Bad Spark Plug Symptoms | A Complete Guide

A bad spark plug can make all the difference as to how an engine performs. Malfunctioning spark plugs affect performance, fuel efficiency and the reliability of your car or power equipment. In most cases, like a single cylinder engine, a non-functioning spark plug will cause the engine not to start or run. If you have a spark plug question, this is where you’ll find the answer.

What is a spark plug? What does a spark plug do? These are some of the more basic questions I’ll be answering. Though you’ll mostly get sound technical advice. How do you test a spark plug? Learn all you need to know about the ignition system. What is a spark plug gap and why is it important? How do you set a spark plug gap? Time to find some answers.

A spark plug is a small, inexpensive, and seemingly insignificant component of all gas engines. Yet, the spark plug is vital. without it, there is no combustion. What’s an internal combustion engine without that all crucial combustion? Even if a spark plug is functioning as it should, we need to understand that it is but a single part of a larger sequence of events. The spark plug is the final step in the ignition system. We need to understand this as whole to effectively diagnose and correct spark plug, or ignition, issues. Let’s start by examining the spark plug and what role it plays in the combustion cycle of an engine.

---

What is a Spark Plug?

A spark plug is an electric component used in all gas-powered internal combustion engines. The purpose is to provide an electric arc (or spark) to ignite compressed fuel and air during the ignition cycle (stroke) of an engine.

The spark plug consists of a threaded metal, cylindrical shell. This is screwed into the engine head, securing the plug whilst providing a perfect seal to ensure full compression within the combustion chamber. Contact between the spark plug shell and the engine head creates a negative grounding. A flat plate protrudes from the shell, creating a grounded cathode. This plate is positioned directly below an electrode.

An heat-protective ceramic housing has a highly insulated conductive inner wire that is connected to the ignition coil or magneto via the spark plug wire (or ignition cord). An electrode, nickel-alloy coated copper, protrudes from the ceramic housing. This electrode acts as a positively charged anode and receives a high-voltage current from the ignition coil (or magneto).

There is a precise gap between the negatively grounded cathode and the electrode. When current passes through the electrode, a short circuit is created between the positive and negative opposing conductors. This creates a spark across the spark plug gap – the space between the electrode and the negative plate below it. The spark is timed to ignite the fuel and air mixture at the exact moment when the engine is sealed, and the gas is compressed. This is known as ignition timing.

The Ignition System

A spark plug cannot function properly unless the entire ignitions system works as it should. A single cylinder, 2-stroke engine is the simplest. Even a 4-stroke, single cylinder engine isn’t too complicated. An engine that has 2 or more cylinders becomes more complicated. I’ll discuss single cylinder ignition systems first and then explain how multi-cylinder engines differ.

The ignition coil is the most important component in an ignition system. This provides the high-volt current needed to create a spark that is powerful enough to ignite the fuel. If the coil isn’t in top condition, the spark will be weak, or there can be no spark at all.

---

How Does An Ignition Coil Work?

We call this device a coil because is it is made up of two coils, a primary and secondary coil. Basically, the coil is an inductive transformer. It is used to increase the voltage up to 100,000V. Though it can only do this for a fraction of a second, that’s all it takes to ignite the fuel in the combustion chamber.

VIDEO | How an Ignition Coil Works

The coil is connected to the alternator, or magneto. Engines with an electric starter will also have a battery as part of this circuit. Every engine has an ignition switch. Handheld power equipment and generators will usually have an “ON” switch, this closes the circuit supplying power to the coil. A car, and power equipment that use a key starter, use the first position of the key switch to do the same. When your dashboard lights come on, it means power is being sent to the ignition coil. Though most modern cars use a computer to manage the final power supply to the coil. More about that later.

Once power is supplied to the coil, the primary winding (or coil) is energized, creating a magnetic field around the coil. Breaker points (in old engines) or a solid state electronic ignition module will break the circuit to the primary coil. In other words, the current flow is temporarily stopped. This break is timed to happen milliseconds before the spark plug needs to ignite the fuel.

When the current to the primary coil is interrupted, the magnetic field around it collapses. This induces an electric current in the secondary coil. Because the secondary coil has multiple windings (many more than the primary coil), the voltage increases exponentially. This high-volt charge is conducted through a super-insulated conductor to the spark plug. In a single cylinder engine the power is conducted directly from the coil to the spark plug.

---

Multi-cylinder Ignition Systems

When an engine has more than cylinder, the power cannot always be supplied directly from the coil to the plug. Though many modern engines have separate coils for each cylinder. Two-cylinder have always used two coils, which work in a similar way to single-cylinder engines. Traditionally, the spark from the coil needs to be distributed to each cylinder at precisely the right time. In a 4-stroke engine this would be when all valves are closed and the fuel is compressed, ready for ignition.

The distributor is quite aptly named because it distributes the spark from the coil to the spark plug that is currently in the compression stroke. This is just before the piton reaches Top Dead Center (TDC). A distributor is a mechanical device that is connected to the camshaft. As the cam spins, a rotor inside the distributor spins at the same speed. The rotor is connected to the coil and has a conductive plate at the top. The distributor cap has conductors leading to each spark plug wire +on the outside. Inside the cap, conductive probes make contact with the rotor as it spins. So, the rotor sends a high-volt current to each spark plug wire as it moves across the corresponding position inside the distributor cap.

The camshaft controls the valves, opening and closing inlet and exhaust valves as the piston reaches a certain position in the combustion cycle, known as valve timing. This coincides with the rotation of the distributor rotor, ignition timing. It’s this mechanical relationship that ensure all engine strokes happen at precisely the right moment. To ensure that the ignition timing is correct, we turn the distributor slightly to make sure the rotor makes contact with the appropriate spark plug wire, at moment when the spark is required.

When people talk about setting or adjusting the timing of a car, this is what they do. A timing strobe is connected to the spark plug wire from the distributor to the No.1 cylinder. Every time the spark moves through the wire, the strobe flashes. A mark on the flywheel shows us when the no.1 cylinder is in the ignition cycle, just before TDC. The strobe flash has to shine onto this mark, for the ignition timing to exact. As we turn the distributor the flash will change. By making fine adjustments, moving the distributor clockwise or counter-clockwise, we get the flash to happen exactly when the timing mark is illuminated by the strobe.

While some engines still use a distributor, even those with computer engine management, many have done away with this mechanical system altogether. Some engines have a coil directly at each spark plug and the power distribution is controlled by the onboard computer. Others may use two or more coils with leads connected to the spark plugs, which are also controlled by a computer. With these systems the timing is adjusted by the computer and will require a Dyno-tester to do initial timing adjustments.

---

How to Test A Spark Plug

The preceding paragraphs may having nothing to do with spark plugs, but it’s important to understand these principles when testing a spark plug. Before replacing a spark plug, it’s important identify the cause of a weak, or non-existent spark. It could be a result of another problem in the ignition system. This could be a faulty coil, distributor, conductive spark plug wire, or, in the case of most modern engines, a computer fault. So, to effectively diagnose an ignition issue, we need to methodically work through every step of the process.

It’s not always easy to determine whether the fault lies with the fuel or ignition systems. You should always start by checking whether fuel is reaching the intake manifold, before embarking on the often tedious process of diagnosing ignition faults.

When it is decided that there is definitely no spark, it’s time to diagnose which part of the ignition system is the cause. Removing the spark plug and testing it, is a very good place to start.

The old-fashioned way of testing a spark plug, requires no specialized equipment. But this method can be dangerous, as you run the risk of a high-voltage current from the coil making contact with you. I’ve experienced this a few times. It won’t kill you, but it is more than unpleasant. Though, for the brave, it is effective, but not conclusive. You can remove the spark plug and hold it against the engine head or block so that the threaded metal shell makes contact with the engine to cause a ground connection. Make sure the appropriate spark plug wire (from the distributor or coil) is connected to the plug. When you start the engine, with the key or recoil starter, you should see a rapid succession of bright blue sparks. No spark, or a spark that isn’t too bright, can indicate a spark plug problem, but you can’t know for sure until you’ve done a little more testing.

Tip from an old dog. When using this primitive method of testing a spark plug, ensure that you make no physical contact with any metal surface of the car. If you do, you will be grounding yourself, and you’ll become the spark plug – a painful experience.

Apart from the risk of quick sharp electrocution, this method of spark plug testing has other pitfalls. By testing a spark plug in this manner, you are merely establishing that there is no spark at the final point – the spark plug. The problem could lie in any of the components that make up the ignition system. The only way to be sure, is to conduct a second test, using a brand new plug that you know is working. If the second spark plug doesn’t work, you have to start looking elsewhere. Are the conductive wires in good condition? Is the distributor making good contact, or is the coil faulty?

The quickest, easiest, and safest way to test a spark plug is to use a spark plug tester. Although a spark plug tester only tells you whether the spark is actually reaching the cylinder, it will tell you immediately whether the spark plug is the cause. If you know the current is reaching the plug, then the rest of ignition system should be fine. A spark plug tester is a simple devices with a wire that connects to the distributor or coil.

You remove the spark plug and insert the spark plug tester into the spark plug opening in the cylinder head. Some may have a boot that fits onto the plug – these are better. When you start the engine, a light in the spark plug tester will tell you if there is power at that cylinder. The light has a calibration, indicating how strong the spark current is. If the light doesn’t ignite, or shows a weak current, then you know the plug is probably not the cause, since there is no power coming from the spark plug wire, or the current is too weak. You can assume the spark plug is fine and check the rest of your ignition system to find the fault.

You can also use a multimeter to test the resistance of the spark plug wire. The recommended resistance between the coil or distributor and the spark plug is 12,000Ω (OHM) per foot. Though it’s best to consult the manual for the engine you’re testing as it may be different.

Whenever you remove a spark plug do a visual inspection first. If the ceramic insulation is cracked, immediately discard the plug. If you notice a black sooty residue on the electrode, clean this off with sandpaper. Also look for pitting or burning on the plug, this can often indicate an incorrect spark plug gap or a spark plug that needs to be replaced. Sometimes, all the spark plug needs a good clean, and may not need replacing. This is often the case with 2-stroke engines. Too much 2-stroke oil in the fuel can cause carbon to collect on the spark plug electrodes. This can also be the case with a high-mileage 4-stroke engine, worn engine rings can cause oil to seep into the combustion chamber.

---

How to Replace a Spark Plug

To change a spark plug you’ll need a few basic tools. A spark plug wrench is an obvious essential item. You’ll need to ensure that this is the correct size. Typically, a spark plug will either require a ⁵⁄₈” or ¹³⁄₁₆”. A lot of power equipment manufacturers supply a simple tube spark plug wrench with their products. This will be the correct size for the spark plug used in that engine. Otherwise, these are cheap to buy at just about any auto parts, hardware, or large department store. However, I prefer a ratchet with a spark plug socket. This means you can use a swivel connector and the right-size extension to access the plug with greater ease and reduce the chance causing any damage to the plug or surrounding components. You’ll also need a spark plug gap gauge. You’ll need to check the spark plug packaging to see what size gap gauge to use.

Before you begin, thoroughly clean the area around the spark plug. When you remove the spark plug, there is a hole that leads directly into the cylinder combustion chamber. Any dirt that enters the cylinder will damage it.

Step 1

Remove the spark plug wire. I always blow hard into the boot of this wire to clean away any dirt or soot that may be inside. It’s important to remove only one spark plug wire at a time. Replace it before you move onto the next plug. This is to prevent accidentally swapping the wires. If a plug is connected to the wrong wire, the firing order will be incorrect, and the engine won’t run properly.

Step 2

With the wire removed and safely out of the way, place the spark plug wrench snugly over the spark plug. Make sure it fits over the plug properly. Turn counter-clockwise to loosen the spark plug.

Step 3

Although a spark plug gap is set at the factory, you should always check it first with the correct spark plug gap tester. Spark plug gap is the distance between the bent metal “hook” at the bottom of the plug and the electrode at the base of the ceramic insulator. By pressing the bottom electrode firmly against a hard surface, like a workbench, you close the gap by applying a mild force down on it. This will close the gap between the two electrodes. If the gap is too small, pry it open with the tip of a screwdriver, test the gap and the adjust it, as described above.

The importance of ensuring the correct spark plug gap needs to be understood. If the gap is too wide, the spark will be weak and cause bad ignition. If the gap is too small, the spark will be too strong. This will cause the electrodes to burn and the spark plug won’t last long.

Step 4

Place the spark plug carefully into the hole and start tightening it by hand, turning clockwise. It’s essential to do this gently, the spark plug should turn easily. If you need to use force to turn the spark plug, it isn’t properly aligned. This means that the thread will be damaged if you continue turning it. If it doesn’t feel right, turn the plug back (counter-clockwise) and try again. It’s not unusual to do this a few times before you get the thread properly aligned. A cross threaded spark plug opening in the engine head is complicated, and usually costly, to repair.

Once the you’ve tightened the plug as far as you can by hand, carefully place the spark plug wrench over the plug. Tighten it down with a firm quarter turn once the plug is seated into position. There’s a sweet spot where the plug is tight enough to ensure an airtight seal of the combustion chamber. However, it is essential not to over-tighten the spark plug, this could damage the plug or the engine head. If you’re uncertain, use a torque wrench, you will usually find the correct torque settings on the spark plug packaging or your owner’s manual.

Step 5

Replace the spark plug wire, making sure that you press the boot firmly into position. You should feel resistance if you try pull back on the spark plug boot. Double check this, as a loose spark plug boot will result in a bad connection.

Move onto the next spark plug and start again at STEP 1.

---

What Type Of Spark Plug Should You Use?

Any spark plug will work fine, regardless of the engine type. Of course, this applies to gas engines – diesel engines don’t use spark plugs. Though, there are various types of spark plugs and some may be better for high performance engines (especially older model cars and motorcycles). Others may simply last longer or improve reliability, as well engine performance.

Copper-Nickel Spark plugs

These are the most common spark plugs because they are the cheapest and have been around the longest. These plugs us a copper conductor with a nickel alloy coating to protect the electrode. Without any type of heat resistant metal coating, the copper will melt.

Copper-Nickel spark plugs don’t last as long as most others that use metals with a higher heat resistance. However, copper is the best conductor and many modern vehicle manufacturers specify these plugs for their engines. This, despite the need for more frequent spark plug changes.

High-compression engines, and performance engines, especially if a turbo charger is used, function more efficiently with copper-nickel plugs. This applies to most vehicles from the mid 1990’s onward.

Single Platinum Spark plugs

Platinum is an expensive metal, making these spark plugs more expensive than copper nickel-plugs. A single platinum spark plug uses a platinum coated main electrode. Platinum is an extremely hard, durable metal and these spark plugs last much longer than copper-nickel plugs.

Double Platinum Spark plugs

On a single platinum plug, only the main electrode is coated with platinum. A double platinum spark plug, means both the main and ground electrode are platinum coated.

Iridium Spark plugs

These are the most expensive spark plugs and offer the best performance. They have a fine-tipped electrode and improved conductivity for a superior spark. These plugs are also more durable than copper-nickel plugs and can often last as long as platinum spark plugs.

Silver Spark plugs

Silver is used in cases where excessive heat is generated inside the engine, due to its superior thermo conductivity. Since these plugs are expensive and don’t last as long as platinum plugs, they are becoming redundant. Modern engines, even high performance models, now have sufficient engine cooling, so as to eliminate the need for high-temperature spark plugs.

However, older model high performance cars, most notably Italian supercars from the 1970’s, 80’s and early 90s, as well most high performance motorcycles from the same era will benefit from the use of silver spark plugs.

---

When Should You Replace Your Spark Plugs?

Most engine manufacturers specify regular intervals for spark plug changes. But, even with a correct service regime, spark plugs can fail. This can be a manufacturing defect or oil accumulating on the spark plug from a worn engine.

Modern cars with a “check engine” light will usually alert you to the problem. One of the first things to do when this light comes on is to check the spark plugs, spark plug wires, distributor, and coil.

High fuel consumption could also be an indication, as weak spark decreases engine efficiency. This is often accompanied by a loss in engine power.

An engine that doesn’t start easily can be as a result of bad spark plugs.

A misfire is usually a sign of bad ignition and a spark plug or spark plugs could be causing this. This can be identified by erratic idling or popping engine noises, often when you accelerate or decelerate.

Note that all these conditions can be an indication bad spark plugs, or another fault, and may have nothing to do with the spark plugs. You always need to follow the steps laid out earlier in the article before deciding it’s time to replace spark plugs.