A cylinder leakdown tester goes a step beyond a standard compression test. Instead of measuring pressure generated by cranking, it injects external compressed air into a cylinder that is stationary at top dead center (TDC) on its compression stroke. The tool features two gauges: one measures the incoming air pressure (usually set to 100 PSI), and the other measures the pressure retained inside the cylinder. The difference between the two numbers shows the percentage of air leaking out of the combustion chamber.
A healthy cylinder will lose less than 10 percent of its air. If the leak is higher, you can find the root cause by listening to where the air escapes. Hissing at the tailpipe means an exhaust valve is not seating. Air whistling through the throttle body indicates an intake valve leak. If you hear bubbles in the radiator neck, the head gasket is blown or the cylinder head is cracked, pushing air into the cooling jacket. Hissing from the oil dipstick tube means air is blowing past worn piston rings.