During its lifetime, an average spark plug might fire 27.5 million to 110 million times. Each time, it vaporises a few molecules off the spark plug electrodes. Eventually, the spark plug gap widens, and the air-fuel mixture no longer ignites efficiently. Other engine problems (poor air-fuel management) might contaminate the spark plug, and in certain circumstances, it may not fire at all.
A spark plug firing.
How the Spark Plug Works
Fire (combustion) requires three components: oxygen (O2), fuel, and heat. In your engine, every time a cylinder pulls an intake stroke, it pulls in air (≈21% (O2). Port-injection engines inject fuel during the intake stroke, while direct-injection engines, gasoline or diesel, might wait until the compression stroke. Finally, heat is provided in one of two ways: one or two spark plugs for gasoline engines, or the heat of compression for diesel engines. The result, as when any time these three components are combined, is a tiny explosion. Cruising at 88km/h, the average cylinder will fire about 1,000 times per minute, or 16 times per second!
Cross section of a spark plug.
In gasoline engines, the heat is provided in the form of a tiny spark. High voltage, 5 kv to 45 kV, depending on the vehicle, is generated in an ignition coil, controlled by a distributor or engine control module (ECM). The charge is conducted to the spark plug via a spark plug wire. The spark occurs when the charge jumps between the spark plug electrodes, 0.25 mm to 1.8 mm across. The heat generated, between 4,700 °C to 6,500 °C, ignites the air-fuel mixture, driving the piston down in the power stroke
The gap junction of a spark plug.
Signs of Spark Plug Problems
How can you tell if your spark plugs need attention? Your engine is a fine-tuned machine, and performance, fuel economy, and reliability are dependent on consistent delivery of air, fuel, and spark. Here are seven signs of spark plug problems.
1. Fuel Economy
If you’re tracking fuel economy, as everyone should, spark plug problems might cause your engine to consume more fuel than usual. As the ECM has no control over spark strength or oxygen content, it adds fuel to compensate for poor combustion.
2. Misfire
A cylinder misfire occurs when the spark plug can’t ignite the air-fuel mixture. This can occur because of worn spark plugs, contamination, cracked spark plug wires, or other problems.
3.Check Engine Light
For many people, the check engine light might be the only sign your engine is having spark plug problems. The ECM is far more sensitive than many drivers and can detect a single cylinder misfire in thousands of good ignitions.
4. Hard Starting
Proper ignition is most difficult when the engine is “cold”. The ECM adds more fuel to account for poor vaporisation, which can be difficult for a worn spark plug to ignite, resulting in hard starting, long cranking, or a no-start condition.
Ever had problems getting your car to start in the morning but it works just fine for the rest of the day? This is a good place to start.
5. Rough Idle
At idle, spark plug problems might be evident as more vibration, basically a cylinder misfire that occurs only at idle.
6. Poor Performance
On acceleration, spark plug are under particular demand to deliver a strong spark to ignite more fuel and generate more power. Faulty spark plugs or weak ignition coils may not keep up with the demand.
7. Hesitation/Surging
Slightly less noticeable, hesitation on acceleration and surging might be described as the engine not “responding” immediately to driver input. Then, a “surge” in power might come unexpectedly. This slight delay in power delivery might indicate a spark plug problem.
A fouled spark plug
How to Fix?
Cleaning and adjusting the gaps of spark plugs is tricky as the gaps have very close measurements (0.25 mm to 1.8 mm).
Just get new spark plugs. Don't get those used ones (Belgium). No one knows how much work they may have done and how much spark they have done. Just get new ones.
Get the iridium tipped plugs. They're the latest technology, seem to last longer and deliver a more consistent spark.
If the problems still don't go away, take a look at your coils.
Being observant is key to keeping a reliable car, and recognising spark plug problems early can improve fuel economy and prevent you from being stranded (my-motor-no-gree-start on a rainy Sunday morning).
If you have any questions about your spark plugs, be sure to check with a trusted professional.
Feel free to look around
Suggestions or questions?
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