As we mentioned, a leakdown tester can help pinpoint problem areas in an engine. The key is to identify the exit path the pressurized air is taking. A few possible escape routes are shown here (arrows). Air passing through the oil fill hole or crankcase breather tube points toward worn rings as the culprit. If you hear it through the carburetor, there's probably a less-than-perfect seal at the intake valve. | 
If pressurized air is making it past the exhaust valve, it will make its presence known in the exhaust system. You have to listen for it, but we've heard of cases in which the air could actually be felt exiting the tailpipe. | 
A leakdown tester can be especially valuable in tracking down a blown head gasket. If air is getting into the cooling system during a leakdown test, you may see air bubbles in the radiator. During a test on another car, we heard a gurgling noise in the coolant passages, and pulling that head revealed a blown gasket. |
Spark plug logistics on a third-gen Camaro require that cylinders 6 and 8 be accessed from beneath the car. Yes, it was a pain, but we learned something from this close-quarters experience; we could hear the sound of pressurized air in the oil pan, indicating that air was getting past the rings. If we'd drained the oil after warming up the car, we might have even felt air exiting from the drain plug hole. It's something else to think about as you play leakdown detective with your own ride. | | |
Possible Causes for High-to-Moderate Cylinder Leakage
The cause of low pressure can be determined by listening for escaping air.* Air escaping from crankcase breather, dipstick tube, or sump plug hole: Defective rings or worn cylinder walls
* Air escaping from carburetor: Defective intake valve
* Air escaping from exhaust system: Defective exhaust valve
* Air bubbles in radiator or air escaping from adjacent spark plug hole: Leaking head gasket or crack in block or head Courtesy of Proform