Figure 3-10. - 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 back to normal. A broken or weak valve spring would cause the pointer to swing widely as the engine is accelerated. A loose intake manifold or a leaking gasket between the carburetor and manifold would show a steady low reading on the vacuum gauge.

Vacuum gauge tests only help to locate the trouble. They are not always 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 test. 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.

There are commercial cylinder leakage testers available, but actually the test may be conducted with materials readily available in most repair shops. In addition to the supply of compressed air, a device for attaching the source of air to the cylinder is required. For a gasoline engine, this device can be made by using an old spark plug of the correct size for the engine to be tested. By removing the insulator and welding a pneumatic valve stem to the threaded section of the spark plug, you will have a device for introducing the compressed air into the cylinder.

The next step is to place the piston at TDC or "rock" position between the compression and power strokes. Then you can introduce, the com- pressed air into the cylinder. Note that the engine will tend 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. A loud hissing of air at the carburetor would indicate a leaking intake valve or valves. Excessive hissing of air at the oil filler tube (crankcase) would indicate an excessive air leak past the piston rings. Bubbles observed in the coolant at the radiator would ind

te a leaking head gasket.

As in vacuum testing, indications are not conclusive. For instance, the leaking head gasket may prove to be a cracked head, or the bad rings may be a scored cylinder wall. The important thing is that the source of trouble has been pinpointed to a specific area, and a fairly broad, accurate estimate of the repairs or adjustments required can be made without dismantling the engine.

In making a cylinder leakage test, remove all the spark plugs so that each piston can be positioned without the resistance of com- pression of the remaining cylinders. The com- mercial testers, such as the one shown in