Cylinder Leakage Testing
Perform a cylinder leakage test by injecting regulated shop air into the cylinder head's spark plug hole, then visually searching and listening for a leak. We know the different places that must be sealed in a combustion chamber. There's the intake valve that leads to the intake manifold. We also have it's companion the exhaust valve that leads to the exhaust tailpipe. There's the piston rings that lead to the crankcase and the PCV valve in the valve cover. Then there’s the seal between the head and the block leading to the engine's cooling system. These are the places to search while the cylinder is under pressure.
So, if we introduce compressed air into the combustion chamber through the spark plug hole, we're be able to listen and see any air escaping through one of these areas.
For instance, if the intake valve were not seating correctly, air would escape past the valve and into the intake manifold. If you opened the throttle plate and heard air escaping during a cylinder leak test, you just identified the intake valve of that particular cylinder as the problem. If air is heard escaping through the tailpipe, the exhaust valve is faulty, probably burned. If air is escaping through the PCV valve, the piston rings are faulty. If air bubbles appear from the radiator neck the head gasket (most likely), cylinder head, or the block's deck are at fault. Always refer to the manufacturer's specifications before performing any automotive test.