Engine Leak-Down Tester
When an engine misfires, runs rough, or shows low compression on a traditional gauge, a leak-down test helps separate mechanical sealing problems from other faults. This controlled-pressure procedure measures how well each cylinder retains air and reveals where pressure escapes. For both gasoline and diesel engines, understanding the test results can point toward a head gasket, valve, or ring concern. The following explanation covers the direct answer and the diagnostic reasoning an owner needs before choosing repair work.
Direct answer
A leak-down tester is a diagnostic tool that pressurizes a cylinder with regulated shop air and compares the amount that escapes against the amount supplied. The gauge reads a leakage percentage, and the hiss of exiting air helps identify the leakage path. Intake or exhaust noise often indicates a valve seating issue, while crankcase venting suggests ring or cylinder wall bypass. Bubbles in the coolant can point toward a head gasket breach, making the instrument useful beyond a simple compression reading.
The tester is commonly found through general parts retailers and tool suppliers, including options for diesel engine use. Diesel versions typically rely on adapters that fit glow plug or injector bores rather than the smaller spark plug openings on gasoline cars. When working with any high-pressure cylinder, the engine must be rotated so the tested cylinder is at top dead center with both valves closed. The reading then provides a repeatable comparison across cylinders, helping narrow the mechanical source of compression loss before any parts are replaced.