Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Cylinder Combustion Pressure Leakage
The combustion pressure leakage test provides an accurate means for determining engine condition.
Combustion pressure leakage testing will detect:
- Exhaust and intake valve leaks (improper seating).
- Leaks between adjacent cylinders or into water jacket.
- Check the coolant level and fill as required. DO NOT install the radiator cap.
- Start and operate the engine until it attains normal operating temperature.
- Turn the engine OFF.
- Remove the spark plugs.
- Remove the oil filler cap.
- Remove the air cleaner hose.
- Calibrate the tester according to the manufacturer's instructions. The shop air source for testing should maintain a regulated air pressure at 552 kPa (80 psi).
- Perform the test procedures on each cylinder according to the tester manufacturer's instructions. Set the piston of the cylinder to be tested at TDC compression.
- During the testing, listen for pressurized air escaping through the throttle body, tailpipe and oil filler cap opening. Check for bubbles in the radiator coolant.
All gauge pressure indications should be equal, with no more than 25% leakage.
FOR EXAMPLE: Input air at 552 kPa (80 psi), the primary gauge factory set at 207 kPa (30 psi) input pressure. The secondary gauge should have no more than 176 kPa (25.5 psi) loss, when connected to the cylinder.
Refer to CYLINDER COMBUSTION PRESSURE LEAKAGE DIAGNOSIS CHART .
| CONDITION | POSSIBLE CAUSE | CORRECTION |
|---|---|---|
| AIR ESCAPES THROUGH THROTTLE BODY | Intake valve bent, burnt, or not seated properly | Inspect valve and valve seat. Reface or replace, as necessary. Inspect valve springs. Replace as necessary. |
| AIR ESCAPES THROUGH TAILPIPE | Exhaust valve bent, burnt, or not seated properly | Inspect valve and valve seat. Reface or replace, as necessary. Inspect valve springs. Replace as necessary. |
| AIR ESCAPES THROUGH RADIATOR | Head gasket leaking or cracked cylinder head or block | Remove cylinder head and inspect. Replace defective part. |
| MORE THAN 50% LEAKAGE FROM ADJACENT CYLINDERS | Head gasket leaking or crack in cylinder head or block between adjacent cylinders | Remove cylinder head and inspect. Replace gasket, head, or block as necessary. |
| MORE THAN 25% LEAKAGE AND AIR ESCAPES THROUGH OIL FILLER CAP OPENING ONLY | Stuck or broken piston rings; cracked piston; worn rings and/or cylinder wall | Inspect for broken rings or piston. Measure ring gap and cylinder diameter, taper and out-of-round. Replace defective part as necessary. |
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.