscilloscopes to diagnose poor running conditions for years, Ion Sense enhances and simplifies this technique, by providing real-time feedback directly to the ECU, while the engine is running.
When servicing, however, Tim Howes, Deputy General Manager of the Supply Chain & Technical Service at NGK Spark Plugs (UK), advises that, “If the coil is mounted directly on the plug, the coil would need to be removed to gain access, giving the opportunity to visually check for perishing/contamination of the rubber parts, plus corrosion, cracks and evidence of current leakage ‘tracking’. It is important to use the correct specialist tools when removing and refitting ignition coils, in order to prevent damage to the insulating materials, circuit boards, windings, connectors, etc.”
Denso adds that, should an ignition coil be identified as defective, the root cause should be determined, to avoid the replacement part failing prematurely as well. Naturally, the vehicle manufacturer’s ignition system instructions should always be referred to in the first instance but disconnect the negative (-) battery terminal and wait at least 90 seconds before removing the coil. Naturally, corner cutting extends to replacement parts. NGK reports that sub-standard coils tend to be cheaper but their inadequacies are not obvious by inspecting the outside, because the quality of the internal windings and potting materials tend to be where savings are made. Lower HT voltage and even internal short-circuiting can ensue. These inadequacies can result in an engine that is more reluctant to start, increased misfiring frequency, raised exhaust emissions and increases the likelihood of damage to the catalytic converter and engine. Both SMPE and ACtronics also advise that low-quality coils can cause severe damage to the engine ECU, due to the incorrect fly-back voltages received.
Yet, ELTA advises that technicians should check the spark plugs first, because they tend to cause suspected coil-related issues, before embarking on an exhaustive diagnosis procedure.
SPARK PLUGS: THE BUSINESS END
It is easy to forget just how much of a hard life a typical spark plug endures. Even at engine idling speeds, each one must ignite the fuel mixture eight times a second, be expected to withstand huge temperature variations in milliseconds, because combustion heat is followed immediately by a cooling effect from air being drawn in through the inlet valve(s), and resist forces that can be the equivalent of fifty times the force of gravity. Should any replacements be fitted that do not combine high mechanical strength, with effective insulators that can contain at least 30,000 volts, misfires could be the least of the problems encountered. The plug could disintegrate, causing catastrophic physical damage to the combustion chamber.
While NGK told us that it has witnessed plugs being mis-sold as premium precious-metal types, the arrest and subsequent charging of an online trader in May, who sold counterfeit spark plugs that could have damaged engines had they been fitted, proves that you should prioritise not only trusted brands but also proven suppliers.
Yet, incorrect fitting can also damage even the best plugs. Antoaneta Spiridon, Motaquip’s Business Line Manager – Spark Plugs, advises that plug threads should not be lubricated, because this reduces the friction at the thread faces, risking overtightening. Distortion of the plug’s metal shell can result, which interrupts the heat transfer path within the plug, causing the electrodes to overheat. These ‘hot spots’ make pre-ignition more of a threat, which is something that you do not wish to promote in certain GDI engines that are prone to Low Speed Pre Ignition. Follow any fitting instructions carefully with GDI engines, so that the ground electrode is positioned correctly, relative to the fuel injector. The use of either a dial, or torque wrench, is essential; use a short as possible extension bar and keep it straight. Tilting the extension bar/socket may exert excessive side pressure on the spark plug and crack the brittle insulator. Incidentally, periodic adjustment of the spark plug gap between services tends to be unnecessary on modern cars, although it is recommended on cars that run on gas.
Never downgrade specifications, either. While older cars may benefit from being fitted with a precious-metal spark plug, due to the more focussed spark offering a more efficient burn, they are mandatory on some newer and high-performance engines. Fitting a more conventional nickel-alloy type risks prejudicing engine efficiency, which is likely to promote an illuminated MIL. Just like a spark plug that is worn-out, fitting replacements of an incorrect specification risks placing extra strain on the ignition coil, something that can then have ramifications for the entire engine management system.
USEFUL DIAGNOSTIC TOOLS