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Cylinder leak-down test

NB leak-down test workflow with precise compression-TDC setup, repeatable cylinder comparison, and clear leak-path interpretation for valve/ring/head-gasket diagnosis.

Difficulty
Advanced
★★★★☆
Est. Time
90-150 min
Models
NB1 & NB2
Last Updated
2026-03-15

Before You Start / Safety

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

Leak-down testing is a precision follow-up test after suspicious compression results.

  • Engine should be warm for realistic sealing behavior.
  • Use regulated compressed air only.
  • Keep transmission in gear/park and secure crank position: engine can rotate if not exactly at compression TDC.

Required Tools

  • Dual-gauge leak-down tester
  • Compressor + regulator
  • Spark plug socket/ratchet/extensions
  • Breaker bar for crank rotation
  • Notes sheet/app for per-cylinder values

Required Parts / Fluids

  • No parts required for testing
  • Optional spark plug replacement if due after test

Step-by-Step Procedure

1) Warm engine and prep for safe cranking work

  • Warm to operating temperature.
  • Shut off engine.
  • Disable ignition/fuel as needed.
  • Remove all spark plugs for easier manual rotation.

2) Set cylinder 1 to compression TDC

This is the most critical step.

  • Rotate crank by hand.
  • Confirm cylinder is at top dead center on compression stroke (both valves closed).
  • Hold crank position before applying air.

If piston is not exactly at compression TDC, air pressure can push engine off position.

3) Connect tester and set reference

  • Install adapter hose by hand.
  • Connect tester and regulated air source.
  • Zero/calibrate per tester instructions before reading leakage.

4) Apply air and record leak percentage + leak path

Record percentage and listen for escape location:

  • Intake hiss -> intake valve sealing issue
  • Exhaust hiss -> exhaust valve sealing issue
  • Oil filler/dipstick/crankcase hiss -> ring/cylinder leakage
  • Coolant bubbling/noise -> head-gasket or crack suspicion

5) Repeat cylinders 2-4 identically

Keep method consistent:

  • same reference pressure,
  • same TDC discipline,
  • same tool setup.

Consistency matters more than a one-off number.

6) Interpret by pattern

  • Even, moderate leakage across cylinders = typically uniform wear.
  • One-cylinder outlier = localized fault likely.
  • Multiple high cylinders = broad wear/timing or shared sealing issue.

Use leak-down together with compression data and observed symptoms.

Practical interpretation framework

Leak-down percentages vary with tester design and pressure baseline, but a common rule-of-thumb is:

  • 0-10%: very good
  • 10-20%: usable/aging depending mileage
  • 20%+: significant leakage; investigate cause

On NB diagnostics, cylinder-to-cylinder spread and leak location are usually more decisive than one absolute percentage.

Verification / Post-service checks

  • Reinstall spark plugs/ignition components correctly
  • Confirm normal start/idle post-test
  • Document all four cylinder results and leak paths for future trend tracking

Sources

  1. MELLENS — Mazda Miata Factory Service Manuals (year/VIN procedures and diagnostic cross-check source). Retrieved 2026-03-15. https://www.mellens.net/mazda/index.html
  2. AutoZone — How To Perform A Cylinder Leak Down Test (workflow and leak-path interpretation context). Retrieved 2026-03-15. https://www.autozone.com/diy/tools/how-to-perform-a-cylinder-leak-down-test
  3. Speedway Motors — How to do an Engine Cylinder Leak Down Test & Interpreting Results (compression-TDC importance and interpretation context). Retrieved 2026-03-15. https://www.speedwaymotors.com/the-toolbox/how-to-do-an-engine-cylinder-leak-down-test-and-interpreting-results/145764
  4. Rick’s Free Auto Repair Advice — How To Perform A Cylinder Leakdown Test (repeatable pressure/method guidance). Retrieved 2026-03-15. https://ricksfreeautorepairadvice.com/how-to-perform-a-cylinder-leakdown-test/