LO 7-9 Outline diagnostic techniques used for evaluating the condition of a piston ring-to-cylinder wall seal

1.9. Cylinder Leak-Down Testing

Cylinder Leak-Down Testing
Cylinder leakage is the percentage of gas leakage past the piston rings. High leakage rates are usually due to worn cylinders and rings. Cylinder leakage testing is an accurate and meaningful measurement of the cylinders’ sealing capabilities. The leak-down test can be performed separately or together with compression testing. Cylinder leak-down testing is more practical for diesel engines than compression testing. During cylinder leak-down testing, air is fed into the cylinder at pressures of 80–120 psi (552–827 kPa), and the engine should be warm to minimize the clearance between the cylinder wall and the piston. With the valves closed, the piston is first positioned at TDC and then the engine is locked there. The cylinder leakage tester is connected to the cylinder and pressurized with regulated shop air. A second gauge on the leakage tester will measure actual cylinder pressure, which varies with the amount of cylinder leakage. A worn cylinder will allow more air leakage past the rings, and the gauge will read a lower pressure. Leakage is calculated as a percentage of measured cylinder pressure compared with regulated air pressure Figure 70. The rate of leakage is compared against the manufacturer’s specification to determine whether the amount of leakage is acceptable. Engine problems can be detected by listening for large volumes of air leaking out of the turbocharger, the intake or exhaust system, or even the oil filler tube. Excess air leakage in these areas may indicate burned or broken valves, damaged pistons, or simply worn cylinders. An engine in good condition should generally show only 5–10% leakage of the pressure of the air supplied to the cylinder after 1 minute. Some engines, in acceptable condition, may have leakage as high as 20%; more than 30% leakage indicates a severe problem Figure 71.

FIGURE 07-70: A leak-down tester supplies regulated air pressure to a cylinder and measures the percentage of regulated air pressure maintained in the cylinder. The percentage of leak-down is an indicator of cylinder and ring condition.

FIGURE 07-71: A cylinder leakage test reports the amount of air pressure retained in a cylinder, which corresponds to cylinder condition. © Snap-On Incorporated.