Question 1

When spark plugs need replacing

Accepted Answer

A spark plug ignites the petrol-air mixture compressed in the combustion chamber via an electrical spark. The plug's central and side electrodes wear with time and the gap between them grows, meaning the spark no longer fires as effectively. The result is rough running, more unburnt fuel entering the exhaust (the lambda sensor reads a lean mixture), rising fuel consumption and an engine warning light.

Manufacturers' stated intervals are 30 000 km for a standard copper/nickel plug, 60 000 km for a platinum plug and 90 000–120 000 km for iridium plugs. These are strict limits — waiting can kill the ignition coils, because a worn plug demands several times the normal voltage from the coil. A broken coil typically costs the same as all four plugs together, so preventive replacement is clearly the more economical choice.

Question 2

How spark plugs are replaced

Accepted Answer

The air intake ducting and any wiring covering the spark plug tops is removed. The ignition coils are extracted carefully by hand via their retaining clips, rotating them a few degrees from their seal before lifting fully. The plugs are removed with a spark-plug socket of the correct length (usually a 16 mm or 14 mm socket plus extension). Any dust or debris in the plug well is blown out with compressed air before the new plug is fitted.

New plugs are checked for gap (typically 0.8–1.1 mm depending on the car), threads receive a small amount of copper or ceramic paste and they're threaded in by hand first to prevent cross-threading. Final tightening is done with a torque wrench to specification (usually 25–30 Nm). Ignition coils are reseated and the engine is started — smooth idle and no misfire are signs of a successful job.

Question 3

Why do spark plugs wear quickly?

Accepted Answer

Premature plug wear almost always comes back to the engine's air-fuel mixture or combustion event. An overly rich mixture (short intake flow, faulty mass airflow sensor) deposits carbon on the electrodes that bleeds spark current and weakens ignition. An overly lean mixture (an intake leak, faulty fuel pressure) raises combustion chamber temperatures, melting electrode material. A third common cause is oil leaking through the rocker cover gasket into the plug well — the oil burns around the electrode and forms a thin coarse surface that disrupts sparking. Regular oil changes, mass airflow sensor cleaning and intake manifold leak tests notably extend plug life.

Spark Plug Replacement in Vantaa