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3.
INTRODUCTION
The use
of a vacuum gauge is often overlooked when determining mechanical condition and carrying
out fault
diagnosis on internal combustion
engines.
Monitoring actual manifold
vacuum
is invaluable when
troubleshooting
engine faults. This can only be
carried
out
by
using
a good quality vacuum
gauge
and this,
coupled with a hand-operated vacuum pump, also allows static testing of all types of vacuum operated
systems.
Set out on the
following
pages
are
applications
that the VS402.V2 vacuum tester kit can be used
for,
but it must
always
be remembered
that
these are
examples
only and reference to manufacturer's service
manuals
should always be
made
for correct testing procedures and specifications. In addition to this,
it
is
always recommended
that
additional
tests,
ie.
compression tests,
cylinder
leakage tests, ignition timing checks
etc.
be carried out to confirm indications of vacuum
gauge
readings.
4.
INLET MANIFOLD SYSTEMS
4.1.
Analysing Engine
Mechanical
Condition
via
Manifold Vacuum
Readings
4.1.1.
The
readings
shown
below are only examples of what may be
observed.
It
is
important to remember that it
is
the action of the needle
rather than
the actual
reading that is
more
important. Varying engine types will run different manifold
vacuum
pressures, depending on camshaft profile, valve
overlap,
timing
etc. so
<;~n
exact vacuum reading cannot be
specified.
The main criteria is
that the needle
reading
is between 16 to 21inHg and steady.
4.1.2.
Manifold vacuum
is also affected by altitude and
it
will drop approximately 1 inHg for
every 1000
feet
above sea level so this
rr.ust
also be considered when assessing
manifold
vacuum actual
readings.
4.2.
Basic
Diagnostic Tests
4.2.1.
Carry out the following procedures:
Step
1. Run engine
until
normal operating temperature
is reached.
Switch
engine
off.
Step
2.
Connect the
vacuum
gauge to a port directly
on
the
manifold
{fig.1) or on
the
carburettor/throttle body below the throttle butterfly.
Step
3.
Start and run the engine at
idle,
observing the
gauge
reading.
4.2.2. Refer to the following
table
to
indicate
any problems and
their possible
causes.
(
<!:>
Jack Sealey
Limited
READING:
16·21
inHg with
needle steady.
CAUSE:
Normal.
READIN
: Readings s lightly lower
han
for
normal engine
including
hen throttle is
suddenly opened
then
released.
AUSE: Wom piston
rings.
Carry
ut compression
test.
READING: When the throtlle is
suddenly opened
then
. . . -
the
needle
should drop
to below
5inHg
then
bounce
up
to
approximately
2SinHg settling back
to
original
reading.
CA
·
Normal.
READING: Reading
slightly
low
and
uctuating slowly.
AUSE: Over lea.l or rich mixture.
heck and
reset
in
accordance
with
manufacturers
specifications.
Original Language
Version
READING:
~
extrwnoly
low
steady.
USE: l.eaD>g
i n l * e -
system,
Faulty manifotd
gasl<et,
cal'buretto.-
base
gasket. split
vacuum hose. EGR valve seized.
READING: Regular
fluctuation
between
normal and low re3dings.
CAUSE: Blown head
gasket between
o
adjacent cylinders. Carry
out
y linder
leakage test
EADING:
Normal
when
first
starte
ut
drops
rapid
ly
when
revs
held
at
OOOrpm.
AUSE:
Restriction
in
exhaust
VS402.V2
I
Issue:
1
-
22/05112
)
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