Figure 3-77.Approximate vacuum gauge readings on a
normal operating engine.
burned breaker points, and spark plugs with the
electrodes set too closely.
A sticking valve could cause the gauge pointer to
bounce from a normal steady reading to a lower reading
and then bounce back to normal. A broken or weak
valve spring can cause the pointer to swing widely, as
the engine is accelerated. A loose intake manifold or
leaking gasket between the carburetor and manifold
shows a steady low reading on the vacuum gauge.
A vacuum gauge test only helps to locate the
trouble. It is not conclusive, but as you gain experience
in interpreting the readings, you can usually diagnose
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.
The commercial 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.
Next, place 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
Figure 3-78.Cylinder leakage tester.