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 VS402 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.
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 an exact vacuum reading
cannot be specified. The main criteria is that the needle reading is
between 16 to 21inHg 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.
fig.1
READING: 16-21 in Hg 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.
R
EADING: Regular fluctuation between
normal and low readings.
CAUSE: Blown cylinder head gasket
between two adjacent cylinders. Carry
out cylinder leakage test.
READING: When the throttle is
suddenly opened then released the
needle should drop to below 5inHg
then bounce up to approx. 25in Hg
settling back to original reading.
CAUSE: Normal
READING: Reading slightly low and
fluctuating slowly.
CAUSE: Over lean or rich mixture.
Check and reset in accordance with
manufacturer’s specifications.
V
S402.V3 | Issue - 1 14/09/16
Original Language Version
© Jack Sealey Limited
READING: Reading low but steady.
CAUSE: Retarded ignition timing.
Confirm using timing light and reset to
manufacturer’s specification.
