M. Boudaghi, M. Shahbakhti, and S.A. Jazayeri, in a paper in the Journal of Engineering for Gas Turbines And Power, describe the term ‘Misfire’ as a ‘lack of combustion in the cylinder.’
The engine uses a combination of oxygen, fuel, and a spark to do its work. A deficiency in one or more of those elements can lead to a misfire. Mechanics classify misfires as ‘Intermittent,’ ‘Dead-miss,’ and ‘Partial.’
- Combustion does not happen in dead-miss misfires.
- Incomplete combustion will occur in a partial misfire.
- Intermittent misfires are difficult to predict because they only occur some of the time.
The events surrounding a misfire are just as important as the severity of the misfire. For instance, some misfires will only occur when you accelerate. The ‘Check Engine’ light will illuminate, and ‘Limp Mode’ will engage as strange sounds emerge from under the hood.
You also have misfires that occur when you idle. Mechanics blame both instances on everything from incorrect air/fuel mixtures and cracked distributor caps to dead throttle position sensors and loose connections.
But what about spark plugs? How do they fit into this equation?
How can a spark plug gap cause a misfire?
- Construction
Spark plugs consist of a center electrode, a ground electrode, and a porcelain insulator. They also feature resistors that combat RFI from high-voltage electricity. Despite their seemingly simple design, spark plugs are complex parts capable of channeling 10 – 30kV, tolerating cylinder pressures of more than 200psi, and withstanding temperatures as high as 3000 degrees C.
- Purpose
The spark plug produces the spark that ignites the air/fuel mixture. Without a spark plug, combustion cannot occur. Without combustion, the engine cannot run. This makes spark plugs a vital aspect of an engine’s operations.
- Spark Plug Gap
Spark plugs have a gap between the center and ground electrodes. It matters because the spark must cross that gap as it jumps from one electrode to the other to ignite the air/fuel mixture. Unfortunately, spark plugs tend to wear out over time because of the stresses they encounter.
Either oil and carbon deposits will foul the spark plugs, narrowing the gap, or the electrodes will corrode and erode, widening the gap. The consequences are usually the same. The plug will produce a weak spark that can’t ignite the air/fuel mixture correctly or consistently. The result is a misfire.
- Diagnosis
The spark plug is the first component mechanics check when misfires occur because it directly affects ignition and combustion. However, changing the spark plugs may not fix the issue if the fault originates from other problems, such as a short circuit, clogged fuel injectors, leaking fuel injectors, a lean or rich air/fuel mixture, etc.
Some drivers use OBDII scanners. When a misfire occurs, the ECM takes note of the phenomenon and records the error code. You can retrieve that error code with a scanner. A manual will tell you what it means. Repair Smith has published a list of standard codes related to misfires.
P0300 and P030X point to a misfiring cylinder. The code will mention the defective cylinder. You can check the corresponding spark plugs to determine whether they are damaged or worn out. You should also extend your inspection to include the plug wires.
What Are The Best Practices For Preventing Misfires Caused By Spark Plug Gap Problems?
Has the ECM detected a misfire? Depending on your model, the vehicle may manifest one or more of the following symptoms:
- The engine’s power will fall.
- The engine may refuse to start.
- The fuel economy will deteriorate.
- You will hear strange sounds, such as popping.
- The engine will jerk and vibrate.
If you blame the spark plugs for the misfires, you can take the following preventative measures:
- Fight the urge to gap platinum and iridium spark plugs unless the need arises. You’re more likely to damage the electrodes.
- If you’re measuring a platinum or iridium spark plug’s gap with a coil-style tool, insert the gauge gently to protect the electrodes from unnecessary harm.
- Don’t use screwdrivers to adjust the gap. The tip is made from delicate precious metals. It will break.
- Remove dirt and debris by loosening the plug and applying low-pressure compressed air.
- Don’t under-torque or over-torque spark plugs.
- Replace spark plugs with fouled electrodes.
- If the spark plug is wet with oil, identify and eliminate leaks.
- Locate the wire that transmits the current from the ignition coil and replace it once you identify defects.
- Test the coil pack with a multimeter. The coil pack provides the current the spark plug uses to ignite the air/fuel mixture. Connect a multimeter to the pins after disconnecting the spark plug wires. You’re testing the resistance. Does it match the resistance associated with your vehicle? If not, get a new coil pack.
- Perform regular maintenance. This allows you to identify and eliminate the factors that widen a spark plug’s gap. You can also clean dirty plugs and replace their damaged counterparts before they cause misfires.
- Clean or replace dirty and worn-out air filters.
Can Misfires Due To Spark Plug Gap Be Resolved With Simple Adjustments?
Sometimes. Manufacturers assign specific gaps to their spark plugs. They expect those gaps to persist for the lifetime of the spark plug. In other words, if a spark plug gap is too wide, it is probably too old, and you should replace it.
Adjustments are a temporary measure. They are only feasible when the plug gap widens earlier than expected because of unpredictable factors, such as an installation error, excess heat from a malfunctioning engine, etc.
Is A Wider Or Narrower Spark Plug Gap More Prone To Causing Misfires?
A wide gap is better than a narrow one because it delivers a reliable spark that consistently ignites the air/fuel mixture. Problems arise when the gap is too broad. The spark can’t cross the distance between the electrodes, especially in situations where the voltage is insufficient.
Ignition and combustion won’t happen. And even if they do, they will occur intermittently and inefficiently. The same problem arises when the gap is too narrow. The spark plug should have the widest gap that allows the spark to efficiently ignite the air/fuel mixture.
What Are The Consequences Of A Spark Plug Gap Causing A Misfire?
Misfires are dangerous. People dismiss them as a nuisance, but they can have tangible consequences, for instance:
- Misfires can cause road accidents. For instance, popping sounds from the engine may startle you as you overtake or navigate a sharp corner. The engine may also stall or jerk unexpectedly.
- Besides causing misfires, wide spark plug gaps will increase your expenses dramatically by ruining the vehicle’s fuel economy.
- It will accelerate the spark plug’s rate of wear and tear.
- Bad spark plugs can damage the catalytic converter.
- The vehicle’s emissions will increase.
This paper from Computational Intelligence and Neuroscience lists several proposed methods for detecting misfires. They include temperature, pressure, and vibration monitoring. However, rather than experimenting with these techniques, you should consult a technician whenever your vehicle manifests common symptoms of a misfire. They are better equipped to diagnose and resolve the problem.