Re: Leak test explanation please!
Cylinder leak-down test: A cylinder leakdown test uses compressed air to pressure up the combustion chamber with the piston locked in the TDC position or in any postion before port opening. The idea is to see how well the cylinder holds pressure by measuring how much loss there is. Typically 10% - 15% loss is acceptable, most of us shoot for the 90%+ range. If there is excessive leakage, using this method you can quickly diagnose the problem by listening to where the air is escaping: If you hear it comming out the exhaust that would indicate bad rings. If you hear it comming out of an adjacent spark plug hole, this would indicate a blown head gasket or warped head.
Some will argue that a leak-down test is more accurate than a compression test but that is not necessarily true. It does, however, allow you to pinpoint the problem if one exists. I will do a compression test first then break out the time-consuming leak-down tester if it fails the compression test.
