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Spark plug inspection (ensure proper firing)

Inspect Mazda MX-5 NB spark plugs for wear, fouling, and heat signs to confirm proper firing and decide whether cleaning, re-gapping, or replacement is needed.

Difficulty
Intermediate
★★☆☆☆
Est. Time
30-60 min
Models
NB1 & NB2
Last Updated
2026-02-26

Before You Start / Safety

  • Inspect plugs only on a fully cool engine.
  • Blow debris out of plug wells before loosening plugs.
  • Work one cylinder at a time so lead routing cannot be mixed up.

High-risk quote: “you should only remove and install spark plugs in a cool engine, because metal expands when hot and thread damage can happen.” https://ngksparkplugs.com/en/resources/spark-plug-installation

Required Tools

  • 16 mm (5/8 in) spark plug socket + extension
  • Ratchet
  • Feeler gauge / wire gauge
  • Torque wrench (recommended for reinstall)
  • Compressed air / blower
  • Good light and magnifier for electrode reading

Required Parts / Fluids

  • Replacement spark plugs for your exact NB year/engine if inspection shows wear/fouling
  • Optional: ignition wire set if boots/leads are degraded

Model-specific notes (NB1 vs NB2)

  • NB1 and NB2 inspection workflow is the same (remove, read, measure, decide).
  • Exact plug part numbers, target gap, and interval vary by market/year and emissions calibration.

Manual-reference quote: “The Manuals are in PDF format. You can use Adobe Reader to read or print them.” https://www.mellens.net/mazda/

Step-by-Step Procedure

1) Record baseline symptoms first

Before pulling plugs, note symptoms: hot-idle stumble, misfire under load, rough start, fuel smell, emissions failure.

2) Remove one plug at a time

Label cylinder order if needed and avoid swapping leads between cylinders.

Community quote: “No mention of a possible mis-wiring of the spark plugs at the coil pack… return the plug wires to their proper places.” https://forum.miata.net/vb/showthread.php?t=279746

3) Read insulator and electrode condition

Use plug appearance as a diagnostic clue, not a standalone diagnosis.

  • Light tan/gray firing end: typically normal combustion.
  • Heavy dry black deposits: rich/weak ignition tendency.
  • Wet oil-like deposits: possible oil-control issue.
  • Glazed/melted signs: overheating/pre-ignition risk.

High-risk quote: “Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and possible engine damage.” https://ngksparkplugs.com/en/resources/read-spark-plug

4) Check gap and compare cylinders

Measure each plug gap and note cylinder-to-cylinder differences. Uneven wear patterns are often more useful than one plug in isolation.

High-risk quote: “A worn spark plug not only wastes fuel but also strains the whole ignition system because the expanded gap (due to erosion) requires higher voltages.” https://ngksparkplugs.com/en/resources/read-spark-plug

5) Decide: reuse, clean/re-gap, or replace

  • Replace if insulator is cracked, electrodes are eroded, or deposits are severe.
  • If reinstalling serviceable plugs, torque correctly and verify seat type.

High-risk quote: “Torque is one of the most critical aspects of spark plug installation.” https://ngksparkplugs.com/en/resources/spark-plug-installation

6) Reinstall and verify firing quality

Reinstall plugs and leads, start engine, then verify steady idle and clean throttle response.

Community confirmation quote: “No.3 clearly looks like it has a more grey insulator and more ashen coloured ground terminal.” https://www.mx5nutz.com/threads/reading-spark-plugs.391244/

Torque Specs / Capacities

  • Cross-checked public references show a common NB spark-plug tightening range near 11–17 ft-lb (15–22 Nm) for typical maintenance context.
  • Always prioritize exact plug manufacturer seat-type guidance and year/VIN workshop data.

High-risk quote: “Spark Plugs 11-17 (132-204 inch/lbs) 15-22” https://theapexdrag.com/mx-5-na-nb-torque-assembly-specs/

High-risk quote: “A new or reinstalled plug with a new gasket will require ⅜-⅝ turn… For reinstalled plugs with a used gasket only a ¼ turn is required…” https://www.championautoparts.com/Technical/Tech-Tips/Spark-Plug-Torque-Recommendations.html

Verification / Post-service checks

  • Stable idle after warm-up
  • No new hesitation under light acceleration
  • No ignition lead arcing/noise in dark inspection
  • No new DTC/CEL after short drive cycle

Uncertainty / Open Questions

  • VIN/market-specific NB gap and interval tables were not consistently extractable in this environment from official PDFs.
  • Use this inspection guide for condition assessment, then confirm final replacement spec from your exact model-year manual + plug catalog.

Images

No reusable images were added in this run.

Image Credits

  • No images used.

Sources

  1. Mellens.net — Mazda Miata Factory Service Manuals. Retrieved 2026-02-26. https://www.mellens.net/mazda/
  2. NGK — How do I “read” a spark plug? Retrieved 2026-02-26. https://ngksparkplugs.com/en/resources/read-spark-plug
  3. NGK — Spark Plug Installation. Retrieved 2026-02-26. https://ngksparkplugs.com/en/resources/spark-plug-installation
  4. Champion Auto Parts — Spark Plug Torque Recommendations. Retrieved 2026-02-26. https://www.championautoparts.com/Technical/Tech-Tips/Spark-Plug-Torque-Recommendations.html
  5. The Apex Drag — MX-5 NA/NB Torque & Assembly Specs. Retrieved 2026-02-26. https://theapexdrag.com/mx-5-na-nb-torque-assembly-specs/
  6. MX5Nutz Forum — Reading spark plugs. Retrieved 2026-02-26. https://www.mx5nutz.com/threads/reading-spark-plugs.391244/
  7. Miata.net Forum — No-Start Troubleshooting Guide. Retrieved 2026-02-26. https://forum.miata.net/vb/showthread.php?t=279746