What this test proves

Compare cylinder pressure behavior while other cylinders run, using an engine-specific approved method.

Tools

  • Tester or pressure transducer specifically approved for running compression
  • Cylinder-disable capability
  • OEM running-compression procedure

Preparation and precautions

  • This is an advanced test. Disable fuel to the tested cylinder as specified to prevent catalyst damage and fuel accumulation.
  • Control ignition hazards, secure adapters and observe equipment pressure/temperature ratings. Do not apply a conventional gasoline procedure to diesel or hybrid operation.

Connections

  • Install the approved adapter and instrument in the test cylinder. Use the required check-valve configuration; an ordinary peak-hold gauge may not show dynamic pressure correctly.

Equipment settings

  • Use the specified idle/speed conditions. For a transducer, choose a rated pressure range and timebase capturing complete engine cycles.

Test sequence

  1. Follow the OEM cylinder-disable procedure.
  2. Record pressure under the permitted running conditions.
  3. Compare with specified or equivalent cylinder data.
  4. Stop before changing connections; restore all systems and inspect for leakage.

Expected signal / readings

Running pressure differs from cranking pressure because cylinder filling and speed differ. Dynamic traces contain compression and pumping events.

Interpreting results

  • Abnormal filling or evacuation requires valve-timing, intake, exhaust and mechanical follow-up.
  • Peak pressure alone does not isolate the faulty part.

Common mistakes

  • Allowing fuel into a disabled cylinder
  • Using cranking compression limits
  • Revving with unsecured test equipment