Leak-down Test (Cylinder Leakage Test)

It is more precise than a compression test. The piston is set at top dead centre on the compression stroke. The results are given as a percentage of leakage. Many technicians combine it with a compression test: compression shows which cylinder is weak, and the leak-down test shows why. Listening at each opening identifies the leak path.

Air escaping through the exhaust, intake, oil filler or coolant shows whether valves, rings or the head gasket are leaking.

The crankshaft can turn suddenly when air is applied, so hold the engine securely. Compare all cylinders rather than relying on one reading.