What is the procedure for a leak-down test and how are results interpreted?

What is the procedure for a leak-down test and how are results interpreted?

• A. Remove valve cover and visually inspect for leaks.
• B. Use a compression gauge on all cylinders sequentially.
• C. Use a vacuum test to identify leaks from the intake.
• D. Pressurize cylinder at TDC with inert air, listen for leaks and observe gauge; identify sources (valves, rings, head gasket) by leakage path.

Answer: (D)

Leak-down testing measures how well a cylinder seals when the piston is at top dead center with both valves closed. By pressurizing the cylinder with inert air at a known pressure and watching the gauge, you can see how much air leaks out and, by listening to where it escapes, identify the source. The procedure is to bring the engine to the compression stroke of the cylinder, disable ignition and fuel, remove the spark plug, and connect a leak-down tester. With the regulator set, air is introduced into the cylinder; the gauge should hold steady if the seals are good. A rapid drop means leakage; listen for where the air is coming out to pinpoint the path: air heard through the intake indicates a valve not sealing, air heard through the exhaust points to an exhaust valve problem, air escaping into the crankcase suggests piston-ring leakage, and air observed in the cooling system points to a head gasket leak or a crack. This path-based diagnosis helps distinguish whether the issue is valves, rings, or the head gasket, guiding the necessary repairs.

Leak-down testing measures how well a cylinder seals when the piston is at top dead center with both valves closed. By pressurizing the cylinder with inert air at a known pressure and watching the gauge, you can see how much air leaks out and, by listening to where it escapes, identify the source.

The procedure is to bring the engine to the compression stroke of the cylinder, disable ignition and fuel, remove the spark plug, and connect a leak-down tester. With the regulator set, air is introduced into the cylinder; the gauge should hold steady if the seals are good. A rapid drop means leakage; listen for where the air is coming out to pinpoint the path: air heard through the intake indicates a valve not sealing, air heard through the exhaust points to an exhaust valve problem, air escaping into the crankcase suggests piston-ring leakage, and air observed in the cooling system points to a head gasket leak or a crack.

This path-based diagnosis helps distinguish whether the issue is valves, rings, or the head gasket, guiding the necessary repairs.