What this test proves

Measure cylinder pressure during cranking. Interpretation depends on engine design, temperature, cranking speed and test procedure.

Tools

  • Compression gauge and correct spark-plug/glow-plug/injector adapter
  • Battery support if specified
  • Engine-specific specifications

Preparation and precautions

  • Use a tester rated for the engine: a gasoline gauge is not a diesel gauge.
  • Follow the required temperature, fuel/ignition disable and plug-removal procedure. Secure the vehicle and observe starter limits.

Connections

  • Fit the correct adapter without damaging threads or exceeding the specified insertion length. Use the tester’s specified valve arrangement.

Equipment settings

  • Zero/reset between cylinders. Use consistent cranking cycles and the OEM throttle position; do not force an electronic throttle open.

Test sequence

  1. Verify adequate and stable cranking speed.
  2. Crank the first cylinder for the prescribed cycles and record pressure buildup and final pressure.
  3. Repeat on all cylinders using the same conditions.
  4. Compare absolute readings and allowable cylinder spread.
  5. Restore plugs, injectors and disabled systems using specified torque and replacement seals.

Expected signal / readings

Readings must meet the engine’s minimum and balance specification. There is no universal compression pressure for all engines.

Interpreting results

  • Low pressure may involve valve sealing, rings, head sealing, timing or poor test conditions.
  • Confirm the cause with leak-down or further mechanical tests; do not diagnose rings from pressure alone.

Common mistakes

  • Using different crank times
  • Ignoring low battery speed
  • Adding oil without an approved wet-test procedure, risking hydraulic lock