Rick's Free Auto Repair Advice

When to change spark plugs

When to change spark plugs

It isn’t every 100K miles

That’s right smarty-pants, not all spark plugs last 100K miles. This myth got started back in the 1990’s when carmakers switched to platinum tipped spark plugs. Those plugs were rated at 100K miles. But those 1990’s engines ran at lower combustion pressure and temperatures. The fuel systems ran at much lower pressures and the ignition systems fired and lower voltages than late model engines. You don’t see single ignition coils or DIS ignition on late model cars. They’re all coil-on-plug.

How late model engines result in shorter spark plug life.

• Smaller 4-cylinder engines achieve emissions compliance using a leaner mixture that burns hotter than older engine. A hotter burn means shorter spark plug life, even though the newer iridium spark plugs have higher melting points.
• Direct injection increases cylinder pressure. DI fuel injectors can inject fuel at up to 2,000-psi. thereby increasing compression pressures.
• Turbo chargers also boost cylinder pressure
• Firing voltages are much higher due to coil-on-plug designs.

when to replace spark plugs

Worn spark plug with rounded center electrode and excessive gap

Spark plug life depends on MANY factors including your particular engine design, ignition system design, the type of plug recommended by the factory and whether the engine is equipped with a turbo. Since there’s no universal engine, there’s no such thing as universal spark plug life.

Spark plug life is listed in your maintenance guide

Never depend on word-of-mouth advice from your buddies. Here’s an example of how much the spark plug change intervals vary:

2010 Ford Fusion: Spark plug change spark plugs at 90,000 miles
2013 Chevy Equinox: Spark Plug change spark plugs at 97,500 miles
2013 Subaru Forrester: spark plug change spark plugs at 60,000 miles

2016 Subaru Forester: Spark plug change at 36,000 miles
2013 Toyota Camry: Spark Plug change spark plugs at 120,000 miles

Can you go beyond the spark plug change interval?

Yes, but not without cost. When you reach 80% of the spark plug’s projected life, the spark plug electrodes are 80% worn. It’s at that point when spark plugs start to misfire. And those misfires cost you in terms of performance, efficiency, and gas mileage. Worse yet, misfires cause carbon deposits in the cylinder head and can damage the expensive catalytic converter. So it’s NEVER a good idea to go longer than car maker recommendations.

Which plugs should you use?

ALWAYS use the same type of plug recommended by the carmaker. If the car came equipped with fine-wire iridium, you MUST use that same style to get the same life and performance. My preference is to stick with the same brand that was installed at the factory.

Can you change spark plugs yourself?

Replacing spark plugs isn’t a hard job. But one thing has changed from the early days of DIY spark plug change: you now need a torque wrench. Over and under torqueing spark plugs is the number 1 cause of spark plug failure, which results in a misfire and can even cause the spark plug to blow out of the cylinder head (taking the threads with it).

Spark plug torque is dramatically reduced in engines with an aluminum cylinder head. Where you could have gotten away with hand tightening on a cast iron head, doing the same thing on an aluminum head can cause serious and expensive damage. Trust me, your hands can’t tell the difference between 15 ft/lbs and 25 ft/lbs., so ALWAYS use a torque wrench.

For more tips on spark plug installation, see this post

©, 2018 Rick Muscoplat


Posted on by Rick Muscoplat

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