by Jim Kerr

Since the beginning of the internal combustion gasoline engine, a source of spark has been needed to ignite the air fuel mixture. Sparkplugs were, and are still today, the answer. Subjected to high heat, extreme pressures, and large temperature changes, sparkplugs have perhaps the most difficult task in the engine, yet they spark millions of times without failure.

Improvements over the years in ignition systems and spark plug design have extended the useful life of a sparkplug. In the Sixties, it was recommended to change sparkplugs every 16,000 kilometres. Ignitionsystems of that period typically produced a maximum of 20,000 volts to fire a spark across the sparkplug electrodes. A sparkplug in good condition requires 8,000 to 12,000 volts to create a spark across the spark plug electrodes while operating in an engine. A worn sparkplug requires much higher voltage to fire; this could easily exceed the voltage available from the ignition systems of that time.

Electronic ignition systems of the 1970’s could produce higher maximum voltages. Some systems could produce 30 to 35 thousand volts, which was enough to fire even partially worn sparkplugs. Thus, recommended change intervals were extended to between 40 to 50 thousand kilometres. Many vehicles on the road today still require sparkplug changes at this interval to ensure maximum vehicle performance and the best fuel economy.

The challenge of meeting emission laws brought about the development of coil pack ignition systems. These systems use one coil to fire two sparkplugs, as compared to the earlier systems that used one coil to fire all the sparkplugs. Coil pack systems can produce 50,000 volts or higher and are capable of firing even badly worn sparkplugs. For best performance, the sparkplug change interval was still around 50,000 kilometres, but many drivers drove the car much further before problems would occur.

Operating an engine with worn sparkplugs causes the coils to produce higher voltages necessary to jump across the worn electrodes. Any areas of weak electrical insulation could allow the spark to jump somewhere else, and the misrouted spark can damage electronic ignition modules, coils, and spark plug wires. Changing the sparkplugs at the recommended intervals was cheap insurance to prevent misfiring and costly parts repairs.

Some of the latest engine designs use one ignition coil for each sparkplug. This is not for higher voltage output, but rather to shorten the path of electricity from the coil to the sparkplug. The shorter the path, the less chance of electrical leakage. The short path also reduced electromagnetic interference in the engine compartment. This is very important as cars continue to increase in the amount of computer wiring and sensors onboard. Electromagnetic interference can cause a computer to malfunction!

The latest improvements in sparkplugs have been the introduction of special high mileage sparkplugs. Both Ford and General Motors have been advertising sparkplugs that will last for 160,000 kilometres. These sparkplugs cost about three to four times as much as regular sparkplugs, and use special alloys for the electrodes that wear very slowly.

You may have seen other sparkplugs for sale that feature multiple electrodes or electrodes with special shapes. These sparkplugs use premium electrode materials for longer life, and their special design helps to ensure a spark will fire across the electrodes. As sparkplug electrodes wear, they round off. Spark jumps much easier from a sharp, pointed surface than a round one, so extra electrodes and special shapes with sharp edges help the spark jump across. These sparkplugs are costly to manufacture, so they cost more as well.

Why should sparkplugs be changed at the recommended mileage when the car seems to be running fine? When a sparkplug begins to fail, it does not stop completely. It may misfire only when you accelerate or when you are driving on the highway. With only one of eight sparkplugs partially misfiring on an eight-cylinder engine, your fuel economy could be reduced by ten percent! At today’s price, the wasted fuel would easily pay for new sparkplugs and give better performance as a bonus.

When changing sparkplugs in an engine, there are several important steps. First, get the correct replacement sparkplugs from your auto parts supplier. Sparkplugs are numbered as to their heat range (operating temperature of the sparkplug tip), thread size and length, and type of gasket or seal to the engine. Installing an incorrect sparkplug could result in expensive engine damage.

Remove each sparkplug wire carefully by pulling on the boot at the sparkplug. Do not pull directly on the plugwire, as it causes internal damage to the wire that requires the plugwire to be replaced.

Before removing the old sparkplugs, blow out any dirt or stones around the plugs that could fall into the engine. Engines with aluminum cylinder heads should be allowed to cool before removing the sparkplugs to prevent damage to the threads in the cylinder head. If a sparkplug starts to bind while it is being turned out, turn it back in and out again a few times to try and break any carbon off the sparkplug threads as you remove it. Forcing the sparkplug out will likely damage the cylinder head!

Check and adjust the gap between the electrodes on the tip of the new sparkplugs before installing them. The gaps vary from engine to engine, and the specifications are usually found on the emissions label under the hood of the vehicle. Bend only the ground or outside electrode if it is necessary to adjust the gap.

Finally, tighten the sparkplugs to the proper torque and reinstall the plug wires. Be especially careful on aluminum cylinder heads to avoid overtightening the sparkplugs. Repairing the threads in the head can be very expensive!

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