Every time a gasoline engine runs, its spark plugs fire thousands of times a minute to ignite the air and fuel in each cylinder. It is a demanding job, and even long-life plugs eventually wear out. This guide covers what spark plugs do, how they fail, the symptoms of worn plugs, and how to tell when it is time for a new set.
What does a spark plug actually do?
A spark plug sits in the cylinder head with its tip inside the combustion chamber. The ignition system sends a high-voltage pulse to the plug, and the electricity jumps a small gap between two electrodes. That spark lights the compressed air-fuel mixture, the burning mixture pushes the piston down, and the engine makes power. Timing matters: the engine computer decides exactly when each plug fires based on engine speed, load and other inputs.
Most modern engines use a separate ignition coil for each cylinder, often mounted right on top of the plug. Older engines may use a distributor or a coil pack with plug wires. Either way, the plug is the last link in the chain.
Why spark plugs wear out
Each spark erodes a tiny bit of metal from the electrodes. Over many miles, the gap gets wider, and a wider gap takes more voltage to jump. Eventually the coil cannot reliably push a spark across it, especially under load, and the cylinder misfires. Plugs can also be fouled by deposits. Black, sooty plugs can point to a rich fuel mixture, while oily plugs suggest oil is getting into the cylinder. Those are clues a technician uses during an engine diagnostic check, because a fouled plug is often a symptom of another problem.
Symptoms of worn spark plugs
- Rough idle. The engine shakes or stumbles at a stop. Our article on why a car idles rough covers other possible causes too.
- Misfires or hesitation. You may feel a jerk or stumble when accelerating, often worse going uphill or merging onto the interstate.
- Hard starting. Weak sparks make it harder to light the mixture, especially on a cold morning.
- Lower fuel economy. Incomplete combustion wastes fuel.
- Check engine light. Misfires are monitored by the engine computer and usually set a code. A flashing light means a severe misfire, and you should ease off and have it checked promptly.
None of these symptoms point only to spark plugs. Ignition coils, fuel injectors, vacuum leaks and sensor problems can cause the same complaints, which is why it pays to diagnose before replacing parts.
How often should spark plugs be replaced?
It depends heavily on the plug material and the engine. Older-style copper plugs wear relatively quickly. Platinum and iridium plugs, which most newer engines use, are designed to last much longer. Many manufacturers list a replacement interval in the maintenance schedule, so check your owner’s manual for your specific vehicle. If you bought the car used and do not know when the plugs were last done, it is reasonable to have them inspected.
Some engines make plug replacement a quick job. On others, such as many V6 engines mounted sideways, the rear plugs sit under the intake manifold, which adds labor. That is one reason it is helpful to know what is due ahead of time and plan for it alongside other routine maintenance.
Spark plugs and ignition coils
Coils and plugs work as a team, and a worn plug can shorten coil life by forcing it to produce higher voltage. When one cylinder misfires, a common test is to swap that coil to a different cylinder and see whether the misfire follows it. That points to either the coil or the plug. Inspecting the old plugs also tells a story about how the engine is running, so they are worth looking at rather than tossing straight in the trash.
Can I replace spark plugs myself?
On some engines, a careful do-it-yourselfer can. The keys are using the exact plug your engine calls for, checking the gap if the plug requires it, threading plugs in by hand first and tightening to the correct specification. Aluminum cylinder heads are easy to damage with cross-threading or over-tightening, and a stripped plug hole is a far bigger repair than the plugs themselves. If access is tight or you are not sure of the procedure, leave it to a shop.
Have your spark plugs and ignition checked in Hermitage
If your engine is misfiring, idling rough or your plugs are due by mileage, bring it to Music City Auto Repair. We will read any stored codes, check the ignition system and explain what we find before any work starts. Request an appointment online or call 615-600-5994.
Frequently asked questions
Can a worn spark plug make the check engine light come on?
Yes. A plug that no longer fires reliably can cause misfires, and the engine computer watches for them. A misfire usually stores a trouble code and turns on the check engine light, and a flashing light means the misfire is severe enough to risk damaging the catalytic converter.
Should all spark plugs be replaced at the same time?
In most cases, yes. The plugs in an engine wear at roughly the same rate, so replacing the full set keeps the cylinders firing evenly. If one plug failed early, it is also worth finding out why, since oil, coolant or a bad coil can damage a single plug.
Can I switch to a different type of spark plug than my engine came with?
It is safer to stick with the plug type, heat range and gap your manufacturer specifies. Engines are designed around a particular plug, and the wrong one can misfire, foul or wear quickly. Your owner's manual or the parts listing for your exact engine will show the correct plug.
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