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Engine compression test (periodic check)

Repeatable NB compression-test workflow with warm-engine prep, WOT crank method, cylinder-balance interpretation, and wet-test escalation logic.

Difficulty
Intermediate
★★★☆☆
Est. Time
45-90 min
Models
NB1 & NB2
Last Updated
2026-03-15

Before You Start / Safety

This guide is for Mazda MX-5 NB (1998-2005).

Compression testing is most useful when method is consistent. Inconsistent cranking speed or throttle position creates misleading numbers.

  • Warm engine first, then shut off for test setup.
  • Disable fuel and ignition before cranking.
  • Keep battery supported/charged for stable cranking RPM.

Required Tools

  • Compression tester with correct spark-plug adapter
  • Spark plug socket, ratchet, extensions
  • Battery charger/booster (recommended)
  • Notes app/sheet for per-cylinder values

Required Parts / Fluids

  • None required for dry test
  • Optional: small amount of engine oil for wet re-test on low cylinder

Step-by-Step Procedure

1) Warm engine and disable ignition/fuel

A warm engine gives more representative sealing values.

After shutdown:

  • disable ignition output,
  • disable fuel delivery,
  • prevent accidental start while testing.

2) Remove all spark plugs

Remove all four plugs so starter can spin engine evenly.

Tip: label removed plugs by cylinder if you will inspect/reuse them.

3) Install compression tester on cylinder 1

  • Thread adapter by hand first.
  • Seat tester firmly but do not overtighten.

4) Crank at wide-open throttle and record

  • Hold throttle wide open during crank.
  • Crank for a fixed pulse count/time each cylinder.
  • Record peak reading.

Consistency rule: same crank duration + same battery condition on every cylinder.

5) Repeat cylinders 2, 3, 4 identically

Do not change method mid-test. If battery slows, recharge before continuing.

6) Interpret cylinder balance first

Most valuable indicator is spread between cylinders, not one absolute number from one gauge.

Practical interpretation:

  • tight grouping = generally healthy sealing consistency,
  • one clear outlier = localized issue likely,
  • all low but even = test-condition issue or generalized wear.

7) Wet re-test low cylinder(s)

If one cylinder is low:

  • add small measured oil amount to that cylinder,
  • repeat test.

If value rises notably, ring/cylinder sealing is suspect. If little change, valve/head-gasket leakage becomes more likely.

8) Escalate to leak-down when needed

Compression test identifies that sealing is poor; leak-down identifies where it leaks.

Follow with leak-down test for definitive fault localization.

Practical test-quality checks

  • Cranking speed remains stable (battery support in use)
  • Throttle was fully open on every cylinder
  • Ignition/fuel remained disabled throughout
  • Same gauge and method used for all four cylinders

Verification / Post-service checks

  • Reconnect ignition/fuel systems correctly
  • Engine starts and idles normally post-test
  • Compression values logged with date/odometer for trend tracking

Sources

  1. MELLENS — Mazda Miata Factory Service Manuals (year/VIN compression procedure and specs reference source). Retrieved 2026-03-15. https://www.mellens.net/mazda/
  2. Flyin’ Miata Help Center — How to perform an engine compression test on your Miata (NB/NA-oriented practical method context). Retrieved 2026-03-15. https://help.flyinmiata.com/how-to-perform-an-engine-compression-test-on-your-miata-HJrIlAggh
  3. MX5Nutz — Compression Test thread (owner practice context: WOT and stable cranking). Retrieved 2026-03-15. https://www.mx5nutz.com/threads/compression-test.145824/
  4. wikiHow — How to Do a Compression Test (general safety and sequence context). Retrieved 2026-03-15. https://www.wikihow.com/Do-a-Compression-Test