Automotive Systems

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Vacuum Gauge Test


When an engine has an abnormal compression reading, it is likely that the cylinder head must be removed to repair the trouble. Nevertheless, the mechanics should test the vacuum of the engine with a gauge. The vacuum gauge provides a means of testing intake manifold vacuum, cranking vacuum, fuel pump vacuum, and booster pump vacuum. The vacuum gauge does not replace other test equipment, but rather supplements it and diagnoses engine trouble more conclusively.

Vacuum gauge readings are taken with the engine running and must be accurate to be of any value; therefore, the connection between the gauge and the intake manifold must be leakproof. Also, before the connection is made, see that the openings to the gauge and the intake manifold are free of dirt or other restrictions.

When a test is made at an elevation of 1,000 feet or less, an engine in good condition, idling at a speed of about 550 rpm, should give a steady reading from 17 to 22 inches on the vacuum gauge. The average reading will drop approximately 1 inch of vacuum per 1,000 feet at altitudes of 1,000 feet or higher above sea level.

When the throttle is opened and closed suddenly, the vacuum reading should first drop about 2 inches with the throttle open, and then come back to a high of about 24 inches before settling back to a steady reading as the engine idles, as shown in figure 3-77. This is normal for an engine in good operating condition.

If the gauge reading drops to about 15 inches and remains there, it would indicate compression leaks between the cylinder walls and the piston rings or power loss caused by incorrect ignition timing. A vacuum gauge pointer indicating a steady 10 inches, for example, usually means that valve timing of the engine is incorrect. Below-normal readings that change slowly between two limits, such as 14 and 16 inches, could indicate a number of problems. Among them are improper carburetor idling adjustment, maladjusted or 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.

Figure 3-77.—Approximate vacuum gauge readings on a normal operating engine.
Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

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