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 engine behavior.

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.