Worn or damaged spark plugs can cause engine misfires, rough idle, slow acceleration and lower fuel economy. Spark plugs create the spark that burns fuel in each cylinder, so when they wear out the engine cannot run smoothly.
What do spark plugs do?
In a gasoline engine, each spark plug sends a spark across a small gap at the right moment. That spark ignites the air-fuel mix in the cylinder and starts the power stroke. A strong, steady spark means complete combustion, smooth power and good fuel economy.
How many spark plugs does an engine have?
Most gasoline engines have one spark plug per cylinder:
- A four-cylinder engine has four
- A V6 has six
- A V8 has eight
- A four-cylinder boxer engine, like those in many Subarus, has four, two on each side
Some engines use two plugs per cylinder, so the count varies. Diesel engines do not use spark plugs. They use glow plugs, which heat the combustion chamber to help the engine start, especially in cold weather.
What are the symptoms of bad spark plugs?
- Misfires: The engine stumbles, jerks or shakes, especially under load
- Rough idle: The engine shakes or sounds uneven when stopped
- Slow acceleration: The car feels weak when you press the gas
- Poor fuel economy: Incomplete combustion wastes fuel
- Hard starting: The engine takes longer to start
- Check engine light: The computer often stores a misfire code
What types of spark plugs are there?
- Copper: A traditional design. Good conductors but they wear faster.
- Platinum: Longer-lasting than copper and common in many newer cars.
- Iridium: Very durable and hold a steady spark for a long time. Common in modern and high-performance engines.
Use the plug type and gap your manufacturer specifies. The wrong plug can cause poor running or even engine damage.
How often should spark plugs be replaced?
It varies a lot by plug type and engine. Some need replacement much sooner than others. Check your owner's manual for your vehicle's schedule, or ask us to look it up. If you notice any of the symptoms above, have the plugs checked even if you are not due yet.
Can a misfire be caused by something other than spark plugs?
Yes. Ignition coils, plug wires, fuel injectors, vacuum leaks and low compression can all cause misfires. That is why a proper diagnosis comes before replacing parts. Our advanced computer diagnostics help find the exact cylinder and cause.
How can you make spark plugs last?
- Follow your vehicle's service schedule
- Use quality plugs that match your manufacturer's specs
- Fix oil leaks and misfires early, since they can foul new plugs
When to see a mechanic
If the check engine light is flashing, the engine has a serious misfire. Ease off and have it checked as soon as you can, because unburned fuel can damage the catalytic converter. Also come in if your engine runs rough, hesitates or uses more fuel than usual. See our engine repair and service page for more.
Engine running rough? R&E Auto Repair is at 930 Rayford Road in Spring, TX. Every vehicle gets a digital inspection with photos and notes sent to your phone, and no work is done until you approve it. If something can wait, we say so. Call 832-585-0229 or contact us to book a visit. We're open Monday through Saturday, 8 AM to 5 PM.