Cylinder Leakage Test (CLT)

The cylinder leakage test can accurately pinpoint causes of leakage. Regulated compressed air is introduced into the cylinder through its spark plug hole (Figure 3.22). The piston is positioned at top dead center (TDC) on the compression stroke, ensuring that both valves are completely closed.
• It is better to perform a cylinder leakage test when the engine is warm and the rings are sealed with oil. Otherwise, a small amount of movement at TDC can allow the piston ring to move off its ring land, allowing leakage
• An acceptable leakage reading on the tester’s gauge is usually less than 10% to 15%, although vehicles with up to 30% leakage might still be performing to the owner’s satisfaction. The owner might not notice the power difference until it is restored after an overhaul, because the loss in power has happened gradually.

If a cylinder shows high leakage, listen to locate the sound of the leaking air. The following lists cylinder leakage test results. Here are some possible locations for leaking air and the likely causes:
• Oil filler = leaking rings or piston
• Manifold intake = leaking intake valve
• Exhaust pipe = leaking exhaust valve

FIGURE 3.22 A cylinder leakage tester is connected to the cylinder

through a hose to the spark plug hole

• Bubbles in the radiator = blown head gasket, or a crack in the head or block, which allows the regulated air to enter the cooling system


The leakage tester offers three advantages over a compression test:
1. The test can be performed on an engine that is removed from a car (such as an engine purchased at a salvage yard).
2. The exact source of leakage can be pinpointed before engine disassembly.
3. A racing camshaft will not affect the results of the test. It would cause lower readings on the compression test, however, because engine vacuum is lower at cranking speeds with a racing cam.


SHOP TIP


If leakage is past the piston rings, the PCV valve could allow air to travel into the intake manifold where it can cause the technician to mistakenly suspect a leaking intake valve. To avoid this situation, remove the oil filler cap, or disconnect the vacuum line to the PCV valve or pinch it with pinch pliers.


NOTES
• If a high mileage engine tests OK on the power balance test and has compression within expectations but has excessive cylinder leakage test results, carbon deposits in the combustion chamber could account for the relatively good compression, although the engine will probably have excessive blowby and may lack power.
• When a cylinder bore has considerable taper wear on the top, the reading can be improved if the piston is moved slightly past TDC into a less worn area.