Leak-Down Test
Reading results
No engine will have perfect sealing with 0 percent loss.
Air psi 15-20 to read loss 2nd gauge at 0psi, turn up air to hear leakage.
The percent of leakage should also be consistent across the cylinders.
The cylinder being tested must be placed at Top Dead Center (TDC).
5 - 10 percent loss indicates an engine in great to good running order.
10 - 20 percent loss can still run OK, but it’ll be time to keep an eye (or ear) on things.
20 + percent loss and it may be time for a teardown and rebuild.
30 + percent loss Major problems.
Any great differences indicate a problem in that cylinder.
TEST RESULTS
- Intake valve : Air whistling out of the intake, carburetor or throttle body indicates a leak at the intake valve.
- Exhaust valve : Air heard hissing out of the tailpipe, turbocharger or exhaust manifold means an exhaust valve leak.
- Piston rings : Whistling or hissing out of the PCV valve, oil filler cap hole or dipstick tube means the air is pushing past the rings. Suspect ring or cylinder wall wear.
- Head gasket : Air bubbles in engine coolant seen at the radiator filler cap could mean air escaping into the coolant past the head gasket.
- Cracked cylinder head : Bubbles in coolant or coolant being pushed up out of the radiator neck can also indicate cracks in the cylinder head or cylinder walls.
