Cylinder-leakage testing really proves itself when you pinpoint the leak source:
• Air escaping out the tailpipe indicates a leaky exhaust valve.
• Air escaping into the intake manifold tells you there's a leaky intake valve.
• Air escaping into the radiator indicates a compression leak into the cooling system through a bad head gasket or a crack in the head or cylinder.
• Air escaping into the crankcase indicates blow-by due to worn rings, a worn or damaged cylinder, or a cracked or burned piston (for further diagnosis, a "wet" leakage test is needed).
WET IT DOWN
With a dry leakage test, you can tell whether the blow-by problem is due to the rings or taper wear in the cylinder by turning the crank to reposition the piston halfway down the cylinder. Cylinder wear is always greatest at the top, so moving the piston down from TDC should reduce the amount of leakage if taper is the problem. If the rate of leakage decreases, you've identified the problem and further diagnosis isn't needed. But if the rate of leakage remains unchanged, it tells you that taper wear is negligible and that the problem is either rings or possibly piston damage. That's where the wet test comes in.
Squirt a little oil into the cylinder through the spark plug hole to temporarily seal the rings. Turn the engine over several times so the oil will spread around the rings. Make sure the piston is back at TDC and repeat the leakage test. If the rate of leakage is now lower, the rings are causing the blow-by problem. No change? Then you've confirmed piston damage.
OPEN AND CLOSE
In the event that a leakage test doesn't show a problem, the fault may lie with the valve train. After all, a perfectly tight cylinder with a worn cam lobe, bent pushrod or damaged rocker arm will run just as bad as an engine with a burned valve. Unfortunately, there's no easy way to check other than yank off the valve cover and take a peek.
Add cylinder-leakage testing to your diagnostic strategies. You’ll quickly learn it makes cylinder diagnosis a more clockwork than guesswork procedure.