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PART
8-1-GENERAL ENGINE SERVICE
8-9
it indicates a sticking valve.
MANIFOLD VACUUM TEST
A manifold vacuum test aids in
determining the condition of an en-
gine and in helping to locate the
cause of poor engine
performance.
To check manifold vacuum
:
1. Operate the engine for a mini-
mum of 30 minutes at 1200 rpm
or until the engine is at normal
operating temperature.
2. On 6-cylinder
engines,
install
an accurate, sensitive vacuum gauge
in the intake manifold fitting.
On a V -8 engine, remove the plug
or power brake line at the rear of
the intake manifold and install an
accurate, sensitive vacuum gauge.
3. Operate the engine at recom-
mended idle rpm, with the transmis-
sion selector lever in neutral.
4. Check the vacuum reading on
the
gauge.
Test Conclusions. Manifold vac-
uum is affected by carburetor adjust-
ment, valve timing, ignition timing,
the condition of the valves, cylinder
compression, the condition of the
positive crankcase ventilation sys-
tem, and leakage of the intake mani-
fold, carburetor, carburetor spacer
or cylinder head gaskets and a
restricted exhaust system.
Because abnormal gauge readings
may indicate that more than one of
the above factors are at fault, exer-
cise caution in analyzing an abnor-
mal reading. For example, if the
vacuum is low, the correction of one
item
may
increase
the
vacuum
enough so as to indicate that the
trouble has been corrected. It is im-
portant,
therefore,
that each cause
of an abnormal reading be investi-
gated and further tests conducted,
where necessary, in order to arrive
at the correct diagnosis of the
trouble.
Table l lists various types of read-
ings and their possible
causes.
Allowance should be made for the
effect of altitude on the gauge read-
ing. The engine vacuum will de-
crease with an increase in
altitude.
HYDRAULIC VALVE
LIFTER TESTS
Dirt, deposits of gum and varnish
and air bubbles in the lubricating oil
can cause hydraulic valve lifter fail-
ure or malfunction.
Dirt, gum and varnish can keep
a check valve from seating and
cause a loss of hydraulic pressure.
An open valve disc will cause the
plunger to force oil back into the
TABLE
1-Manifold Vacuum Gauge Readings
.
Gauge Reading
Engine Condition
18 inches or over-All engines.
Normal.
Except where otherwise
specified.
Low and steady.
Loss of power in all cylinders possibly
caused by late ignition or valve
timing,
or
loss of compression due to leakage around
the piston rings.
Very low.
Intake manifold, carburettor spacer or cyl-
inder head gasket leak.
Needle fluctuates steadily as
A partial or complete loss of power in one
speed increases.
or more cylinders caused by a leaking valve,
cylinder head or intake manifold gasket,
a defect in the ignition system, or a weak
valve spring.
Gradual drop in reading at
Excessive back pressure in the exhaust
engine idle.
system.
Intermittent fluctuation.
An occasional loss of power possibly
caused by a defect in the ignition system
or a sticking valve.
Slow fluctuation or drifting
Improper idle mixture adjustment or car-
of the needle.
buretor, spacer or intake manifold gasket
leak or restricted crankcase ventilation
system.
valve lifter reservoir during the time
the push rod is being lifted to force
the valve from its seat.
Air bubbles in the lubricating sys-
tem can be caused by too much oil
in the system or too low an oil level.
Air may also be drawn into the
lubricating system through an open-
ing in a damaged oil pick-up tube.
Air in the hydraulic system can
cause a loss of hydraulic pressure in
the valve lifter.
Assembled valve lifters can be
tested with tool 6500-E to check the
leak down rate. The leak down rate
specification (Page 8·68) is the time
in seconds for the plunger to move
the length (Page 8-68) of its travel
while under a 50 lb. load. Test the
valve lifters as follows:
1.
Disassemble and clean the lift-
er to remove all traces of engine
oil. Lifters cannot be checked with
engine oil in them. Only the test-
ing fluid can be used.
2. Place the valve lifter in the
tester, with the plunger facing up-
ward. Pour hydraulic tester fluid
into the cup to a level that will
cover the valve lifter assembly. The
ftuid can be purchased from the
manufacturer of the tester. Do not
use kerosene, for it
wiU
not provide
an accurate test.
FIG. 4-Piacing SJeel Ball in
Valve Liftttr Plunger
FIG. 5-Adjusting the Ram
Length
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