4. OPERATION
T
he 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 VS4021 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, i.e. compression tests, cylinder leakage tests, ignition timing checks etc. be
carried out to confirm indications of vacuum gauge readings.
5. INLET MANIFOLD SYSTEMS
5.1.
ANALYSING ENGINE MECHANICAL CONDITION VIA MANIFOLD VACUUM READINGS
5.1.1.
The readings shown below are only examples of what may be observed.
IMPORTANT:
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 an exact vacuum reading cannot be
specified.
The main criteria is that the needle reading is between 16 to 21 inHg
and steady.
5.1.2.
Manifold vacuum is also affected by altitude and it will drop
approximately 1inHg for every 1000 feet above sea level so this must
also be considered when assessing manifold vacuum actual readings.
5.2.
BASIC DIAGNOSTIC TESTS
5.2.1.
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. Refer to the following table to indicate any problems and their
possible causes.
READING: Reading low but steady.
CAUSE: Retarded ignition timing.
Confirm using timing light and reset
to manufacturer’s specification.
VS4021.V2 | Issue 2 (F,2,3) 26/02/19
Original Language Version
© Jack Sealey Limited
fig.1
READING: When the throttle is
suddenly opened then released the
needle should drop to below 5inHg
then bounce up to approx. 25 inHg
settling back to original reading.
CAUSE: Normal.
READING: 16-21 inHg with needle
steady.
CAUSE: Normal.
READING: Reading extremely low but
steady.
CAUSE: Leaking manifold intake
system, faulty manifold gasket,
carburettor base gasket, split vacuum
hose, EGR valve seized.
READING: Reading slightly low and
fluctuating slowly.
CAUSE: Over lean or rich mixture.
Check and reset in accordance with
manufacturer’s specifications.
R
EADING: Regular fluctuation
between normal and low readings.
CAUSE: Blown cylinder head gasket
between two adjacent cylinders. Carry
out cylinder leakage test.
