[dtw]

Compressed air is introduced through the spark plug hole; twin gauges allow you to calculate the actual and percentage pressure drop in the cylinder (IIRC the first gauge measures input pressure at the tester, the baseline reading, and the 2nd gauge measures pressure in the cylinder). The pressure drop is the amount of leakdown. By listening carefully, the leakage can actually be heard through the breather or at the head. The location of the leakdown 'hiss' indicates ring wear (breather) or valve guide (head) wear. Leakdown tests are somewhat subjective depending on technique and equipment, but large or outlying leakdown values (when compared to other cylinders) can help pinpoint problems. Leakdown is a static test (engine is not turned over).

Best results are obtained by considering leakdown in conjunction with compression test results. A best practice (generally anyway) is to conduct both tests back to back. Otherwise, results of either test independently may be inconclusive. Compression tests can corroborate the results of a leakdown test. (If a cylinder leaks down at 11% versus 3% on all others, you can develop the expectation that compression test results on the same cylinder may also be poor due to bad sealing).