A leak-down tester is a measuring instrument used to determine the condition of internal combustion engines by introducing compressed air into the cylinder and measuring the rate at which it leaks out.

Compression testing is a crude form of leak-down testing which also includes effects due to compression ratio, valve timing, cranking speed, and other factors. Compression tests should normally be done with all spark plugs removed to maximize cranking speed. Cranking compression is a dynamic test of the actual low-speed pumping action, where peak cylinder pressure is measured and stored.

Leak-down testing is a static test. Leak-down tests cylinder leakage paths. Leak-down primarily tests pistons and rings, seated valve sealing, and the head gasket.

Leak-down will not show valve timing and movement problems, or piston movement related sealing problems. Any test should include both compression and leak-down.

Testing is done on an engine which is not running, and normally with the tested cylinder at top dead center on compression, although testing can be done at other points in the compression and power stroke. Pressure is fed into a cylinder via the spark plug hole and the flow, which represents any leakage from the cylinder, is measured. Leak-down tests tend to rotate the engine, and often require some method of holding the crankshaft in the proper position for each tested cylinder. This can be as simple as a breaker bar on a crankshaft bolt in an automatic transmission vehicle, or leaving a manual transmission vehicle in a high gear with the parking brake locked.

Leakage is given in wholly arbitrary percentages but these “percentages” do not relate to any actual quantity or real dimension. The meaning of the readings is only relative to other tests done with the same tester design. Leak-down readings of up to 20% are usually acceptable. Leakages over 20% generally indicate internal repairs are required. Racing engines would be in the 1-10% range for top performance, although this number can vary. Ideally, a baseline number should be taken on a fresh engine and recorded. The same leakage tester, or the same leakage tester design, can be used to determine wear.

In the United States, FAA specifications state that engines up to 1,000 cu in (16 L) engine displacement require an 0.040 in (1.0 mm) orifice diameter, 0.250 in (6.4 mm) long, 60-degree approach angle. The input pressure is set for 80 psi (550 kPa), and 60 psi (410 kPa) minimum cylinder pressure is the accepted standard.

While the leak-down tester pressurizes the cylinder, the mechanic can listen to various parts to determine where any leak may originate. For example, a leaking exhaust valve will make a hissing noise in the exhaust pipe while a head gasket may cause bubbling in the cooling system.