Spark plugs are a critical component of your vehicle’s engine ignition system. Their primary function is to create an electrical spark that ignites the fuel and air mixture inside the engine’s combustion chamber.
When the fuel and air mixture is compressed inside the combustion chamber, the spark plug generates a high-voltage spark* that ignites the mixture, creating a controlled explosion that drives the engine’s pistons. This process is repeated thousands of times per minute to keep the engine running.
Want to guess how many volts in a spark plug?
The voltage required to create a spark in a spark plug ranges from 20,000 to 50,000 volts. However, in high-performance engines, the voltage requirement can be as high as 100,000 volts.
Spark Plug Resistance
Spark plug resistance can be compared to a narrow water pipe section restricting water flow. Similarly, spark plug resistance restricts the flow of electrical current used to generate the spark that ignites the fuel in your car’s engine.
The resistance of a spark plug can have a significant impact on the performance of your vehicle:
- Too much resistance and the spark plug may not be able to deliver the necessary voltage to ignite the fuel and air mixture in the combustion chamber, resulting in poor engine performance, reduced fuel efficiency, and potentially damaging the catalytic converter.
- On the other hand, if the resistance is too low, the spark plug may allow too much current to flow, which can cause excessive heating and potentially damage the ignition system or even cause a fire.
In addition to affecting the performance of your vehicle’s engine, the resistance of the spark plug can also impact the plug’s lifespan. If the resistance is too high, the spark plug may wear out more quickly, while low resistance can cause the spark plug to overheat and potentially damage the electrode or the insulator.
So, how much resistance should a spark plug have?
The degree to which a spark plug impedes the flow of electrical current through it is measured in ohms (a unit of electrical resistance.) Typically, the resistance of a spark plug should be between 5,000 and 10,000 ohms. However, the exact resistance value may vary depending on the make and model of your vehicle and the specific type of spark plug being used.
It’s important to note that the resistance of a spark plug can be affected by a number of factors, including the condition of the spark plug wires, the temperature and humidity of the engine compartment, and the age and condition of the spark plugs themselves.
Spark Plug Metals
Platinum and iridium have become a buzzword for ignition durability – as good conductors of heat and electricity. Both metals also can resist chemical corrosion and electrical erosion. That is why platinum and iridium are used on the electrodes of many spark plugs today.
Some plugs have a solid platinum or iridium center electrode, while others have a small button of platinum welded onto the tip of the center electrode or both electrodes (single platinum vs. double platinum). Some even use an alloy of platinum and iridium in the center electrode.
Platinum and iridium are used because they minimize electrode wear. Every time a plug fires, a tiny amount of metal is vaporized and erodes from the surface of both electrodes. The center electrode typically suffers the most wear because it runs hotter than the side electrode.
As the electrodes wear, the air gap across which the spark must jump becomes wider and wider. The gap on a standard spark plug grows about 0.00063 to 0.000126 inch for every 1,600 kilometres of normal driving. And the wider the gap, the greater the voltage needed to jump the gap.
The firing voltage requirements typically creep up about 500 V for every 16,000 to 24,000 kilometres of driving on standard plugs with conventional electrodes. Eventually, the plugs may need more volts to fire than the coil(s) can produce, resulting in an ignition misfire. On OBD II-equipped vehicles, too many misfires will cause the Check Engine light to come on.
Using platinum or iridium almost eliminates electrode wear. Platinum and iridium are expensive metals, but they can double or even triple a spark plug’s normal service life, from 48,000 to 72,000 kilometres for a standard plug, up to 160,000 kilometres or more. Most aftermarket spark plug suppliers do not make specific mileage claims for their platinum or iridium spark plugs but say to follow the OEM replacement intervals, usually 160,000 kilometres.
Though long-life platinum and iridium spark plugs cost more than standard spark plugs, OEMs use them because they reduce the risk of misfire (which helps protect the catalytic converter) and they reduce the need for maintenance, which allows them to offer 100,000-mile “tune-up” intervals.
One important point to keep in mind about platinum plugs is that not all platinum plugs are the same. There are differences in electrode configurations, design, and durability. Some provide better fouling resistance than others, and some are not recommended for use in turbocharged or supercharged engines.
Spark Plug Fouling Resistance
Regardless of the type of alloy used in the electrodes, all spark plugs must be able to resist fouling. The trick here is to design the plug so that the electrodes run hot enough to burn off any deposits but not so hot that they cause pre-ignition or detonation. The electrodes need to reach about 700 degrees F quickly to burn off carbon deposits. But if the electrodes get too hot (above 1,500 degrees F), they can ignite the fuel before the spark occurs, causing pre-ignition and detonation. For most plugs, the ideal operating temperature is around 1200 degrees F.
The temperature of the electrodes is controlled by the length of the ceramic insulator that surrounds the center electrode and the design of the electrode itself. Ceramics do not conduct heat very well, so an insulator with a relatively long nose will conduct heat away from the electrode more slowly than one with a relatively short nose. The longer the path between the electrode and the surrounding plug shell, the slower the rate of cooling and the hotter the plug.
Many plugs have a copper core center electrode. Copper is an excellent conductor of heat, and allows the plug to dissipate heat quickly under load yet remain hot enough at low speed and idle to burn off fouling deposits.
A spark plug’s “heat range” (heat rating), therefore, depends on the ceramic insulator’s length and the center electrode’s design. The heat range must be carefully matched to the engine application. Otherwise, the plugs may experience fouling problems or run too hot and cause pre-ignition/detonation problems. Most plugs today have a relatively broad heat range, so they reach a self-cleaning temperature quickly but do not get too hot under load. This allows plug manufacturers to consolidate applications and use fewer plugs to cover a wider range of engines.
When replacing spark plugs, the heat range must be correct for the engine application. Always follow the vehicle or spark plug manufacturer’s recommendations. If the plugs are too cold, fouling may occur if the vehicle spends a lot of time idling or is used only for short trips (especially during cold weather). If the plugs are too hot, the engine may experience preignition and detonation under load or during hot weather.
In some situations, a slightly hotter or colder plug may be installed than the one normally recommended. Switching to a slightly hotter plug can reduce fouling in an older engine that uses oil. A hotter plug can also reduce fouling in vehicles that spend a lot of time idling or are used only for short-trip, stop-and-go city driving. But a hotter plug should not be used unless an engine is experiencing a fouling problem because of the increased risk of preignition and detonation. Switching to a slightly colder plug can reduce the risk of preignition and detonation in performance applications (especially turbocharged and supercharged engines), in vehicles used for towing or in those that are driven primarily on the highway.
Spark Plug Electrode Design
Many spark plugs today have unique electrode designs such as V-split, grooved or clipped ground electrodes, multiple ground electrodes, fluted center electrodes, V-notched center electrodes, etc. Though each plug manufacturer takes a slightly different approach and claims various benefits for their design, the basic idea is to make it as easy as possible for the spark to jump the gap and ignite the fuel mixture. A spark jumps more easily from a sharp edge than a rounded blunt edge. That is one reason why new plugs require less firing voltage than old ones with worn electrodes.
The electrodes on some spark plugs are also designed to “unshroud” the spark for easier ignition. This allows the flame kernel to expand more rapidly and reduces the quenching effect that could cause a misfire.
Spark Plug Replacement
Over time – as the spark plug is exposed to high heat, high pressure, and electrical arcing – its electrode wears down, leading to reduced performance, misfires, and other engine problems.
Normal spark plug wear presents as:
- A worn electrode causes a gradual reduction in the spark plug’s performance.
- Build-up of deposits from the fuel and oil in the engine, which can interfere with the spark and reduce performance.
- Fouling occurs when the spark plug becomes coated in a layer of carbon or other material, which can lead to engine misfires.
- Increased gap between the electrode and the center electrode potentially results in misfires. Most spark plugs are pre-gapped. Even so, the gap may have to be reset for some engines because of consolidations.
CAUTION: If your engine has an aluminum cylinder head, wait until the engine has cooled to replace the spark plugs. If you try to remove the spark plugs while the engine is hot, there is a greater risk of damaging the spark plug hole threads in the head.
Spark Plug Wires
Good plug wires are just as important to ignition performance as spark plugs. Three basic types of wires may be used:
- Distributed resistance wire. This type of wire has a fibreglass core impregnated with latex graphite. It provides the maximum amount of radio frequency interference (RFI) suppression. RFI occurs when the high voltage passes through the plug wires. Creating a controlled amount of resistance in the wire (3,000 to 12,000 ohms per foot) suppresses RFI and prevents sensitive onboard electronics from picking up false signals that could cause drivability problems.
One of the drawbacks of carbon core suppression wires is that internal resistance creates internal heat. Over time, this ages the carbon core, causing resistance to increase. And, as resistance goes up, so does the chance of a misfire. - Inductance (mag) wire. This type of wire has a spiral wound core of copper/nickel alloy wire. RFI is suppressed primarily by the magnetic field formed by the loops of wire wrapped around the core rather than the resistance of the wire itself. Mag wire has less total resistance (about 500 ohms/foot) than suppression wire, reducing the current needed to fire the plugs. But its main advantage is improved durability over the long run.
- Fixed resistor wire. This type of wire has a steel or copper metallic core with a fixed resistor in the plug boot to control RFI.
Diagnosing Plug Wires
Plug wires should always be inspected when the spark plugs are changed and when there is a misfire complaint. Start with a visual inspection for obvious damage, such as burned or cracked insulation, chafing, contact with the exhaust manifold, loose plug boots or terminals, etc. Any wires that are burned or damaged must be replaced. The same goes for wires with loose or damaged boots or terminals.
Next, start the engine, then look and listen for arcing while the engine idles. A snapping or cracking noise would tell you secondary voltage is finding a shortcut to the ground. Observing the engine in the dark may help you see where the voltage leaks. Any fireworks visible along the cables’ length or at the ends would tell you new wires are needed.
Still can’t find a bad wire? Look at the secondary firing pattern on an oscilloscope. A bad plug wire with excessive internal resistance may cause an intermittent or steady misfire that is usually most noticeable under load. This will cause an increase in the affected cylinder’s firing voltage. An open plug wire or spark plug will cause the firing voltage for that cylinder to spike to the coil’s maximum output.
If the firing voltage is high in a cylinder, turn the engine off and measure the plug wire’s resistance end to end with an ohmmeter. Refer to the manufacturer’s specifications. If the resistance exceeds specifications, the wire needs to be replaced.
A shorted ignition cable or grounded spark plug will cause a drop in the firing voltage. Rubbing a grounded probe along the length of each plug wire while the engine is idling may help you find any weak spots in the insulation.
High secondary resistance is indicated when one cylinder in the superimposed display has a firing line higher than the rest and a shorter spark duration. Bad plug wires, worn spark plugs, and lean fuel conditions may cause high secondary resistance.
To further isolate the cause, the KV demand for the affected cylinder should be compared to the other cylinders. If the required firing voltage is 20% or higher than the rest, the problem is either too wide a plug gap or a lean fuel condition. But if the firing voltage is roughly the same as the other cylinders, the likely cause is high resistance in the plug wire or spark plug.
Replacing Plug Wires
If more than one plug wire has excessive resistance, replace the entire set.
Handle cables with care during installation. Do not jerk, force, twist or bend sharply.
Start with the longest or shortest wire first, and change one wire at a time to avoid mixing up the firing order.
Listen for the “click” to ensure the plug boots and end terminals are fully seated.
Follow the original cable routing to avoid crossfire problems. Cables for cylinders that fire consecutively should not be routed parallel or be near one another. Keep them separated by several inches or cross them.
All cables should be supported by wire looms, and kept away from exhaust manifolds and sharp edges.
Or you can just come to our car repair shop in Hamilton, and we can take care of your car – spark plugs included – for you! Contact CRS Automotive Hamilton today to make an appointment.
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