Section Il. ENGINE TEST AND PERFORMANCE ANALYSIS

TM 10-3930-633-34

Section Il. ENGINE TEST AND PERFORMANCE ANALYSIS

(3) Operate the engine at recommended idle

6-6. General

RPM, with the transmission selector lever in

The performance checks and tests outlined in

neutral.

this section will aid in diagnosing engine con-

(4) Check the vacuum reading on the gage,

dition and determine the extent and location of

and compare with table 6-1.

repairs required to bring the engine up to a

b. Test Results. Manifold vacuum is affected

serviceable condition.

by the carburetor adjustment, valve timing,

6-7. Intake Manifold Vacuum Test

valve lash, ignition timing, condition of the

a. Test Procedures. A manifold vacuum test

valves, cylinder compression, condition of the

aids in determining the condition of an engine and

positive crankcase ventilation system, and

in helping to locate the cause of poor engine

leakage of the intake manifold, carburetor,

performance. To check manifold vacuum:

carburetor spacer or cylinder head gaskets.

(1) Operate the engine for a minimum of 30

c. Readings. Because abnormal gage readings

minutes at 1200 RPM or until the engine is at

may indicate that more than one of the above

normal operating temperature.

factors are at fault, exercise caution in analyzing

(2) Connect an accurate, sensitive vacuum

an abnormal reading. For example, if the vacuum

gage to the intake manifold vacuum line or on the

is low, the correction of one item may increase the

fitting in the intake manifold. (See figure 6-1.)

vacuum enough to indicate that the trouble has

been corrected. It is important, therefore, that

each cause of an abnormal reading be investigated

and further tests conducted, where necessary, in

order to arrive at the correct diagnosis of the

trouble.

6-8. Compression Test

a. General. A cylinder compression test aids

in determing the condition of the valves, rings

and head gasket. This test should always be

performed at the recommended intervals to help

determine if any major engine repairs are

necessary.

Figure 6-1. Manifold Vacuum Test Connection.

6-2