Spark plugs: when to change them and why "still working" is not an argument

A spark plug does not fail in one moment. It degrades gradually, and the real problem is that it degrades together with the coil — and the coil costs several times more.

What happens to a plug

The electrode erodes and the gap widens. Bridging a wider gap needs higher voltage.

The consequences, in order:

1. The coil works at its limit — and wears faster.
2. Under load the spark is insufficient — hesitation appears; covered in the hesitation article.
3. Rough running when cold — covered in its own article.
4. Misfires and catalyst destruction.

The main point: plugs are preventive maintenance for coils, not a separate expense.

Types and intervals

Nickel (copper core) — cheapest, shortest life. Replace to the schedule, and their schedule is short.

Platinum — longer life.

Iridium — the longest life, a fine centre electrode, and the best spark formation.

The selection rule: fit the type the engine manufacturer specified. Iridium in place of nickel on an engine designed for nickel does not deliver the promised life, because the ignition system and schedule were designed differently. Nickel in place of iridium guarantees a shorter interval.

What matters most: the correct heat range and geometry. A plug with “better specifications” but a different electrode reach is a route to piston contact or overheating.

When to change early

• Rough running when cold has appeared.
• Hesitation under load.
• Fuel consumption has risen with no other cause.
• The engine runs on gas — plug life is shorter there.
• A lot of short journeys — carbon builds faster; covered in the short trips article.

What an old plug tells you

A removed plug is a diagnostic document.

Light brown deposit — normal.

Black dry soot — rich mixture.

Black and oily — oil in the chamber.

White with a melted electrode — overheating or detonation.

Melted — serious detonation.

The full range of conditions is in the spark plug reading article.

In practice: plugs in different conditions across cylinders are the fastest clue to which cylinder is the problem.

Mistakes when changing them

Overtightening. Stripped threads in an alloy head mean a head repair. Use the specified torque and a torque wrench.

Under-tightening — the plug cannot shed heat into the head and overheats.

Changing them on a hot engine — a risk of damaging the threads.

Dirt in the plug well — blow it out before unscrewing, or it goes into the cylinder.

Ignoring oil in the well — that is the cam cover gasket, and it is what kills coils; covered in the ignition coil article.

Do not change one at a time — plugs of the same age behave alike, so replacing one means returning a month later.

What to watch in diagnostics

Per-cylinder misfire counters — before and after replacement.

Fuel trims — worn plugs change the combustion process.

Ignition timing — with poor combustion the module may adjust it.

In short

• Plugs are preventive maintenance for coils, not a separate expense.
• Fit the type the manufacturer specified, not the one with “better specs”.
• The condition of removed plugs identifies the problem cylinder immediately.
• Overtightening in an alloy head means a head repair.
• Change them as a set, because they degrade together.

Download the app — misfire counters and fuel trims are read with an ELM327 adapter over Bluetooth SPP on Android.