Spark Plugs

How a spark plug ignites the mixture

The ignition coil stores magnetic energy and releases it as high voltage. When voltage is sufficient to ionise gas in the plug gap, current flows as a spark. The resulting hot plasma starts a flame kernel, whose stability depends on mixture, pressure, turbulence, electrode shape and ignition energy.

Required voltage rises with cylinder pressure and gap. Turbocharging, lean mixtures and deposits can increase demand, while worn electrodes enlarge the effective gap. A weak coil or damaged boot may then leak voltage elsewhere before the plug fires.

Ignition sequence

1. The engine controller calculates ignition timing from speed, load, temperature and knock data.
2. The coil primary circuit builds magnetic energy.
3. Current is switched off and secondary voltage rises rapidly.
4. Voltage reaches the plug through a coil boot or ignition lead.
5. The gas in the electrode gap ionises and conducts.
6. A flame kernel grows across the combustion chamber.
7. The plug sheds firing-end heat through its insulator and shell into the head.

Spark-plug designs

Heat range and operating temperature

Heat range describes how readily the firing end transfers heat to the head. A “hotter” plug retains more heat to resist deposits; a “colder” plug removes heat faster under high thermal load. It does not describe spark energy. Too hot risks pre-ignition and electrode damage, while too cold promotes fouling.

• Insulator nose length: controls the heat path from firing tip to shell.
• Engine load: raises combustion and electrode temperature.
• Mixture and timing: affect flame temperature and deposit formation.
• Boost pressure: increases cylinder density and firing voltage.
• Oil/coolant entry: creates deposits and abnormal heating.
• Short journeys: may not bring plugs to self-cleaning temperature.

Construction

Centre electrode and resistor

The terminal connects through a conductive core and often a suppression resistor to the centre electrode. The resistor limits radio-frequency interference and protects electronics. An incorrect non-resistor plug can disturb vehicle systems.

Ceramic insulator

High-alumina ceramic contains voltage while conducting heat. Cracks, carbon tracking or impact damage provide leakage paths. Dropped plugs and misaligned sockets should be inspected rather than fitted automatically.

Shell, threads and seat

The steel shell clamps the plug into the head and transfers heat. Flat-seat plugs use a gasket washer; taper-seat plugs seal directly. Thread reach must match exactly so exposed threads do not collect carbon or interfere mechanically.

Ground electrode and gap

The ground strap shape exposes the spark while carrying heat. Gap is measured with a suitable wire gauge. Coin-style wedges can load a fine centre electrode and should not be forced against precious-metal tips.

Materials and performance

Selecting the correct plug

Gap, torque and installation specifications

A plug transfers heat correctly only when its seat is clamped. Too little torque can leak combustion and overheat the shell; too much can stretch threads, crack ceramic or distort the plug. Torque depends on thread, seat, head material and whether the washer is new.

Many plated plugs are installed dry. Anti-seize lowers friction and can produce excessive clamp load at the same wrench setting. Use it only when the plug or engine manufacturer specifies an adjusted procedure. Dielectric grease, where allowed, belongs sparingly inside the boot, not on electrodes or threads.

Reading removed plugs

Diagnosis

1. Record fault codes, misfire counters and conditions before clearing.
2. Inspect coils, leads, boots, connectors and plug wells for oil or water.
3. Compare plugs cylinder by cylinder while preserving their positions.
4. Measure gap and inspect electrodes, ceramic and sealing seat.
5. Swap components only through a controlled diagnostic plan.
6. Test compression, injectors and mechanical timing where misfire remains.
7. Correct oil, coolant, mixture or ignition causes before fitting new plugs.
8. Verify misfire counters under safe operating conditions after repair.

Fault symptoms and urgency

• Flashing engine lamp: stop heavy load; active misfire can overheat the catalyst.
• Severe shaking: reduce operation and diagnose ignition, fuel and compression.
• Hard starting: inspect plugs but also battery speed, fuel and sensors.
• Knock or pre-ignition: stop before piston and valve damage develops.
• Plug loose in head: switch off and inspect threads and sealing seat.
• Broken electrode: check the cylinder for foreign-object damage.

Removal and replacement

Work at the temperature specified for the engine, commonly cool. Remove dirt from plug wells, release coil connectors safely and use a plug socket that holds the plug without side load. If resistance is abnormal, stop and manage corrosion or thread deposits rather than forcing the aluminium head.

Check new part numbers and gaps, start every plug by hand through an extension and feel for free thread engagement. Torque in one smooth operation. Refit coil boots fully, route wiring and test the engine before replacing covers.

Common mistakes

• Choosing by thread size while ignoring heat range and reach.
• Assuming heat-range numbers are universal across brands.
• Applying anti-seize without an approved torque adjustment.
• Gapping a fine-wire electrode with a wedge tool.
• Installing plugs into dirty wells.
• Starting threads with a powered tool.
• Overtightening taper-seat or small-thread plugs.
• Replacing plugs without diagnosing oil or coolant deposits.
• Mixing plugs of different specification across one engine.
• Driving with a flashing misfire warning.

Modifications, service intervals and MOT

Modified boost, fuel or ignition can change heat and gap requirements, but a colder plug is not a cure for poor calibration. Changes should be based on combustion evidence and competent tuning, with insurer disclosure where applicable.

Follow time or mileage intervals for the exact plug. Long-life electrodes reduce wear but do not prevent deposits or seized threads indefinitely. During the UK MOT, emissions and warning-lamp faults caused by misfire can lead to failure. Catalyst damage and unsafe running are reasons to repair sooner than the test date.

Spark plug FAQs

Q: How often should spark plugs be changed?
A: Follow the engine's specified interval for the exact plug type.

Q: Does a hotter plug make a stronger spark?
A: No. Heat range describes heat transfer, not ignition energy.

Q: Can plugs be changed without checking the gap?
A: Verify against service data even when supplied pre-gapped.

Q: Should anti-seize be used?
A: Only when specified; it changes thread friction and clamp load.

Q: Are iridium plugs always better?
A: They offer durable fine electrodes, but only the approved specification is suitable.

Q: What causes black spark plugs?
A: Rich mixture, cold running, weak ignition, oil or wrong heat range may contribute.

Q: Can one plug be replaced?
A: Diagnose the cause; service replacement is normally completed as an engine set.

Q: Why is there oil in the plug well?
A: A cover or tube seal may leak externally; oily firing tips suggest internal consumption.

Q: Can a loose plug damage the head?
A: Yes. Combustion leakage can erode threads and overheat the plug.

Q: Why does misfire worsen under boost?
A: Higher cylinder pressure raises firing-voltage demand, exposing gap, coil or mixture faults.

Q: Can wrong plugs damage an engine?
A: Incorrect reach or heat range can cause mechanical contact, pre-ignition or deposits.

Q: Is a flashing engine light urgent?
A: Yes. Active misfire can rapidly overheat the catalytic converter.

Q: Can spark-plug faults affect the MOT?
A: Misfire can cause emissions and warning-lamp failures.