CYLINDER LEAKAGE TEST
Another aid in
locating compression leaks is the cylinder leakage tester. The principle involved is that
of simulating the compression that develops in the cylinder during operation. Compressed
air is introduced into the cylinder through the spark plug or injector hole, and by
listening and observing at certain key points, you can make some basic deductions.
testers, such as the one shown in figure 3-78, have a gauge indicating a percentage
of air loss. The gauge is connected to a spring-loaded diaphragm. The source of air is
connected to the instrument and counterbalances the action of the spring against the
diaphragm. By adjusting the spring tension, you can calibrate the gauge properly against a
variety of air pressure sources within a given tolerance.
In making a
cylinder leakage test, remove all spark plugs, so each piston can be positioned without
the resistance of compression of the remaining cylinders.
the piston at TDC or "rock" position between the compression and power strokes.
Then you can introduce the compressed air into the cylinder. Note that the engine tends to
spin. Now, by listening at the carburetor, the exhaust pipe, and the oil filler pipe
(crankcase), and by observing the coolant in the radiator, when applicable, you can
pinpoint the area of air loss. Aloud hissing of air at the carburetor indicates a leaking
intake valve, or valves. Excessive hissing of air at the oil filler tube (crankcase)
indicates an excessive air leak past the piston rings. Bubbles observed in the coolant at
the radiator indicates a leaking head gasket As in vacuum testing, indications are not
conclusive. For instance, a leaking head gasket may prove to be a cracked head, or bad
rings may be a scored cylinder wall. The important thing is that the source of the trouble
has been pinpointed to a specific area, and a fairly broad, accurate estimate of repairs
or adjustments required can be made without dismantling the engine.