310T Engine and Supporting Systems Practice Exam

A leak-down test checks how well a cylinder seals by pressurizing it with a regulated air source while the piston is at the top of the compression stroke so the valves are effectively closed. The standard approach is to thread a gauge adapter into the spark plug hole, connect a leak-down gauge, and apply steady air pressure (usually around 80–100 psi). By watching the gauge and listening for where the air escapes, you learn the condition of the cylinder’s sealing surfaces. If air leaks out through the intake or exhaust paths, the valves aren’t sealing properly. If air escapes into the crankcase (you can hear it or feel pressure at the dipstick), the piston rings aren’t sealing well. If air bubbles into the cooling system (radiator or overflow), the head gasket is compromised. This method pinpoints valve sealing, piston-ring sealing, and head-gasket integrity, making it the most informative way to assess cylinder sealing compared with other quick checks.