Ford Thunderbird 1957 - Manifold Vacuum Test; Engine Compression Test; Test Conclusions; Manifold Vacuum Gauge Readings

Section
2
-Tune-Up
1-9
Manifold
Vacuum
Test
A
test
of
manifold
vacuum
is
a
valuable aid
in
determining
the
condition
of
an
engine
and
also
in
help
ing
to
locate
the
cause of
poor
engine
performance. To
test
manifold
vacuum:
1.
Operate
the
engine for
a
minimum
of
V^-hour at
1200
rpm.
2.
Install
an
accurate,
sensitive vacuum
gauge
on
the
fuel
pump
end of
the
fuel
pump
vacuum
line.
3.
Run
the
engine at
recommended idle
rpm.
4.
Check
the
vacuum
reading
on
the gauge.
TEST
CONCLUSIONS.
Manifold
vacuum
is
affected
by
carburetor
adjustment,
valve
timing,
the
condition of
the
valves,
cylinder
compression,
and leakage
of the
manifold,
carburetor,
or
cylinder
head
gaskets.
Because
abnormal
gauge
readings
may
indicate
that
more
than
one
of
the above
factors is at
fault,
exercise
caution in
analyzing
an
abnormal 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
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.
Table
3 lists
various
types
of
readings
and
their
possible
causes.
This table
is
merely
a
guide,
however,
and not
a
firm
standard.
Allowance should be
made for the affect
of altitude
on
the
gauge
reading. The engine
vacuum
will
decrease
with an
increase
in
altitude.
Engine
Compression
Test
1.
Be sure the
battery
is
good.
Operate
the
engine
for
a
minimum
of
V2
hour
at
1200
rpm.
Turn
the
igni
tion
switch
off,
then
remove
all
the
spark
plugs.
2. Set
the throttle
(primary
throttle
plates
only
on
4-barrel
carburetor)
and
choke
in the
wide
open
posi
tion.
3.
Install
a
compression gauge
in No.
1
cylinder.
4. Crank
the engine
until
the
gauge
registers
a
maxi
mum
reading
and record
the
reading.
Note
the
number
of compression
strokes
required
to
obtain
the
maximum
reading.
5. Repeat the
test
on each
cylinder,
cranking
the
engine the same
number
of strokes
for
each
cylinder
as
was
required
to
obtain
a
maximum
reading
on
No.
1
cylinder.
TEST
CONCLUSIONS.
A
variation of 10
pounds
from
specified
pressure is
satisfactory.
However,
the
compression of
all
cylinders
should
be
uniform
within
10 pounds.
A
reading
of
more
than
10 pounds
above
normal
in
dicates excessive
deposits
in the
cylinder.
A
reading
of
more
than
10 pounds
below normal
indi
cates
leakage
at the
head
gasket,
rings,
or valves.
A low even compression in
two adjacent
cylinders
indicates
a
head gasket
leak.
This
should
be
checked
before
condemning
the rings or
valves.
To
determine
whether the rings or the
valves
are
at
fault,
squirt
the
equivalent
of
a
tablespoon of
heavy
oil
in
the
combustion
chamber,
then
crank
the
engine
to
Table
3
Manifold
Vacuum
Gauge Readings
Gauge
Reading
Engine Condition
18-20 inches
(6-cylinder
engine)
19-20
inches
(8-cylinder
engine)
Normal
Low and
steady
Loss
of
power in all
cylinders
caused
possibly
by
late
ignition or
valve
timing,
or
loss
of
compression
due
to
leakage
around
the
piston
rings.
Very
low
Manifold, carburetor,
or
cylinder
head
gasket
leak.
Needle
fluctuates
steadily
as
speed
increases.
A partial or
complete
loss
of
power
in
one
or
more
cylinders caused
by
a
leaking valve,
leaking
head
or
manifold
gasket,
a
defect
in
the
ignition
system,
a
weak valve
spring.
Gradual
drop
in
reading
at
engine
idle. Restriction
in the
exhaust
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
of the
needle. Improper
idle
mixture
adjustment, carburetor
or
manifold
gasket
leak,
or
possibly
late
valve
timing.